Structural And Functional Aspects Of The Biology Of Submerged Aquatic Macrophyte Communities In The Lower Chesapeake Bay Vol III Interactions Of Resident Consumers In A Temperate Estuarine


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      ,<;•    &OOL3
                                                                               PB03-233395

                                                                              EPA-600/3-83-047
                                                                              June 1983
                                                         BIOLOGY OF


                                           SUBMERGED AQUATIC MACROPHYTE  COMMUNITIES


                                                IN THE  LOWER CHESAPEAKE  BAY





                                                         Volume III
                                    Interactions of Resident  Consumers  in  a Temperate
                                     Estuarine Seagrass  Community: Vaucluse Shores,
                                                      Virginia,  USA.
                      Region HI Librory
                Environmental Protection Agency              Robert  j.  orth
                                    "  '                    and
                                                   Jacques  van Montfvans
           it *. Environmental Protection Agency
           r;.r,;<;n III information Resource

           *•-'''•'. ^ "     .      .        '    Virginia Institute of Marine Science
           rVi CheSJlUtMreei           .   ,        and School of  Marine Science
           • I: liviipfcia, PA  WWI     •   ./         College of  William and Mary
                                                .Gloucester  Point, VA  2.3062



                                                   Contract No.  R805974
                                                      Project  Officer
                                                     Dr. David Flemer
                                          U.S.  Environmental  Protection Agency
                                                  Chesapeake Bay  Program
                                                     2033 West Street
                                                   Annapolis,  MD   21401
                                                 KPRODUCSO 8r
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                                                     US DfP/UtKFH! Of COKMCRCf      r          '     ••--'!, ^<-.,~f
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                                                                          r.:-:M :•;:::/)
                                                                          i.'i r.- • -,ii; Street

                                                                          K--  -  :-M, PA
                                                                     EPA Report Collection
                                                                     Regional Center for Environmental Information
                                                                     US EPA Region III
                                                                     Philadelphia, PA 19103

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Regional Center tot Environmental Information
            US EPA Region III
               1650 Arch St.
           Philadelphia, PA 19103

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TECHNICAL REPORT DATA
(Please read Imtnit /:c'*n on the rri crsc hi jure c
1. REPORT NO.
EPA-600/3-83-047
4. TITLE AND SUBTITLE
BIOLOGY OF SUBMERGED AQUATIC MACROPHYTE COMMUNITIES IN
THE LOWER CHESAPEAKE BAY: Vol. III. Interactions of
Resident Consumers in a Temperate Estuarine Seagrass
Community; Vaucluse Shores, Virginia, USA
3 RECIPIENT'
PBS
5. REPORT DATE
June 1983
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
Robert J. Orth and Jacques van Montgrans
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Virginia Institute of Marine Science
Glr-'cester Point, VA 23062
10. PROGRAM ELEMENT NO.
11. CONTRACT/GRANT NO.
Grant R805974
12. SPONSORING AGENCY NAME AND ADDRESS
Chesapeake Bay Program
U.S. Environmental Protection Agency
2083 West Street
Annapolis, MD 21401
13. TYPE OF REPORT AND PERIOD COVERED
14. SPONSORING AGENCY CODE

EPA/600/05
15. SUPPLEMENTARY NOTES
16. ABSTRACT
As a part of the Functional Ecology Program on Chesapeake Bay submerged aquatic
vegetation (SAV), this study investigated infaunal and epifaunal trophic dynamics.
The work was conducted in conjunction with other major aspects of the program (see
Volumes I, II, IV) and respresnts the culmination of four years of intensive field and
laboratory investigations.
The main study area established for investigating the functional ecology of
resident consumers in the lower Chesapeake Bay was a large grass bed located at
Vaucluse Shores on the bayside of Virginia's eastern shore. Vaucluse Shores was chosen
as the study site because: (1) the site had been previously studied and background
information was available; (2) the bed is well established and historically stable;
(3) the area is relatively remote and unperturbed; (4) the bed contained the two
dominant lower Bay macrophyte species, Zostera marina and Ruppia maritima; and (5) the
bed was large enough to simultaneously accommodate varied studies and sampling
regimes. This bed was intensively mapped in 1978 and 1979, and permanent transects
were established for sampling reference points.
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EPA Form 2220-1 (Rev. 4-77) PREVIOUS EDITION is OBSOLE IE ^
-' -S. 12.?A Region III
\.;.'jit»nal Center for Environmental
fn formation
r'mia.-Jelphia, PA 19103

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                      NOTICE

This document has been reviewed in accordance with
U.S. Environmental Protection Agency policy and
approved for publication.   Mention of trade names
or commercial products does not constitute endorse-
ment or recommendation for use.
                       11

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                               TABLE OF CONTENTS

                                                                         Page

ACKNOWLEDGMENTS 	     V

PREFACE	    VI

CHAPTER 1.  Structural analysis of benthic communities associated with
            vegetated and unvegetated habitats by J. van Montfrans and
            R. J.  Orth	     1
               Abstract 	     2
               Introduction 	     3
               Methods and Materials	     5
               Results	     7
               Discussion	    31
               References	    35

CHAPTER 2.  Predator exclusion experiments in a Chesapeake Bay grass
            community by R. J. Orth and J. van Montfrans	    39
               Abstract	    40
               Introduction 	    41
               Study Site, Materials and Methods	    42
               Results	    47
               Discussion	    74
               References	    78

CHAPTER 3.  Predator-prey interactions in an eelgiass ecosystem in the
            lower  Chesapeake Bay, Virginia by R. J. Orth and J. van
            Montfrans	    81
               Abstract	    82
               Introduction 	    83
               Materials and Methods	    83
               Results	    86
               Discussion	    89
               References	    93

CHAPTER 4.  Secondary production of some dominant macroinvertebrate
            species inhabiting a bed of submerged vegetation in the
            lower  Chesapeake Bay by R. J. Diaz and T. Fredette	    95
               Abstract	    96
               Introduction 	    97
               Methods	    97
               Life Histories of Production Species 	    99
               Results and Discussion 	   114
               References	   122

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                         TABLE OF CONTENTS (continued)
                                                                         Page
CHAPTER 5.  Preliminary effects of grazing by Bittium varium on
            eelgrass periphyton by J. van Montfrans, R. J. Orth and
            S. Vay	   124
               Abstract	   125
               Introduction 	   126
               Methods and Materials	   128
               Results	   129
               Discussion	   137
               References	   159

CHAPTER 6.  Aspects of waterfowl utilization in a mixed bed of
            submerged vegetation of the lower Chesapeake Bay by
            E. Wilkins	   163
               Acknowledgments	   164
               Abstract	   165
               Introduction 	   166
               Methods	   169
               Results	   175
               Discussion	   205
               Summary	   214
               References	   216

CHAPTER 7.  Trophic relationships in a grass bed based on
            values by J. van Montfrans and R. J. Orth	   221
               Abstract	   222
               Introduction 	   223
               Methods and Materials	   224
               Results and Discussion 	   224
               References	   231
                                      IV

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                                ACKNOWLEDGMENTS

     The work for this volume depended upon  the  cooperation  and coordination
of and could not have been achieved without  the  talents  of the  many
scientists, graduate students,  technicians and summer  aides  at  this Institute.
In particular, we want to thank Tom Fredette, Glenn  Markwith, Page  Mauck,
Cindy Miekly, Cliff Ryer, Linda Schaffner, Terry Stahl,  Lee  Stone,  Tarn Vance,
Anna Vascott, Stephanie Vay and Elizabeth Wilkins.   We are also indebted  to
Dr. Don Boesch for his initial  efforts in the organization of our program.

     Carole Knox, Shirley Sterling and Nancy White are gratefully acknowledged
for their efforts in typing the many drafts  of this  report.  Joe Gilley,  Bill
Jenkins, Sylvia Motley, Kay Stubblefield and Ken Thornberry  provided  expert
assistance in areas of the art work, photographic work and reproduction
aspects.

     Mr. William Cook, who was  the former project officer of this program,  was
a key figure for insuring the smooth operation of the  administrative  aspects
of the grant for which we are deeply thankful.

     Our thanks also go to the many people in the Chesapeake Bay Program, both
at the state and federal levels and to the Citizens  Program  for allowing  the
Submerged Aquatic Vegetation Program to become a reality as  a major area  of
research in the Chesapeake Bay.

     The final copy of this report was prepared  by the VIMS  Report  Center to
whom we owe gratitude.

     Finally, a hearty thank you to those whose  contributions we may  have
overlooked.

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                                     PREFACE

     One of  the most  notable  features  about  habitats  with submerged aquatic
vegetation (SAV)  is  the  characteristically high  density of the associated
fauna.  Included  are  epibiota  and  infauna  which  are represented by a diverse
and complex  assemblage of micro- and macroalgae,  protozoans,  hydrozoans,
anthozoans,  turb^ll^rians,  gastropods,  isopods,  araphipods, polychaetes,
oligochaetes, bivalves,  decapods and barnacles.   Many of these groups exhibit
distinct seasonal  pulses of abundance  depending  on their individual spawning
periods.  The epibiotic  community  within grass beds is quite  distinct from the
communities  in adjacent  unvegetated  areas.   Due  to the lack of a suitable
substrate, there  is  usually very little epifauna  in bare sand or mud areas.
The  pibio a primarily use  the  blades  as a substratum for attachment (i.e.
barnacles, algae,  hydroids, etc.)  or feeding platform in the  case of micro
herbivores grazing on the microalgae that  colonize the blades.  Thus, grass
beds provide substiates, protection  and food resources which  allow for the
maintenance  of high  densities which  in turn  attract and trophically support
numerous migratory utilizers of SAV  habitats,  i.e. crabs,  fishes and
waterfowl.  These  features  are  fundamental to the resource value of SAV beds
on a world wide basis.

     The fact that numerous species  of  epibiota  associated with SAV may not be
totally dependent  for their survival on the  presence  of grass (since many can
exist on almost any  biotic  or abiotic  substrate)  does not  detract from the
importance of SAV.  Submerged vegetation is  a renewable resource, unlike  many
other substrates,  and so persists  through  time.

     The infaunal  community also appears to  benefit from the  presence of  SAV.
This community is  quite distinct from  that of adjacent unvegetated areas.
There is a tremendous increase  in  the  numbers  of  species and  individuals  in
vegetated habitats which is in  part  related  to increased sediment stability,
microhabitat complexity, greater food  supply and  decreased predation pressure.
The motile community consisting of larger  macroinvertebrate species (e.g.
shrimp, crabs, and fish) is also diverse and distinct from surrounding
unvegetated areas.

     Migratory waterfowl species such  as Canada  geese,  redheads and widgeon
are closely associated with beds of  submerged  grasses because of the
importance of the grass as  a food  resource.   Abundances of certain waterfowl
species which depend on submerged  grasses  for  food have declined in the
Chesapeake Bay in conjunction with the  decline of Bay grasses during the  last
15-20 years.

     The trophic function of SAV communities and  the  refuge factor that SAV
provides appear to be the key to understanding the role these habitats play in

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supporting  living resources  of  direct  importance  to man.   These two attributes
are so functionally  interrelated  that,  although  it  may be necessary to
separate the two for  the  purpose  of modelling  the  system, they must be
addressed with a unified  research  approach.

     Our study on the  functional  ecology of  resident  consumers as a part of
the Functional Ecology Program  on  Chesapeake Bay  SAV was  concerned with an
investigation of infaunal  and epifaunal  trophic  dynamics.  This work was
conducted in conjunction  with the  other  major  aspects of  the program (see
Volumes I,  II, IV) and represents  the  culmination  of four years of intensive
field and laboratory  investigations involving  many  dedicated co-workers.

     The main study  area  established  for investigating the functional ecology
of resident consumers  in  the lower Chesapeake  Bay  was a large grass bed
located at  Vaucluse  Shores on the  bayside  of Virginia's eastern shore.
Vaucluse Shores was  chosen as the  study  site because: 1)  The site had baen
previously  studied and background  information  was  available; 2) the bed is
well established and  historically  stable;  3) the  area is  relatively remote and
unperturbed; 4) the  bed contained  the  two  dominant  lower  Bay macrophyte
species, Zostera marina and  Ruppia maritima; and 5) the bed was large enough
to simultaneously accomodate varied studies  and  sampling  regimes.  This bed
was intensively mapped in  1978  and 1979, and permanent transects were
established for sampling  reference points.

     Our initial effort in examining  the functional ecology of resident
consumers was to determine the  structural  aspects  of  the  grassbed community
compared to unv^getated areas (Chapter  1).   We subsequently conducted predator
exclusion experiments  to  determine the  role  of predation  in structuring the
biotic community in  grassbeds (Chapter 2)  and  examined in greater detail
predator-prey interactions in vegetated  habitats  (Chapter 3).   Having
established which species were  numerically dominant,  we calculated the
secondary production  of those species which  were  trophically or functionally
important (Chapter 4).  We then focused  our  attention on  one dominant species,
an herbivorous grazer, and examined its  role in  controlling epiphytic fouling
on Zostera marina (Chapter 5).  Because  waterfowl have been the least studied
trophic components of  the grass bed systems  in the  lower  Bay,  we determined
the intensity of utilization by wintering  waterfowl of the Vaucluse grass
system (Chapter 6).   We also measured  the  impact  of feeding by one species
(Buffleheads) on the  density of macroinvertebrate  population densities.
Finally, we tried to  place into perspective  major  trophic links in the
Vaucluse Shores grassbed by  examining natural  carbon  isotope ratios (•'••'C to
"C) in some of the dominant species  (Chapter  7).   Such an approach enabled us
to determine the sources  of  primary production utilized by the resident
consumers.

     We have written  each chapter  as  a unit  to alljw  for  easier presentation
of the data ami to facilitate'the  submission of discrete  sections to peer
reviewed scientific journals.   Chapter 5 has already  been accepted for
publication in Aquatic Botany and  other  chapters are  being redrafted for
publication at a later date.  Although some  chapters  lack comprehensive
statistical analyses  and data interpretation,  our goals are to thoroughly
conduct such revisions prior to publishing our findings.   These products will
be available in journals at  a future  date.
                                   vii

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                     CHAPTER 1

    STRUCTURAL ANALYSIS OF BENTHIC COMMUNITIES
ASSOCIATED WITH VEGETATED AND UNVEGETATED HABITATS

                        by

               Jacques van Montfrans

                        and

                  Robert J. Orth

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                                   ABSTRACT

     Three distinct habitats  in  the  lower  eastern  Chesapeake  Bay (Vaucluse
Shores at the mouth of Hungars Creek) were compared  based  on  a structural
analysis of the associated  fauna.  These habitats  included a  grassbed (Zostera
marina and Ruppia maritima),  large sand patches  within the grassbed,  and  an
offshore sand bar system.   Within the vegetated  habitat,  comparisons  were made
of the fauna associated with  pure stands of  "L_. marina, pure stands  of
JR. maritima and mixed stands  of  both species.

     Generally there was a  trend towards a greater species diversity  (Shannon
H1) and abundance of infaunal species and  individuals  in  the  vegetated
habitats than in the two sand habitats.  Many  species  which occurred  as  one of
the top ten in each habitat persisted throughout the course of the  study
(July, 1978 - Nov. 1979) and  were characteristic of  the habitat  examined.
Although infaunal abundances  were concentrated at  the  sediment surface in all
three habitats, the grassbed  supported a larger  number of  individuals deeper
in the sediments than did the other  two habitats.

     The epifaunal component  of  the  vegetated habitat  comprised  a unique  and
diverse assemblage of species which  was similar  between each  area investigated
(i.e. Zostera marina, Ruppia  maritima, and mixed stands).   Few seasonal
patterns in epifaunal abundance  were evident in  the  data.   Vegetated  areas
provided greater habitat heterogeneity and were  therefore  capable of
supporting a greater overall  diversity of  species  than nonvegetated habitats.

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INTRODUCTION

One of the most notable featuras of the shallower margins along the
Chesapeake Bay is the presence of submerged macrophytes in many ar°as. Over
8,400 hectares (20,750 acres) of the nearshore mesc- and polyhaline and
sublittoral zone is vegetated by Zostera marina and/or Ruppia maritima (Orth,
et al. 1979). This submerged aquatic vegetation (SAV) adds a third dimension
to an otherwise relatively flat saudy bottom and provides a food source,
substrate and refuge for numerous species allowing .for the maintenance of high
faunal densities. The large stock of organisms inhabiting grassbeds is
thought to be fundamental to the resource value of SAV.

Most invertebrates assimilate energy fixed by seagrasses via a detrital
pathway (Fenchel, 1977; Klug, 1980). These detritivores are in turn utilized
by resident and migratory consumers such as crabs, fishes and waterfowl,
thereby providing an important trophic link between primary producers and
species in higher trophic levels (Carr and Aosras, 1973: Brook, 1975, 1977;
Adams, 1976; Stontr, 1979; Stoner and Livingston, 1980; Zimmerman, 1979;
Nilsson, 196'JO . The transfer of energy from SAV to migratory waterfowl
species including swans, geese and some ducks, is more immediate through a
direct consumption of the macrophytes (Verhoeven, 1978; Bayley et al., 1978;
Cottarn and Munro, 1954).

The biotic community within grassbeds can be quite distinct from that of
unvegetated areas. The epifaunal and infaunal components are represented by a
diverse and complex assemblage which includes macto- and microalgae,
protozoans, nematodes and other meiofauna, hydrozoans;, bryozoans, polychaetes,
oligochaetes, mollusks, crustaceans and several other groups. Many species
exhibit distinct seasonal pulses of abundance depending on their individual
spawning periods (Stevenson and Confer, 1978).

Numerous epifaunal spe.cies are generally not found on sandy bottoms
unless a suitable substrate such as large shells are present. The fact that
the epifauna may not be totally dependent on the presence of grass but is able
to exist on a variety of non-living substrates does not diminish the
importance of seagrasses as a habitat for these species. Unlike many inert
substrates, marine grasses represent a renewable resource for colonization.
This quality accounts in part for the high faunal diversity and density found
in grassbeds from one year to the next.

The infaunal community of grassbeds is also quite distinct from that
found in adjacent unvegetated areas with substantially greater numbers of
species anu individuals found in vegetated areas. This increase may be
related to greater sediment stability, microhabitat complexity and/or food
supply (Orth, 1977; Thayer, Adams and La Croix, 1975). Orth (1977) found the
i

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infauna  of  a Chesapeake  Bay  "L_.  marina bed  to increase in density and diversity
from  the  edge  of  the  bed to  its center.  A similar positive correlation
existed  with species  abundance, as  well  as divers-'ty, and the increasing size
of the bed.  He related  greater faunal abundances to the sediment stabilizing
function  of eclgrass.  Orth  (1977)  showed  experimentally that through
decreasing  the stability of  sediments by clipping blades of grass near the
sediment  surface  and  by  simulating  wave  action,  the density and diversity of
the infauna was decreased.   Natural biological disturbances such as cownose
ray activity had  similar affects  (Orth,  1975).  The vertical component
provided  by seagrasses which is differentiated into leaves, stems, rhizomes
and roots increases microhabitat  complexity and  supports a greater faunal
diversity than is  found  in unvegetated bottoms (Kikuchi 1980).  Furthermore,
numerous  species  of animals  which do not feed directly on the seagrasses are
thereby  able to exist  in such vegetated  habitats (Thayer et al. 1978).

      The  natant community associated with  SAV is diverse and quite distinct
from  that of surrounding unvegetated areas (Orth and Heck, 1980; Heck and
Orth, 1980; Kikuchi,  1974).   Many species  comprising this community rely on
the macrophytes during certain  critical  life history stages.  Hardwick (1973)
found that  the West Coast herring,  Clupea  harengus pallasi, used eelgrass
leaves for egg attachment.   East  coast species which use grassbeds in a
similar manner include the halfbeak, Hyporhampnus sp. and the rough
silverside, Meaibras martinica (John Olnsy, pers. comm.).  The toadfish Opsanus
tau uses  the rhizomes  as attachment sites  for its eggs  as well (Orth, pers.
comm.).   One of the more complete studieo  of eelgrass fuh communities was
conducted by Adams (1976a,b,c)  in North  Carolina.  He found the highest fish
biomass when temperature and eelgrass biomass were greatest.  Further, food
produced  within the grassbed could  have  accounted for approximately 56% by
weight of the diet of  the fish  in this community.  The  high fish production
was due  to juveniles whic'.i had  higher growth efficiencies than older fishes.
They  accounted for 79-84% of the  total annual fish production.  In addition to
fish, natant invertebrates such as  shrimp  and blue crabs are found in
considerably greater abundance  as both juveniles and adults in eelgrass beds
than  in  sandy habitats (Lippson,  1970;  Heck and  Orth, 1980).  Changes in
eelgrass  abundance are even  thought to cause variations in the commercial
catch of  blue crabs.  Thus,  it  appears  that  grassbeds provide numerous
advantages to a variety  of species  and constitute a valuable natural•resource
in the Chesapeake Bay.

      In addition  tv> the  biological  benefits  provided by submerged raacrophytes,
the plants serve  other functional roles  such as  buffering erosion by trapping
sands and pumping nutrients  from  the sediments to the leaves and eventually to
the water column.  These lunctions  are generally not achieved by artificial
substrates which  further emphasizes  the  importance of SAV in the marine
environment.

     When studying the dynamics  of  a particular  habitat, a knowledge of both
the structural complexity and functional aspects of the system are desirable.
Both  serve as a means for evaluating the habitat for management  purposes,
particularly if other habitats  have  been similarly studied so that comparative
data are available.  The objective  of this section is to compare the
macroinvertebrate assemblages associated with the different habitat types

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 found  at  the  Vaucluse  Shores  study site.   These include the grassbed proper
 (eelgrass,  widgeongrass,  and  mixed vegetation), inshore unvegetated sand
 patches within  the  grassbed,  and  an offshore series of sand bars and troughs
 which  separate  the  vegetated  area from the deeper waters of the Chesapeake
 Bay.

                              METHODS AND  MATERIALS

     Routine  sampling  was  scheduled to coincide with major biological events
 in  the grassbed  and  adjacent  areas.   These events included the arrival of
 major  predators  in  the system (early spring),  the partial defoliation of Z.
 marina (mid-sunnier), and  the  predominant  larval settling periods (spring and
 fall).  Such  timing, rather  than  quarterly sampling, would yield the beet data
 on  the structural  characterization of the grass bed and adjacent habitats.

     Three  habitats  (Fig.  1)  were sampled six  times (July and October, 1978
 and April,  June  and  September and November,  1979) to determine quantitative
 and qualitative  differences  in their associated fauna.  The habitats included
 an  offshore sandbar  system (outside  sand  or  OS),  sandy patches within the
 grass  bed (inside  sand or  IS) and the grass  bed (G> proper (Fig. 1).
 Vegttated areas  included  an  inshore  Ruppia maritima Lone, a Zostera marina
 zone offshore and  a  mixed  stand of vegetation  in between.  Samples were
 considered  to be from  the  mixed area if the  least abundant plant species
 comprised a minimum  of 15% of the total vegetational dry weight biomass in the
 sample .

     Initially,  10  stations were  established in each habitat.   However, data
 analysis  from the  first sampling  indicated that 5 rather than 10 stations
 adequately  represented the infauna in each of  the two sand habitats.  One
 sediment  (3.8 cm^)  and three  macroinfaunal (0.007 m^ each) cores were taken at
 each station.  Prior to taking infaunal and  sediment cores in the grass bed an
 epifaunal sample was taken at each station by  clipping and collecting grass
 from the  area to be  sampled.   Coring was  then  conducted within the clipped
 area.  Coring was chosen as the appropriate  sampling method because the root
 and rhizome mat  in  the grassbed made sampling  with other devices difficult.
 In order  to maintain gear  comparability,  identical cores were used in
unvegetated habitats.  All samples were taken  while diving with SCUBA.

     Vertical distribution of infauna was examined in July 1978.  A 35  cm long
plexiglass  core 9.4 cm in  diameter (0.007 ra^)  was used to collect infaunal
 samples.  One such core sample was taken  at  each  station.   The top 10 cm of
each sample was sectioned  vertically into 2  cm intervals and  the remaining
material was divided into  5 cm intervals.  Based  on these data, it was
determined  that a sample depth of 15 cm adequately collected  the infauna.

     Before sieving and preservation samples were held for at  least 30 min. in
 labelled  plastic bags  containing  isotonic MgCl2 as a relaxant.   This kept many
of the .ET^sller polychaetes and oligochaetes  from  fragmenting  and/or crawling
through the sieve.   All infaunal  samples  were  washed through  0.5 mm mesh
sieves and  the retained material  was preserved  in 10% buffered  seawater
 formalin.  A vital  stain (Rose  Bengal)  was added  to facilitate  laboratory
sorting.  All invertebrates in each  sample were removed from  the remaining

-------
    R = RUPPIA

    2 = ZOSTERA

    S=SAND

    /= MIXED

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Fig. 1.  Study site showing vegetation zones, the inside sand station (IS)

       and the outside sand station (OS).

-------
sediments  and  associated plant debris using forceps while microscopically
examining  spoonfulls of each sample placed in a petri dish.

      Epifaunal  samples  were  collected by clipping plants to within 2-3 cm of
the  sediment surface and easing the blades into a collecting bag with a 0.5 mm
mesh  bottom (Marsh  1973).   Samples were kept in water and processed live by
stripping  all  epifauna  from  the blades and preserving them in 102 buffered
seawater  formalin containing the vital stain Rose Bengal.  The remaining plant
material was sorted  to  species (Ruppia, Zostera and algae), oven-dried at 80°C
for  at  least 48  h and then weighed to the nearest 0.1 g.

      Numerical  abundance histograms were plotted by area for both species and
individuals  found in the infaunal and epifaural (grassbed only) community.
Species diversity was calculated for each area using the index of Shannon
(Pielou,  1975).  The index H1  is expressed as:


                              H1  =Z  Pi Iog2 Pi


where s =  number of  species  in the sample and Pi = proportion of the i-th
species in the  sample.   This index is commonly used for comparative purpo'^s
and  includes both a  species  richness (the number of species in a community)
component  and  an evenness  (how equitably the individuals are distributed
between the  species) component.

                                     RESl'LTS

     Cumulative  species  curves for vertically sectioned cores taken in each
habitat approached a plateau after the number of species from the top 15 cm of
sediment had been plotted  (Fig.  2).  Although most species  in each habitat
were  found in  the top 15 cm  of sediment,  the composition and numbers of
individuals  of  the dominant  taxa differed from one area to  the next (Table 1).
Infaunal abundances  were concentrated at  the sediment surface in all three
habitats,  but  the grassbed supported a larger number of individuals deeper in
the sediments  than did  the other two habitats (Table 1).

     Generally  infaunal  species  means per core were twice as great for
vegetated  areas  as those for unvegetated  habitats (Fig. 3).  Samples from the
sandy habitat had a  mean of  between 4 and 11 species per core whereas means
for vegetated areas  ranged from  14 to 30  species per core.   With the exception
of the September and  November,  1979 sample dates,  the number of infaunal
species of  the  inside sand habitats showed slightly higher  abundance than the
more dynamic offshore sand habitat.  Within the three vegetated areas (Ruppia,
Xostera, and mixed)  fewer  species  were uaually associated with Ruppia than
were  found  in the Zostera  or mixed zones.

     Seasonal trends  in  species  abundances were not readily apparent.   Mean
numbers of species per core  were  depressed in all  habitats  during September,
1979.  Otherwise, distinct seasonal pulses in species abundances were obscure.

-------
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INSIDE  SAND--
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        4.0     6.0     8.0     10.0     15.0    20.0    25.0 DEPTH (in cm.)
Fig.  2.   Cumulative  species curves of vertically sectioned cores from

          three habitats.       .  •

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              JULY  1978
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-------
     The .lean number  of  infaunal  individuals  per core generally followed the
trends seen  for species,  with  those  in the sandy area being lower than those
from the vegetated bottom (Fig. 4).   One  exception occurred in July, 1978,
when densities of  individuals  (x  of  about 300 per core) at the inside sand
stations exceeded  that  found  in any  other habitat.  Typically, samples had
means of around 20 individuals per core and in all but the least two sample
periods, data from the  inside  sand habitat showed a greater mean abundance of
individuals  than that  from the more  dynamic sand bar area.  In general, the
range in numbers of individuals per  core  in the nonvegetated areas was greater
than that for the grassbed as evidenced by the wider scatter of individual
sample data  points.   This  implies that the sand-associated fauna were more
patchily distributed  than the grassbed fauna.

     There was no consistent  pattern for  mean infaunal individual abundances
between the  three vegetated areas (Fig. 4).  On July and October, 1978 and
November 1979, the greatest number of individuals were found in the mixed
habitat whereas the Ruppia zone showed greatest abundances in April and
September 1979.

     Seasonal trends  in  the number of individuals were clearly evident in the
vegetated area (Fig. 4).   Mean abundances (between 500 to 1000) were
considerably increased during April  1979  with the lowest means evident the
following September (between 50 and  110).  The months of October and September
showed the lowest mean abundances of individuals from 1978 and 1979 with
between 100-200 and 50-110 individuals, respectively.

     Mean species diversities (Shannon H1) for the infauna varied widely over
the study period ranging  from 1.6 to 2.2  in the outside sand area; 1.4 to 2.7
in the inside sand habitat; and 2.0  to 3.7 in the grassbed (Fig. 5).  In
general diversities were  higher in the grassbed than in the nonvegetated
areas.  No consistent patterns in species diversity were seen between either
the two sandy habitats or  between the three vegetated zones.

     An examination of the top ten infaunal species in each habitat for the
six sample dates (Tables  2-7) shows  that  these species comprised between 77%
(Zostera 10/78) and 98%  (IS 7/78) of the  total community.   Each habitat had
several species which persisted through time  as one of the top ten species.
In many cases these species were  locally  common to each of the habitats
examined.  For example,  the small bivalve,  Gemma gemma was consistently
abundant in both the  sandy habitats  and occurred sporadically in both the
Ruppia and mixed zones but was overshadowed by other numerically abundant
species in the Zostera area.  The polychaete  Scolelepis squamata generally
persisted as one of the  top ten species in both unvegetated habitats although
it was more regularly found in the inside sand area.   Heteromastis filiformis
(Polychaeta) and Oligochaeta spp. were consistently abundant ir. the IS habitat
as well as in all three vegetated sediments but not at the dynamic OS
stations.  The inside sand (IS) -stations  were characterized by the parasitic
gastropod Oaostomia spp.   Acanthohaustorius millsi, a burrowing amphipod, was
found only in the OS samples.  Species which  characterized the vegetated
habitats included the errant polychaete Nereis succinea, the generally
epibenthic isopod,  Edotea  triloba, and several species which are common in the
                                       11
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         designations are as follows:   OS = outside sand; IS =  inside sand;
         R = Ruppia maritima; Z = Zostera marina; M = mixed vegetation.
         Closed circles indicate the diversity  for each  core.   Boxes
         represent the mean diversity  for each  area.
                                     13
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 epifaunal  community such as Polydora ligni, Streblospio benedicti (Polychaeta)
•and  the  isopod,  Erichsonella attenuata.

      The epifaunal  component of the vegetated habitat comprised a somewhat
 unique assemblage of species.  Only one epifaunal clip was made in the mixed
 area on  October  1978,  and June, September and November, 1979.  Therefore these
 histograms represent single point data and do not indicate mean abundances as
 indicated  in other  histograms.   With this fact in mind, some comparisons
 between  epifaunal communities can be made.

      The largest mean  number of species per grass clip (25) vas found in the
 Ruppia habitat during  July, 1978, when mean species abundances in the Zostera
 (  17) and  mixed  ( 18)  habitats  were also relatively high (Fig. 6).
 Approximately ten or fewer species were present in September, 1979, epifaunal
 samples.   Zostera samples contained slightly higher species numbers on three
 of the six sample dates (October 1978, June, November, 1979) than either
 Ruppia or  mixed  samples.   During April and September, 1979, mean species
 abundances were  almost equal between these habitats.  No seasonal trends were
 evident  in the species abundance data.

      A logarithmic  plot of the  mean abundance expressed as individuals per m^
 of vegetated bottom indicates the presence of large numbers of individuals in
 the  epifaunal community (Fig. 7).  Numbers ranged from approximately 2000 per
 nr in the  Ruppia area  (November, 1979) to almost 60,000'in Zostera samples
 (June, 1979). The  variation within a jingle sample date was sometimes almost
 as great (June,  1979)  with no consistent patterns of individual abundances
 from one habitat to the next appearing in the data.

      The amount  of  vegetation expressed as grams dry weight/m* varied
 temporally,  yet  patterns  of relative mean individual abundances per gram cf
 vegetation (Fig. 8) were  similar to those plotted on an aerial basis alone
 (Fig. 7).   Only  the July  1978 data showed a slight variation from the latter
 pattern.   Mean numbers of individuals varied from almost 30 per gram of grass
 (Ruppia, November 1979) to a maximum of about 7,400 per gram (Ruppia, April,
 1979).   No clear seasonal pattern was seen.  Between habitat patterns were
 also  difficult to ascertain although generally there was a decreasing trend in
 the  number of individuals per gram of Ruppia than for the other two vegetated
 habitats.                                                             :
                                                                      \
      Shannon diversity (H1) calculated for epifaunal samples generally tended
 to be higher for the Ruppia area than for the mixed or Zostera areas (Fig.  9).
 Lowest diversities  were seen during September and November, 1979,  in all but
 the  Ruppia area  which  experienced its lowest diversity in April,  1979.
 Seasonal patterns in species diversity were obscure but in general when
 progressing  from spring thrcugh summer to fall,  the diversity of the epifauna
 associated with  both Zostera and mixed stands of vegetation decreased whereas
 diversity  in the Ruppia zone tended to increase.

     An  examination of the top  ten epifaunal species associated with each
 habitat  (Tables  8-13)  revealed  that they were present as one of the top ten
 species  L. all habitats without regard to vegetation type. For example, the
 isopod Erichsonella attenuata,  barnacle Balanus itnprovisus, gastropods
                                       20
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97b
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•
-
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OCT. 1978



T
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R Z M R Z
h- AREA


-
-
APRIL 197

»
.
M R Z M
tn
iii

o
          4.0-]
          3.0-
          2.0-
           1.0-
JUNE 1979
SEPT. 1979
NOV.  1979
                 R   Z   M
                             R   Z    M

                             AREA
                                    R   Z    M
Fig. 9.  Epifaunal species diversity (Shannon H )  for each vegetated zone
         in the grassbed.   Treatment designations  are as follows:   R =
         Ruppia maritima;  Z = Zostera marina; M =  mixed vegetation.   Closed
         circles indicate  the diversity for each core.   Boxes represent
         the mean diversity for each area.
                                     24
-------
TABLE 8.  EPIFAUNAL SPECIES RECORDED IN THE THREE DIFFERENT TYPES OF VEGETATED
          AREAS (RUPPIA, ZOSTERA, MIXED) AT THE VAUCLUSE SHORES STUDY SITE,
JULY, 1978.
RUPPIA
Species
1. Caprella penantis
2. Erichsonella attenuata
3. Balanus improvisus
4. Idotea balthica
5. Bittium varium
6. Nereis succinea
7. Paracaprella tenuis
8. Ampithoe longimana
9. Cymadusa compta
10. Gastropoda
Total
Total Sample
ZOSTERA
Species
1. Crepidula convexa
2. Bittium vnrlum
3. Idotea balthica
4. Erichsonella attenuata
5. Polydora ligni
6. Balanus improvisus
7. Ampithoe jongimana
8. Paracaprella tenuis
9. Nereis succinea
10. Dorldella obscura
Total
Total Sample
MIXED
Species
1. Crepidula convexa
2. Bittium varium
3. Balanus i-provisus
4. Caprella penantis
5. Erichsonella attenuata
6. Ampithoe longimana
7. Idotea balthica
8. Gastropoda
9. Folydora ligni
10. Nereis succinea
Total Sample
o,
'o
28.26
17.26
10.74
8.18
7.03
6.27
3.58
3.45
3.07
1.92
89.76
100.00
81.19
8.03
3.11
1.35
1.07
0.88
0.80
0.61
0.44
0.44
97.92
100.00
40.77
15.68
10.57
6.08
5.55
4.93
3.74
2.64
2.20
". 2'. 03
94.19
100.00
///gram
34.05
20.80
12.94
9.86
8.47
7.55
4.31
4.16
3.70
2.31
56.38
62.81
///gram
89.95
8.90
3.44
1.49
1.19
0.98
0.88
0.67
0.49
0.49
103.89
111.20
///Rram
28.87
11.10
7.48
4.30
3.93
3.49
2.65
1.87
1.56
- 1.43
66.68
70.79
///m2
1106.70
676.04
420.65
320.49
275.42
245.38
140.22
135.21
120.18
75.12
3515.41
3916.46
///m2
5882.41
581.86
225.17
97.6'
77. 7i
63.77
57.79
43.84
31.88
31.88
7093.95
7244.64
3088.59
1187.41
800.50
460.29
420.26
373.57
283.51
200.12
166.77
153.43
7134.45
7574.53
                                         25
-------
TABLE 9.  EPIFAUNAL SPECIES RECORDED IN THE THREE DIFFERENT TYPES  OF VEGETATED
          AREAS (RUPPIA, ZOSTERA. MIXED) AT THE VAUCLL'SE SHORES STUDY SITE,
          OCTOBER, 1978.
RUPPIA
Species
1. Crepidula convexa
2.
3.
4.
5.
6.
7.
8.
9.
10.
Bittium varium
Ampithoe longimana
Erichsonella attenuata
Paracaprella tenuis
Nereis succinea
Caprella penantis
Cymadusa compta
Astyris lunata
Balanus improvisus
Total
Total sample
ZOSTERA
Species
1 . Crepidula convexa
2.
3.
4.
5.
6.
7.
8.
9.
10.
Bittium varium
Atnphithoe longimana
Balanus improvisus
Erichsonella attenuata
Paracaprella tenuis
Astyris lunata
Caprella penantis
Anadara trans versa
Nereis succinea
Total
Total sample
40.58
18.85
9.98
9.09
4.88
3.10
3.10
2.66
1.77
1.77
95.78
100.00
"'.
81.89
6.79
2.80
2.08
1.59
1.38
1.06
0.95
0.30
0.15
98.99
100.00
f/gram
16.37
7.60
4.03
3.67
1.97
1.25
1.25
1.07
0.72
0.72
38.65
40.35
///gram
211.65
17.56
7.24
5.37
4.12
3.56
2.74
2.47
0.78
0.38
256.14
258.75
///m2
916.64
425.76
225.40
205.37
110.20
70.13
70.13
60.11
40.07
40.07
216.88
226.44
///m2
15177.7.6
1259.35
519.14
385.08
295.23
255.29
196.82
176.85
55.62
27.10
18348.24
38535.45
       MIXED
Species
                                            ///gram
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
Crepidula convexa
Bittium varium
Balanus improvisus
Erichsonella attenuata
Ampithoe longimana
Paracaprella tenuis
Nereis succinea
Astyris lunata
Cymadusa compta
Anad a ra transversa
Total
Total sample
                                  78.85
                                  11.58
                                   3.44
                                   2.84
                                   0.67
                                   0.67
                                   0.45
                                   0.45
                                   0.45
                                   0.37
                                  99.77
                                 100.00
///in

10471.85
 1538.52
  456.59
  377.19
   89.33
   89.33
   5.9.56
   59.56
   59.56
   49.63
13251.12
13281.67
                                         26
-------
TABLE 10.  EPIFAUNAL SPECIES RECORDED IN THE THREE DIFFERENT TYPES OF VEGETATED
           AREAS (RUPPIA, ZOSTERA, MIXED) AT THE VAUCLUSE SHORES STUDY SITE,
APRIL, 1979.
RUPPIA
Species
1.
2.
3.
it.
5.
6.
7.
8.
9.
10.


Polydora llgnl
Crepldula convexa
Bittium varlum
Caprella penantis
Paracaprella tenuis
Gammarus mucronatus
Erichsonella attenuate
Balanus Iraprovisus
Astyris lunata
Cymadusa compta
Total
Total sample
X
93.55
1.42
1.32
1.11
0.75
0.71
0.63
0.16
0.12
0.12
99.89
100.00
#/grara
6103.23
92.90
85.16
72.26
49.03
46.45
41.29
10.32
7.74
7.74
6516.12
6523.30
tf/m2
30516.13
464.52
425.81
361.29
245.16
232.26
206.45
51.61
38.71
38.71
32580.65
32616.53
        ZOSTERA
Species

1.  Polydora 1igni
2.  Crepldula convexa
3.  Caprella penantis
4.  Gammarus nmcronatus
5.  Bittium varium
6-  Paracaprella temiis
7.  Microprotopus raneyl
8-  Cerapis tubularis
9.  Nereis succinea
10. Astyris lunata
                Total
                Total sample
///gram

423.48
 54.78
 15.20
 14.65
  5.82
  3.28
  1.64
  1.00
  0.45
  0.45
520.75
521.64
///n.

11433,84
 1478.98
  410.28
  395.54
  157.23
   88.44
   44.22
   27.02
   12.28
   12.28
14050.11
14074.04
         MIXED
Species
///gram
                                                         t/m
1.
2.
3.
4.
5.
6.
7.
8.
9;
10.
?olyjora ligni
Crepidula convexa
Gammarus mucronatus
Caprella penantis
Bittium varium
Paracaprella tenuis
Astyris lunata
Microprotopus raneyi
Balanus iriprovisus
Erichsonella attenuate
Total
Total sample
80.16
7.93
2.74
2.19
1.92
1.64
0.96
0.96
0.68
0.41
99.59
100.00
371.47
36.77
12.68
10.14
8.87
7.61
4.44
4.44
3.17
1.90
461.49
463.39
11701.43
1158.16
389.37
319.49
279.56
239.62
139.78
139.78
99.84
59.90
14526.93
14586.74
                                    27
-------
TABLE 11.  EPIFAUNAL SPECIES RECORDED  IN THE THREE DIFFERENT TYPES OF VEGETATED
           AREAS (RUPPIA. ZOSTERA, MIXED) AT THE VAUCLUSE SHORES STUDY SITE,
	JUNE, 1979.	

         RUPPIA
Species

1.  Balanus improvisus
2.  Polydora ligni
3.  Caprella penantis
4.  Erichsonella attenuata
5.  Nereis succinea
6.  Etylochus elliptieirs^
7.  Bittium varium
8.  Cymadusa compta
9.  Astyris lunata
10. Paracaprella tenuis
                 Total
                 Total sample
///gram

55.44
 59.1
 3.01
 2.84
 0.83
 0.35
 0.24
 0.18
 0.12
 0.12
69.04
70.16
                                                         tf/m
        ZOS12RA
Species
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
Balanus improvisus
Polydora ligni
Gammarus mucronatus
Stylochus elliptic us
Crepidula convexa
Nereis succinea
Erichsonella attenuata
Bittium varium
Caprella penantis
Astyris lunata
Total
Total sample
91.28
2.68
1.62
1.24
1.11
0.68
0.21
0.19
0.19
0.16
99.36
100.00
                                             /'/gram

                                             397.80
                                              11.67
                                                .08
                                                .39
                                                .83
                                                .95
                                               0.90
                                               0.83
                                               0.83
                                               0.71
                                             432.99
                                             435.78
            */m£

            52907.24
             1552.71
              941.04
              716.23
              643.04
              392.10
              120.24
              109.79
              109.79
               94.10
            57586.26
            57957.19
       MIXED
Species                            /o

1.  Balanus ipprovisus            86.61
2.  Crepidula convexa              4.69
3.  Gammarus mucronatus            2.23
4.  Erichsonella attenuata         2.01
5.  Polydora ligni             '    1.79
6.  Stylochus elliptic us           0.78
7.  Nereis succinea               • 0.45
8.  Astyris lunnra      _        •  ' 0.45
9.  Ampithoe longimara             0.33
10. Nassarius pbsoletus            0.33
                Total             99.67
                Total sample      100.00
                                             ///gram
                                             180.05
                                               9.74
                                                .64
                                                ,18
                                                .71
                                                .62
                                               0.93
                                               0.93
                                               0.70
                                               0.70
                                             207.20
                                             207.89
            20165.20
             1091.42
              591.72
              467.75
              415.78
              181.90
              103.94
              103.94
               77.96
               77.96
            23205.57
            23282.40
                                         28
-------
TABLE 12.  LPIFAUNAL SPECIES RECORDED IN THE THREE DIFFERENT TYPES OF VEGETATED
           AREAS (RUPPIA, ZOSTERA, MIXED) AT THE VAUCLUSE SHORES STUDY SITE,
           SEPTEMBER, 1979.

       RUPPIA
Species
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
Erichsonella attenuata
Balanus improvisus
Crepidula convexa
Cymadusa compta
Gastropoda
Mollusca
Nereis succinea
Bittium varium
Paracaprella tenuis
Stylochus ellipticus
Total
Total sample
                                  55.84
                                  16.23
                                   9.09
                                   3.90
                                   3.57
                                   3.57
                                   3.25
                                   1.95
                                   0.65
                                   0.32
                                  98.37
                                 100.00
                                            it
                                            23.15
                                             6.73
                                             3.77
                                               .62
                                               .48
                                               .48
                                               .35
                                             0.81
                                             0.27
                                             0.13
                                            40.79
                                            41.47
1620.46
 471.06
 263.80
 113.06
 103.63
 103.63
  94.21
  56.53
  18.84
   9.42
2854.64
2901.94
       ZOSTERA
Species
1. Crepidula convexa
2. Gastropoda
3. Balanus iraprovisus
4. Erichsonella attenuata
5. Idotea balthica
6. Bittium varium
7. Doridella obscura
8. Stylochus ellipticus
9. Astyris lunata
10. Ampithoe longimana
Total
Total sample
MIXED
Species
1. Crepidula convexa
2. Erichsonella attenuata
3. Balanus improvisus
4. Gastropoda
5. Nereis succinea
6. Idotea balthica
7. Paracaprella tenuis
8. Bittium varium
9. Ampithoe longimana
10. Anadara transve^sa
Total
Total sample
96.02
1.24
0.97
0.57
0.27
0.19
0.17
0.13
0.10
0.08
99.74
100.00
o
96.74
0.82
0.54
0.49
0.38
0.38
0.33
0.16
0.11
0.05
100.00
100.00
                                            ///gram

                                            278.90
                                              3.59
                                              2.82
                                              1.66
                                              0.77
                                              0.55
                                              0.50
                                              0.39
                                              0.28
                                              0.22
                                            289.68
                                            290.44
                                            ///gran

                                            180.32
                                              1.52
                                              1.01
                                              0.91
                                              0.71
                                              0.71
                                              0.61
                                              0.30
                                              0.20
                                              0.10
                                            186.39
                                            186.39
/'An

18685.96
  240.61
  188.78
  111.05
   51.82
   37.02
   33.31
   25.91
   18.51
   14.81
19407.78
19458.37
                                                        ///n.

                                                        15868.56
                                                          133.87
                                                           89.25
                                                           80.32
                                                           62.47
                                                           62.47
                                                           53.55
                                                           26.77
                                                           17.85
                                                            8.92
                                                        16404.03
                                                        16404.03
                                        29
-------
TABLE 13.  EPIFAUNAL SPECIES RECORDED IN THE THREE DIFFERENT TYPES OF VEGETATED
           AREAS (RUPPIA, ZOSTERA. MIXED) AT THE VAUCLUSE SHORES STUDY SITE,
           NOVEMBER, 1979.
           RUPPIA
Species
I.  Erlchsonella attenuata
2.  Crepidula convexa
3.  Cymadusa compta
4.  Paracaprella tenuis
5.  Nereis succlnea
6.  Gastropoda
7.  Balanus improvisus
8.  Oxyurostylis smithi
9.  Cammarus mucronatus
10. Caprella penantis
                   Total
38.11
28.11
   84
   14
   59
   89
   89
 1.89
 1.35
 1.35
92.16
          tf/gram
                                            9.26
                   Total sample  100.00
   83
   90
   25
   12
 0.46
 0.46
 0.46
 0.33
 0.33
22.40
24.31
 675.84
 498.49
 139.00
  91.07
  81.48
  33.55
  33.55
  33.55
  23.97
  23.97
1634.47
1773.51
        ZOSTERA
Species
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.


Crepidula convexa
Balanus improvisus
Idotea balthica
Doridella obscura
Ampithoe longimana
Erichsonella attenuata
Polydora ligni
Caprella penantis
Gastropoda
Gammari's mucronatus
Total
Total sample
88.19
2.66
2.56
1.09
1.07
0.95
0.58
0.54
0.45
0.29
98.38
100.00
          ///gram
                                             243.60
             tf/rn

             28013.67
               844.34
               811.61
               346.90
               340.35
               301.08
               183.27
               170.18
               144.00
                91.63
             31250.03
             31764.62
        MIXED
 Species
                                             ///gram
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.


Crepidula convexa
Erichs. 'uella attenuata
Balanus improvisus
Polydora ligni
Gastropoda
Doridella obscura
Idotea balthica
Nereis succinea
Bittium varium
Stylochus ellipticus
Total
Total sample
96.13
1.74
1.18
0.26
0.20
0.17
0.13
0.04
0.04
0.02
99.91
100.00
506.42
9.17
6.19
1.38
1.03
0.92
0.69
0.23
0.23
0.11
526.37
526. bn
51401.82
931.19
628.55
139.68
104.76
93.12
69.84
23.28
23.28
11.64
53427.16
53475.29
                                         30
-------
Crepidula convexa and Bittium  varium and  the  polychaete Nereis succinea were
all commonly ranked among  the  top  ten  species in  all  vegetated habitats.
Seasonal pulses of abundance for J5. inprovisus  (June,  1979)  and Polydora ligni
(April, 1979) were evident  in  the  data with possibly  similar evidence for
Crepidula convexa and Bittium  varium.

     The results presented  here  indicate  that the grassbed and each of the
sandy habitats are distinct with respect  to their associated fauna.
Differences between the vegetated  habitats are  less clear  cut.  The number of
species and individuals was greatly increased in  the  vegetated habitats
indicating their importance to the Chesapeake Bay fauna.

                                   DISCUSSION

     A primary understanding of  the structural  aspects  of  vegetated habitats
is necessary to further examine  the functional  roles  of the  organisms within
these habitats.  Additionally, a comparison of  grass  bed community structure
with that of nonvegetated  areas  can provide greater insight  into the
importance of the various  habitats surveyed.

     We have clearly demonstrated  an increased  abundance of  infauna,
particularly oligochaetes,  at  depths of 6  to  15 cm below the sediment surface
in vegetated areas compared to exposed sandy  bottoms.   Infauna living in grass
beds apparently deri\2 some advantage  from the  root and rhizome system of the
plants.  The redox potential discontinuity layer  (RPD)  in  grass beds is
farther below the sedinent  surface than in unvegetated  habitats and probably
allows organisms to survive deeper in  the  sediments.  Oxygenation of surficial
sediment by the rhizosphere increases  the  two dimensionality of the habitat  in
which grass bed associated  infauna live.  Virtually no  information exists on
the rhizofauna of marine grasses although related studies  in salt marshes have
demonstrated the rhizosphere to be important  in determining  the vertical
distribution of associated  infivna (Bell  et al. 1978; Teal and Wieser, 1966).

     Additional advantages  provided by vegetation to  the infaunal community
include increased sediment  stability and  protection from predation.
Seagrasses have been shown  to  stabilize seuiments by  their current baffling
and wave damping action (Hartog, 1970; Taylor and Lewis, 1970;  Zieman, 1972).
Orth (1977) experimentally  demonstrated that  such sediment stability caused
increased infaunal diversity and density.  T.r> addition, observations made en
blue crab feeding prompted Orth  (1977) to suggest that  such  activities were
hampered by the presence of the root and  rhizome  layer  in  Zostera beds.   A
similar hypothesis suggesting  the  protection  provided by the rhizosphere to
the infauna was made by Riese  (1977).  All three  factors acting together
(increased oxygenation of  the  sediments,  greater  sediment  stability and
protection from predation) probably account for the high infaunal densities
and species abundances observed in our study.

     Besides the functional advantages provided by the  root  and rhizome system
of aquatic vegetation, decomposing leaf litter  supplies a  rich detrital food
source for the grass bed infauna (Thayer, et  al.  1977;  Fenchell,  1972).   For
species living among the plants, this  food source is  readily accessible and
                                       31
-------
probably enhances  the  structural  magnitude and diversity of the grass bed
infaunal community  to  the  degree  observed in our study.

     The only exception  to increased  individual abundances in the grass bed
infauna occurred during  the  July,  1978 sample date when the inside sand
habitat showed  the  greatest  abundance of individuals.   This high level was
caused by a Gemma gemma  recruitment  pulse bringing the total number of
individuals to  just  over 33,000 per  meter squared.  Similar increases in the
small gastropod Odostomia  spp. were  also observed.  This species is parasitic
on Gemma gemma  (Abbott,  1974)  and  showed a positive response to an increased
food source.

     Population dynamics of  species  comprising the infaunal community are
evident in our  data  and  emphasize  the continually changing nature of the
dominance hierarchy  over time.  Also  evident is persistence of certain
abundant species in  the  habitats  studied imparting some degree of constancy to
the community associated with  each habitat.   The amphipoo Acanthohaustoris
millsi^ and polychaete  Paraonis fulgens were  numerous and almost exclusively
found in the dynamic offshore  sand bar (OS)  habitat.  Both were always present
as one of the top ten  in this  habitat for the duration of our study.
Acanthohaustorius millsi,  a  burrowing amphipod, is a suspension feeder on
microscopic algae and  detr:.':us and is commonly found at the low tide mark as
well as subtidally  on  sand/  beaches  (Dexter, 19r-9),  It is associated with
dynamic medium  sar.ds such  ar.  those of the sand bar area.  Similarly,  Paraonis
fulgens, described  as  both a  rio.i-selective burrowing deposit feeder and a
highly selective diatom  grazer ^Fauchald and Jumars, 1979) was also found in
all OS samples.  It  constituted the most abundant species at all sample times
except during July  1978  when  it was  third in abundance.  These species are
both adapted for living  in shifting  sediments and are  apparently unable to
compete successfully in  the more  stable protected sandy bottom within the
grass bed .

     The small bivalve,  Gemm?  gemma,  is able to exist  in both the more
turbulent offshore  sandbar habitat and the sandy patches within the grass bed.
Species which more  accurately  distinguish the bare cand zones inside  the
vegetated area  include the opportunistic capitellid polychaete,  Capitella
capitata, as well as the spionid polychaete, Scolelepis squamatus.
Surprisingly,  neither  of these species was among the ten most numerous ones in
grass bed infaunal  samples although the latter species was occasionally found
on the offshore sandbar.   Capitella capitata builds tubes at or near  the
sediment surface and Scolelepis squamatus builds loosely constructed  burrows
in sandy substrates  (Fauchald  and  Jumars, 1979).  Capitella capitata  is known
to be an r- strategist (high  fecundity and short life  span) and, based on our
data is recruited in the late  summer  and early fall.  Scolelepis squamatus
persisted throughout the study in  the  inside sand habitat.

     The grass bed  infaunal community contained a more varied group overall of
species comprising  the top ten during  the course of our study than  that
exhibited for either of  the sandy  habitats.   This fact reflects  the diverse
and dynamic nature of  the  community with a variety of  species recruiting and
subsequently decreasing  in abundance  through time.   Those species which
occurred most  abundantly and regularly included the polychaete Heteromastus
                                      32
-------
filiformis, Polydora  ligni,  Nereis  succinea and numerous unidentified
oligochaetes.  Heteromastus  filiformis  is  a deep burrowing opportunistic
capitellid that was abundant  in  the  grass  bed  proper and in sand patches
within  the vegetated  zone  as  were  unidentified oligochaetes.  Both P_. ligni
(Spionidae) and _N.  succinea  (Nereidae)  were numerous in the grass bed but were
rarely  encountered  in the  two sandy  habitats.   The former species has been
described  as  the most, abundant  species  in  estuarine waters of the Chesapeake
Bay (Wass, 1965) with peak abundances on Zostera marina occurring in April and
May (Marsh, 1970).  Orth (1971)  found peaks in larval abundance in the York
River to occur in April.   Despite  its distinct seasonal pulses, _P_. ligni was
the most abundant polychaete  throughout the year on fouling panels in Hampton
Roads,  Virginia (Calder, 1966).  In  our study, P^.  ligni persisted as one of
the top ten species in all three types  of  vegetated areas during the entire
study.  Nereis succinea  likewise was constantly abundant.  This highly
opportunistic species has  been described as a  surface deposit feeder as well
as an omnivore feeding on  prey  (i.e. other Nereis) and silt-clay to sand sized
particles  and plant detritus  (Dauer, 1980). Not only was 1^. succinea commonly
found infaunally, it  also  was an abundant  epifaunal community member.

     The isopods Edotea  triloba  and  Erichsonella attenuata were also numerous
and routinely present in infaunal  samples.   Edotea is epibenthic and was
rarely  found  in the epifauna  whereas Erichsonella  was common in be
communities.

     The infaunal community  of eelgrass beds at Vaucluse Shores was similar to
those of Guinea Marsh grassbeds  studied by Orth (1973).  Polychaetes
(Heteromastus filiformis,  Streblospio benedicti, Nereis succinea, Polydora
1igni and  to a lesser extent  Spiochaetopterus  oculatus and Scoloplos robustus)
were among the most numerous  species present in both studies.  Oligochaetes
and the isopod Edotea triloba were  similarly abundant.  It appears that an
infaunal community  occurs within grassbeds  which distinguishes those areas
from non-vegetated  sand bottoms  and  furthermore characterizes them in terms of
general dominance hierarchies  and  species  abundances.

     The epifaunal  community  was comprised  of  numerous species all of which
utilized the grass  blades  as  a substrate and a feeding area.  The barnacle,
Balanus improvisus, gastropods,  Bittium varium and Crepidula convexa, isopod,
Erichsonella attenuata -and amphipod, Caprella  penantis were all abundant in
grassbed epifaunal  samples.   They occurred  consistently among the top ten
species in all vegetated habitats without  regard to vegetation type.   Some
species such as Bittium varium and Crepidula convexa were more abundant on
Zostera marina.  Their  prevalence on eelgrass  is probably related to the
morphology of the plant, with the wide  bladed  species preferred to those such
as Ruppia maritima with narrow blades.   Bittium varium feeds extensively on
eelgrass periphyton (see-grazing section,  this report) and its great  abundance
in the epifauna is  probably related  to  the  readily available food supply.
Although juvenile Crepidula, convexa .are thought  to be microalgal grazers as
well, adults switch to a filter  feeding mode (Hoagland, 1975) and therefore
use grassblades primarily as  an  attachment  platform from which to feed.
Caprella penantis is  the most common caprellid amphipod along the east  coast
of the United States  and occurs  abundantly  from Long Island to the Chesapeake
Bay.   It is nonspecific in its habitat  preference,  occurring on a wide  variety
                                      33
-------
of substrates including algae,  sponges,  alcyonarians  and  particularly hydroids
(McCain, 1968).  Erichsonella attenuata,  an  isopod,  is  characteristically
associated with eelgrass (Schultz,  1969)  although  in  our  study it was also
common on Ruppia maritima.  Little  is  known  about  the biology of this species.
It is probably omnivorous and occurs  in  such  great  abundance  in grassbeds
because of the large quantities of  suitable  food and  habitat.

     Not surprisingly, several  of the  dominant  species  found  in the  Vaucluse
Shores epifauna were also present in  a York  River  epifaunal community studied
by Marsh (1973, 1976).  Of the  5 most  abundant  species  found  by Marsh (1973),
four (Bittium varium, Crepidula convexa,  Erichsonella attenuata and  Ampithoe
longimana) were also dominants  at: Vaucluse Shores.  Paracerceis caudata an
epifaunal isopod was commonly found in the Chesapeake Bay during the time of
Marsh's study in 1967 and 1968 but  declined  drastically in abundance after the
devastating passage of hurricane Agnes in June  of  1972  (Orth, 1976).  Their
populations do not yet seem to  have recovered which accounts  for the absence
of P_. caudata during our study.  Some  species which were  abundant for brief
periods during the year in Marsh's  (1975) study were  regularly abundant in the
Vaucluse Shores epifauna.  These include  the  barnacle,  Balar.us improvisus and
epifaunal polychaete, Polydora jigni.  Both  the ascidian,  Molgula manhattensis
and saccoglossan, Ercolania fuscata were  periodically abundant in the York
River but were uncommon in Vaucluse Shores samples.   Reasons  for this fact are
unclear.

     We have demonstrated and confirmed  the  existence of  a diverse and
abundant infaunal and epifaunal community associated  with a vegetated habitat
in the lower Chesapeake Bay.  Marine  grasses  create greater physical
complexity resulting in a more heterogeneous  habitat  that is  capable of
supporting larger numbers of invertebrate species  and individuals than
adjacent sandy areas.  Fish are attracted to  these meadows because of the
preponderence of invertebrates which  are  heavily preyed upon  (Adams,  1976a,
b,c;  Nelson, 1979; Young and Young  1978;  Stoner, 1979;  Orth and Heck,  1980).
Many of the fish (e.g. spot and speckled  trout) which frequent  grassbeds  are
commercially harvested or are important  recreational  species.   Grassbeds  are
also an important refuge for both juvenile as veil as older blue crabs,
Callinectes sapidus, during the soft  shell phase of their  molt  cycle.   Thus
the demise of these habitats may have  serious consequences to many species
although tne effects of such declines  may take  years  to be felt.
                                                                      \
-------
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Adam-;, S. M.  I976b.  The ecology of eelgrass, Zostera marina  (L.) fish
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Dexter, D. M.   1969.   Structure  of  an  intertidal  sandy-beach  community
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Fauchald, K. and P. A. Jumars.   1979.  The diet of worms:  a study of
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Hardwick, J. E.  1973.  Biomass  estimates  of  spawning herring Clupea
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Hartog, G. den.  1970.  The seagrass of the worl\d.   Verh.  Kon. ned. Ak.
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Hoagland, E. K.  1975.  Patterens of evolution and niche partitioning in North
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Marsh, G. A.   1976.   Ecology  of  the  gastropod  epifauna  of  eelgrass  in a
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McCain, J. C.   1968.  The Caprellidae  (Crustacea: Amphipoda)  of  the
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Nilsson, L.  1969.   Food consumption of  diving ducks  wintering at the  coast
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Orth, R. J.  1973.   Benthic infauna  of eelgrass, Zostera marina, beds.
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Orth, R. J.  1975.   Destruction of eelgrass, Zostera  marina,  by  the cowr.ose
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Orth, R. J.  1976.   The demise and recovery of eelgrass, Zostera marina,
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Orth, R. J.  1977.   The importance of sediment stability in seagrass
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Orth, R. J., K. A. Moore and H. H. Gordon.  1979.  Distribution and abundance
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     Rep. No.  600'8-79/029/SAV 1.

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     3:278-288.
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Pielou,  E.  C.  1975.   Ecological diversity.   Wiley-Interscience, New York.
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Reise,  K.  1977.  Predator exclusion experiments in an intertidal mudflat.
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Schultz, G. A.   1969.  How to Know the  Marine Isopod Crustaceans.  Wm. C.
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Stoner, A. W.  1979.  Species-specific predation on amphipod Crustacea by
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                                     38
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                            CHAPTER 2




PREDATOR EXCLUSION EXPERIMENTS IN A CHESAPEAKE BAY GRASS  COMMUNITY




                                by




                          Robert J.  Orth




                                and




                       Jacques van Montfrans
                                39
-------
                               ABSTRACT

     The effects of predators on  the  density  of eelgrass  epifauna and
infauna and sand infauna was studied  using  predator  exclusion
techniques.  A large  topless pen  (20  nr)  and  smaller cages  (0.25 m^)
within the pen as well as outside  the pen were  set up in  a  bare  sand
and an adjacent grass habitat to  test the hypothesis that predation
has a significant effect on the structure of  associated faunal
communities.

     The grass habitat consistently had more  infaunal species per core
than the sand habitat for all treatments.   There were no  distinct
differences among the grass treatments for  species infaunal numbers
but in the sand, species numbers were higher  in pen  and cage
treatments compared with the control.  Except  for the sand  cage
treatments, there was no difference between the pen  and cage
treatments for number of species  in both  habitats.

     Density of individuals in the grass  habitat, treatments was
generally higher than the sand habitat treatments except  for the June
cage treatments.  In  the sand area, infaunal  densities were always
higher in the cage and pen treatments  compared  to the control while in
the grass habitat only the September  cage and pen treatments were
higher than the control.

     Epifaunal densities in the grass  habitat were generally higher in
cage and pen treatments than the control.   Species response to these
treatments were variable and controlled by  the  abundarce  of grass in
the treatment.

     The results of this work support  recent evidence  for the
importance of predation for the structuring of  benthic communities
both in vegetated and non-vegetated habitats.                         j
-------
                              INTRODUCTION

     The structure and  organization  of  any  biological  community is
determined by a number  of  internal and  external  factors,  both abiotic
and biotic.  Most abiotic  factors are usually  external and cannot be
controlled by the community.  Log damage in intertidal regions
(Dayton, 1971), wave stress and  storm swells (Annala,  1974;  Grigg and
Maragos, 1974), anchor  ice  (Dayton,  et  al.  1969,  1970) fire  (Odum,
1971) and pollution stress  (Wihlm, 1967; Wihlm and  Dorris, 1966,  1968)
have all been shown to  alter  the structure  of  the affected community.

     Predation and competition are the  two  most  often  cited  biological
factors that control not only the structure but  the function of the
community.  Competition for a resource, e.g. space  or  food,  can either
be through interference or  exploitative competition (Peterson, 1980)
while predation can be  size selective (Brooks  and Dodson,  1965),
species selective (Dayton,  1971; Patrick, 1970)  or  non-selective  with
respect to species or size  of prey.

     Predation in marine soft bottom benthic communities  have recently
been shown to have dramatic effects  on  the  density  and diversity  of
the infauna (Virnstein, 1977) and has bsen  an  implied  factor for
structuring benthic communities  associated  with  seagrass  beds (Nelson,
1979; Heck and Orth, 1980a; Orth and Heck,  1980; Reise, 1978).  Most
of the above studies have relied on  manipulative tools, e.g.  cages,  to
examine the effects of predators.  Though manipulative techniques have
piovided ecologists with some detailed  insights  into community
dynamics, they are not without their problems, e.g.  separation of cage
effects from predator effects (Virnstein, 1978,  1980).

     Eelgrass beds in the Chesapeake Bay have  an associated  infaunal
and epifaunal community that has a significantly higher density and
diversity than adjacent, unvegetated communities (Orth, 1975,  1977;-
see also Section 1 of this report).  It is  this  dense  assemblage  of
invertebrates that undoubtedly serve as a food source  for  not  only
invertebrate predators (e.g. blue crabs) but also vertebrate predators
(e.g. fish and waterfowl).   The density of  some  of  these  predators i'l
these vegetated areas is sometimes extremely high (Orth and  Heck,
1980) suggesting that their impact on epifaunal  and  infaunal  density
may be important in reducing their density.

     As part of the EPA Chesapeake Bay Program's Functional  Ecology .of
Eelgrass Study in the Lower Chesapeake Bay,  the  objective  of this
study was to determine what impact predation,  excluding waterfowl, has
on the overall density and diversity of both epifauna  and  infauna.
                                   41
-------
                    STUDY  SITE,  MATERIALS AND METHODS

     All  studies  involving the  predator exclusion experiments were
conducted  in  a  large  bed  of submerged grasses located in the
Chesapeake Bay  off  Church Neck  on the Delmax'va Peninsula, Virginia
(Fig.  1).  This is  the  site of  (he  intensive functional ecology of
eelgrass  program  being  used by  Vi'iS  .scientists in conjunction with the
Chesapeake Bay  Program, Subnv: • t_a Aquatic Vegetation sub-program.
This site has been  referred to  as the Vaucluse Shores site and is
situated  approximately  37'25'N  latitude, 76°51'W longitude.

     This site  is characterized by  five distinct habitat types:  1) a
large  unvegetatcd intertidal  sand flat  adjacent to the shoreline; 2) a
Ruppia maritima dominated community  ranging in depth from mean low
water  (MLW) to  approximately  0.3 m below MLW; 3) a mixed bed of R_.
mari tima  and  Z. .narina  located  from  0.3 to 0.6 m below MLW; 4) a T^.
marina dominated  community located  from 0.6 to 1.5 m below MLW; and 5)
a .second  intertidal sandbar separating  the grass bed from the main
Chesapeake Bay.

     Sampling transects were  set up  during an initial intensive
mapping of SAV  (Orth  et al.,  1979).   Our experiments were conducted
along  Transect  C.

     In order to  assess the effects  of  predation on the epifauna and
infauna, we used  two  different  types of exclosures.  One exclosure
consisted of  a  large  circular topless pen, 5 ;n in diameter (20 ra'' in
area).  The other exclosure was a smalltr, square completely enclosed
cage (0.25 m^ in  area).

     Two pens were  constructed,  one  in  the mixed Ruppia-Zosterc bed
and another in  the  adjacent inshore  sandy area (Fig. 1).  The pens
were made of 4.3 ra  long salt  treated wooden pilings placed 1.5 m into
the bottom.   Initially, thick-wall galvanized pipes (240 cm x 2 cm)
were placed between the equally spaced  wooden pilings to provide shape
(Fig.  2).  The  pipes  inadequately supported the weight of the netting
that was placed around  the  pipes  and were later replaced with 10 cm x
10 cm  x 360 cm wooden posts.  Pens were encircled by a piece of black
plastic 0.63 cm r.iesh  netting with a  uv  re.tardant (Conwed Corp Plastic
Netting f/OV3010).   The  netting,  which was 324 cm wide,  was attached to
the posts at a height of  240 cm above the bottom.   Thus, the top of
the pen were always above  the water.  Excess netting was stapled aloru-
the bottom with 18 cm long  wire  staples to form a^i  84 cm wide skirf
which  extended outward  from each  pen.   The skirt prevented predators
from burrowing and gaining  access into  the  pen.   An entrance into each
pen was constructed by  sewing a  5 cm wide x 324 cm  long strip of
VELCRO to one end  of  the  netting  with the opposing  piece attached to  a
piling.

     Smaller square cages measuring  50  cm on a side and  50 cm high
were constructed of reinforcing  rod  frames  covered  with  the same
plastic netting as used on  the  pens.  Each  cage had 30  cm long legs
                                   42
-------
         R = RUPPIA
         Z = 20STERA
         S = SAND
         /= MIXED

         P=PENS
          = TRANSECTS
      CHESAPEAKE
         BAY
Fig.  1.   Location  of  study site for predator exclusion experiments.
                                    43
-------
which were  pushed  into  the  bottom  anchoring  the  cage.   A top attached
with VELGRO strips  on  three sides  allowed  easy access  into each cage.
Panels  simulating  only  the  sides of  cages  were similarly constructed
to simulate some of  the  cage  effects but still allow predator access.

     The use of a  large  topless pen  and  smaller  cage in combination
was chosen  for several  reasons.  Traditional  caging experiments in
both soft and hard  substrates have relied  on  small  cages (less than
1 m^) with  different size meshes.  Cage  effects  such as current
reduction and sediment  deposition  are usually associated with the use
of a cage,  and similarly would affect larval  recruitment (Virnstein,
1978, 1980).  The use of appropriate controls such  as  two-sided cages,
and topless cages have  avoided some  of these  problems.   However, for
motile  predators,  these  controls are still not appropriate.   We felt
that by building a  large enough pen  with no  top,  cage  effects would be
considerably reduced from that of  the smaller cage.  The comparison
between the cages and cage  controls  inside and outside  the pen would
allow us to further distinguish some of  the complex  cage effects.

     Triads  of experimental treatments were randomly arranged in
triplicate  both within  and  outside of the  pens in each  of the two
habitats.   A triad  consisted  of three experimental  treatments:  a
complete cage enclosing  0.25 m  of bottom  area,  and  open cage with no
top and paralle sides of 0.25 m^,  and an uncaged  control area (Fig.
2).  One of  the three triads  per experimental  condition (sand; sand
plus pen; grass, grass  plus pen) was designed to  be  destructively
sampled after an appropriate  time  interval.

     Pens were constructed  in each of the  two areas  (grass and sand)
in late April, 1979.  Prior to setting out the cages,  infauna and
epifauna were sampled inside  and outside the  pen  in  both the sand and
grass (see  below for sampling methodologies).  After the faunal
sampling, the cages were placed randomly in the  pen  along each
experimental triad as discussed above.  One of each  of  the three
triads  were destructively sampled in June  12-14,  September 11-13 and
November 13-16,  1979.

     Two days prior to the  first sampling  period, a  strong Northeast
storm destroyed the netting on the pens.   Sampling  for  fauna was
conducted according to our  design after which  the pens  were  rebuilt.
In addition to the smaller  mesh, a backing of heavier,  large mesh
(13 mm)  netting (Conwed  Corp. Plastic  Netting  #OV1580)  was added for
support.

     Problems were also  encountered  with the  cages  in  the sand area.
Despite  the bottom edges of the cages  being placed  approximately 5  cm
below the sediment surface, large blue crabs  burrowed under  the edges
and had  gained entrance  to  these cages.  Because  of  this disturbance,
all sand cages were removed and replaced by new ones and positioned
over a portion of the bottom that had  been uncaged.  A  24 cm wide
skirt  was also placed around each sand area cage.  Cages in  the grass
area were not disturbed  by  crabs.   These cages were  left in  place to
-------
                                                     K.y

                                                     i  i  Complete tag*

                                                     Ii   Op«n cog*

                                                     •    No  cog*
Fig.  2.   Design of predator exclusion .experiments showing the construction
         of the large  pen  and'the placement of experimental triads.  One
         triad consists  of a complete cage, an open cage, and an area with
         no cage;  there  are three triads per la^ge pen.
                                    45
-------
follow  the development  of  the  community over  time.   In addition, new
cages were placed  in  the grass  area  to parallel those that were placed
in the  sand area.

     During each sampling  period,  both epifauna and infauna were
sampled  similarly.  Samples  collected  within  the pen were done as
carefully as possible to minimize  the  disturbance to adjacent
vegetation.  Prior  to the  sampling,  one triad was randomly designated
to be sampled.  Epifauna were  collected first,  by clipping the shoots
as close as possible  to the  sediment surface  and gently placing these
blades  into a collecting bag with  0.5  mm mesh bottom.   If the
epifaunal sample was  to be taken from  a caged area, the cage was first
removed before sampling.   All  samples  were  taken from an area of
approximately 0.1 m^.   After epifaunal samples  were taken, additional
grass was clipped and removed  to allow for  adequate space to take the
infaunal samples.

     After collection,  epifaunal samples  were kept  in water and
processed live by stripping  all epifauna from the blades and
preserving them in  10%  buffered seawater with formalin containing the
vital stain Rose Bengal.  The  remaining plant material was sorted
according to species  (Kuppia,  Zostera,  algae),  oven dried at 80 C for
at least 48 hours and then weighed to  the nearest 0.1 g.

     Infaunal samples were collected at the same location as the
epifaunal samples.  Initially,  10  cores for infauna were taken with a
plexiglas corer (0.007  m^) to  a depth  of 15 cm.   Subsequently, it wao
found that seven rores  were  adequate to describe the density and
diversity of infauna.   Infaunal cores  were  placed in labeled plastic
bags containing isotonic MgCl2  as  a  relaxant.   This prevented smaller
polychaetes and oligochaetes from  crawling  through  the sieve.  All
infaunal samples were washed through a 0.5 mm mesh  sieve and then
retained material was preserved in 10% buffered  seawater and formalin.
Rose Bengal was also used  to facilitate laboratory  sorting.

     All individuals were  identified to species  if  possible  using the
most recent keys for identifying marine invertebrates.

     Three sediment cores were  taken from each  treatment each sampling
period and sediment grain size  analysis conducted on each sample
according to Folk (1961).

     Sediment traps, consisting of three  small  jars  strapped to a
wooden top with funnels clamped to the  wooden top to allow suspended
sediments to accumulate in the jar,  were  used  to assess  any
differences in sedimentation rates in  the pens  and  cages in  the sand
area only.

     Dyed sediments were placed inside  the pen  and  cages both in the
grass and sand area to  monitor sediment mobility in each of  these
treatments.
-------
     Larval  traps, made  of  one  gallon  jars  attached to stakes inside
and outside  pens, were also used  to  assess  variations in larval
recruitment  rates that may  be due  to cage effects.

     Because of the excess  fouling that  can occur on exposed objects,
the pens and cages were  cleaned regularly to prevent buildup of
excessive epiphytic algae by rubbing all sides  with plastic cloth
brushes constructed of the  same material the cages  were made of.

     Minnow  traps and small crab  pots  were  maintained inside each of
the pens to  catch small  fish and  crabs that may have entered the  pen
at small sizes and grown to sizes  where  they could  not exit.  Cages
were examined weekly to  remove,  fish  or crabs that entered the cages.

     Numerical abundance histograms  were plotted by area for both
species and  individuals  found in  the infaunal and epifaunal (grass bed
only) community.  Species diversity  was  calculated  for each area  using
the index of Shannon (Pielou, 1975).   The index H1  is expressed as:
                        H1 = £     Pi  Iog2 Pi
where s = number of species on the sample and  Pi  =  proportion of the
i-th species in the sample.  This index  is commonly used  for
comparative pu-poses and includes both a species  richness  (the number
of species in a community) component  and an  evenness (how equitably
the individuals are distributed between  the  species) component.

     Because the blue crab, Callinectes  sapidus is  such a  dominant
predator in the grass bed, we examined the stomachs of blue  crabs  for
feeding analysis.  Eighty-three blue  crab stomachs  were analyzed in
1978.  Individuals were collected with a 4.87  m (16 ft) otter trawl
with 19 tarn (3/4 inch) wings and a 6.3 tarn (1/4  inch) cod end  liner.
The trawl was pulled for a period of  2 min.  at a  speed of  2  to 3
knots.  Collected crabs were subsampled  and  those selected were
immediately weighed, measured, sexed, and the  molt  stage  noted.
Stomachs were removed in the field and preserved  in 10% buffered
seawater formalin with" the vital stain, Rose Bengal.  Each stomach  was
carefully dissected in the laboratory and the  contents enumerated and
identified when possible.

                                RESULTS

     The pen and cagp experiments in  the sand  and grass areas  affected
both the epifaunal and infaunal components during the course of  this
study.   Because of the large data set collected for both components,
the results for the infauna, and epifauna will  be presented separately.
It must be stressed that this separation is  artificial and done  only
for simplification of the discussion.  However, there are  undoubtedly
important interactions that occur between both components  and  any
possible interaction will be analyzed in the discussion section.
                                    47
-------
I.  Infauna

     Despite the cages  and  pens being  present  for only two months,
there were some dramatic differences in  the  response of the infauna to
several of the treatments.

     In comparing the sand  area to  the grass area for the June data,
several effects were evident:  1.   The mean  number of species per core
for all the treatments  in grass were higher  than  the comparable
treatments in sand  (Fig. 3).  Within the grass  area, there were no
differences among the treatments whereas in  the sand, the cage (C) and
pen + cage (PC) treatments  had more species  than  the other treatments.
The pen (P) and pen + open  cage (PO),  in turn,  had more species than
the control (S) and open cage (0).  2.   The  mean  number of individuals
per core for the grass  treatments were greater  than the sand
treatments except for the cage (C)  and pen + cage treatment (PC) (Fig.
4\ where similar numbers were recorded.  There were no differences
among all of the grass  treatments whereas the sand cage (C) and pen +
cage (PC) were greater  than the other  four sand treatments.  3.
Comparison of diversity between the two  habitats  (Fig.  5) was not as
distinct as with number of  species  or  individuals since the H1  index
is sensitive to not only species number  but  the distribution  of the
individuals among those species.  Diversity was,  in general,  highest
in the grass areas as compared to the  sand area,  with the pen + (PC)
cage treatment having the highest diversity  of  the sand treatments.
Lowest diversity was found  for the  sand  control (S) and open  cage (0)
in the sand area.  4.  The  pattern  of  species dominance was very
different for the sand  and  grass habitats (Table  1, 2).  This was
particularly evident for the soft shell  c'ara, Mya arenaria, whose high
densities only in the sand  cage (C) and  sand cage + pen (P+C),
contributed to the high densities of individuals  for the^c two
treatments.  Gemma gemma was also found  in increased abundance in
these two treatments as compared to the  other sand treatments.   Gemma
was the dominant species in the other sand treatments but densities
v:ere not as high in the cage treatments.  The grass infauna was
dominated by large numbers  of the capitellid polychaete,  Heteromastus
filiformis, the spionid polychattes, Poiydora ligni and Streblospio
benedicti, and oligochaetes.  The cirriped, Balanus improvisus, was
recorded in these samples.  Though  Balanus is epifaunal,  many were
present on old leaf material that had settled on  the bottom after the
barnacle covered leaves had sloughed off from tne  plants.   Mya     \
arenaria was present in the grass treatments but  their  numbers  were ',
low compared with the densities found in several  of the cage
treatments.  Many of the dominant grass  infaunal  species  were present
in the sand area but in vary low densities and  there were  no  species
that were restricted to either of the two habitats.

     Data for the September sampling date revealed  similar trends  to
the June data:   1.   Ihere were more species in  the  grass  habitat than
the sand (Fig.  3).   The numbers of  species per  core for the pen (P)
and cage (C1) treatments in the sand were not different from  each
other but were  higher than  the control.  The species  per  core in the
                                    48
-------








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Fig.  4.   Number of individuals per core recorded  from infaunal samples for
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                                     50
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-------
TABLE 1. LIST OF TOP TEN DOMINANT SPECIES RECORDED FROM INFAUNAL CAGING EXPERIMENT SAMPLES FOR THE SAND HABITAT FOR JUNE. NUMBERS
REPRESENT PER CENT COMPOSITION THAT EACH SPECIES COMPRISES OF THE TOTAL SAMPLE AND ITS DENSITY PER M2 . TOTALS FOR THE TOP
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                                              53
-------
grass treatments were variable  as  evidenced  by the  distribution of
each of  the cores  for each  treatment.   Tha mean number of species per
core for the older cage  (C)  and pen  +  cage (PC) treatments were
greater  than the other  treatments  while the  grass control (G) had the
fewest but these are probably not  significantly different.  Species
number was reduced in the control  as compared  to the  June data while
the new  cage (C1) and pen +  cage  (PC1)  treatments had less mean number
of species per core than the comparable older  treatments.  2.  Numbers
of individuals were again higher  in  the grass  habitat than the sand
but the  differences were not as large  as in  June.   The density of
infauna  for the- sand treatments were greater than the control as was
the case for the grass habitat.  The density of infauna in the grass
control  (G) was also similar to the  sand treatments.   Overall,
densities of infauna in  the  grass  and  sand were less  than that
observed in June for all treatments.   3.  Mean diversity (H1) was
higher in the grass habitat  than  in  the sand habitat.  There was no
difference among the grass  treatments  and control while in the sand,
H1 was higher in the treatments than the control though there was
considerable overlap of  the  H1  for the  samples.  4.   Dominance
patterns of species in each  of  the two  habitats for the different
treatments varied from the previous  sampling period (Tables  3. 4).
Because  the caged areas  in  the  sand were completely disturbed, after
the June sampling by blue crabs,  the pattern represent trends recorded
for only two months.  The grass  cages,  because they were not
disturbed, allowed for a comparison over a longer time scale.  In
addition, the newer cages, also set up  in June to compliment the sand
studies, allowed comparisons of recruitment  on a shorter time scale.
In the sand area, the bivalve Gemma gemmr. was  one of  the dominant
species  (Table 3) in the pen (Piand cage treatments  (C1, FC1) but  was
absent from the control area (S).  Species present  in the control area
were only small, tube dwelling  polychaetes in  very  low abundances.
Another bivalve, Mulinia lateralis, was  only abundant in the cage (C1)
and pen + cage (PC1)treatments.   In the  grass  treatnents (Table 4)
Gemma was very abundant, whose  densities were,  in most cases, higher
than those densities recorded in June.   The  polychaete,  Heteronastus
filiformis, was much less abundant in June,  though  it was still one of
the top  10 species.  Balanus was no longer a dominant species (except
in the old cage (C), a result of the old  leaves  having either been
washed out of the area or dying as the  leaves  to which they  were
attached decayed.  Oligochaetes were also abundant  as they were in      \
June with approximately similar densities (one  exception,  they were     \
not dominant in the cage treatment).                                     j

     One particularly interesting species found  more  abundantly in  the
caged treatments, especially those initiated in  April (C and PC) than
the pen (P) or control (G) was  the oyster, Crassostrea virginica
(Table 5).   Crassostrea is usually not  found setting  in  grass beds  but
was found to have actually set  on pieces of  eelgrass  along tne bottom
of the cages.

     Crepidula convexa,  an epifaunal gastropod was  abundant  in all  the
treatments.  Its presence as an  infaunal component  is a  reflection,
                                   54
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TABLE 5.  DENSITIES (PER M2)  OF THE OYSTER,  CRASSpSTREA  VIRGINICA. FOUND  IN
          THE GRASS HABITAT FOR EACH OF THE  EXPERIMENTAL TREATMENTS AND CONTROL
          FOR SEPTEMBER AND NOVEMBER.

Control (G)
Old cage (C)
New cage (C1)
Pen (P)
Pen + old cage (PC)
Pen + new cage (PC1)
September
21
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1760
642
10621
1491
November
41
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-
104
9317
1222
                                         57
-------
probably,  of  its  spillover  from  the  epifauna  where  it was  present in
very  large numbers.

      The patterns  discussed  above  for  the  infauna collected in June
and September was  evident  in most  cases  through November.   1.   The
number of  species  was  again  higher  in  the  grass habit't  compared with
the sand habitat  (Fig.  3).   Species  number was  higher in the sand
treatments compared with  the control while in the grass, there were no
distinct differences among  the treatments,  although there  was  a lot of
variation  between  cores of  similar  treatments.   Except for the old
cage  treatment, the mean number  of  species for  each of the grass
treatments and control  was higher  compared to September  and in June.
2.  The number of  individuals per  core was higher in all grass
treatments compared with  the sand  treatments  (Fig.  4).  Density in the
sand  control was  less  than  the three experimental treatments which
among themselves were  not different.   Densities in  the grass
treatments were similar and  were higher  than  densities found in the
grass habitat in September.   3.  Diversity (H1) was also higher in the
grass habitats compared to  the sand  area.   The  sand treatments
incorporating the  cages had  a higher H'  than  the pen or  control while
the mean diversity for  each  grass  treatment was not different.  4.
The dominant species in the  sand area  in November was similar  to that
found in September (Table 6).  Gemma was the  dominant species  in the
pen + cage (PC1),  pen  (P), and new  cage  (C1)  treatments  and Capitella
(polychaete) was dominant in the control (S)  with Gemma  the second
most  abundant species.  The  spionid  polychaetes, Strebfospio and
Polydora,  increased in  abundances  over the September densities.

      Species dominance  in the grass  treatments  for  November (Table 7)
were, in most cases, not similar to each other.  Gemma was the
dominant species in the new  cage (C1)  as it was  in  September and was
also  abundant in the pen + new cage  (PC1),  pen  (P)  and grass control
(G).  Densities of Crassostrea (Table  5) were similar to that  recorded
in September for the new cage (C1) and was  also very abundant  in the
old cage (C) and pen + new cage  (PC1)  treatments.   It was  not  recorded
in •'he new cage treatment (C1) where,  in September,  the  new cage
contained  1760/m .  Crepidula was still abundant in all  treatments and
control.   The oligochaetes increasad in density over September's
density in  the new cage (C1), grass control,  old cage +  pen (PC) and
old cage (C).  Densities of  Polydora ligni  and  Streblospio benedicti
increased  in all treatments  compared with  September as was  the case
for the sand treatments discussed above.

II.  Epifauna

     Because of the experimental design, only one sample of eelgrass
could be taken at  the time of sampling.  All  samples  were  roughly
estimated  to be approximately 0.1 m^ in order to make sample size
equivalent.

     In June, the  penned treatments (P, PO and  PC)  had more species
per sample  (Fig. 6) than the unpenned  area while the  caj-.t.d  area (C)
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                     30-1
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                     20-
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                     30-i
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                      0
                            NOVEMBER
                           G    P    C    C    PC   PC

                               TREATMENT
Fig.  6.   Number of epifaunal  species per sample for each treatment and control
         during the three sampling periods.  Treatment designations are as
         follows:   G=grass control; P=*pen; C=cage; P0=pen + open cage; PC=
         pen i  cage;  0=open,  topless cage; C*=new cage set; PC =new cage
         •f pen  set.
                                    61
-------
had slightly more  species  than  the  open  cage  (0)  and  control  (G).   In
contrast  to species  number the  open cage treatment  had  more
individuals/g of grass while  the  cage  (C)  and pen + cage  (PC) had  the
least number of individuals/g grass (Fig.  7).   These  differences are
primarily attributed  to  the differences  in the density  of the
barnacle, Balanus  improvisus  (Table 8) which  was  very abundant in  the
open cage.  Seven  of  the top  ten  dominant  species were  shared among
all treatments including the  control while Balanus  was  the dominant
species and Crepidula the  second  or third  most abundant species in
each treatment.  The  top three  species in  each treatment  constituted
90% or greater of  all individuals recorded for each sample.  This
overwhelming dominance by  a few species  was the cause for the overall
low H' (diversity) for the treatments  in June (Fig. 8).   There were
also no major differences  in  H' among  the  treatments  for  June.

     In September, all treatments had more species  per  sample than the
control (Fig. 6) with the  cage  (C)  treatment  having the greatest
number.   Except for  the cage  treatment,  the grass control,  pen and pen
+ cage had fewer species than that  recorded for June.  The new cage
(C1) and  new cage  +  pen  (PC1) had slightly fewer  species  than the
comparable older treatments.  This  situation  paralleled the response
of the infauna to  the new  cage  sets also for  the  September sampling
data.

     The  density of  epifaunal individuals  per  gram  of -grass in
September was much greater in all treatments  when compared to the
control (Fig. 7).  The highest  density was  found  in the new cage (C1)
and pen + new cage (PC1).  These  very high densities  were caused by
the large numbers  of  Crepidula, which  in five  of  the  six  treatments
made up 98% or greater of  the total epifauna  (Table 9).   Balanus and
the isopod Erichsonella were  either the  second and  third  species in
all the treatments.   Dense concentrations  of  Crepidula  were observed
covering  the blade from the tip to  the base especially  in the new  cage
treatment, but were not evident anywhere outside  the  experimental  area
in as dense concentrations.   In addition,  compared  to June  when seven
of the ten species were found in  all treatments,  only four  of the  top
ten were not found in all  treatments.

     The very low  diversities of  this sampling period,  which  were
lower than those calculated in June, were  a result  of the  overwhelming
dominance by Crepidula.  The  slightly higher  diversity  value  for the
cage (C) treatment was due to the fact that Crepidula comprised only
90% of the total sample compared with 98%  or  greater  in the other
treatments.

     By November,  there appeared  to  be no  difference  in the total
number of species  in  each  sample  (Fig. 6).  However,  there  were large
differences in the number  of  individuals among  the  different
treatments (Fig. 7).   The  lowest density was  found  in the  cage (C)  and
pen •*• cage (PC) treatments.  This low density  of  individuals  was due
primarily to the fact that the abundance of grass in  these  older cages
was very  low.   The oyster  set that  had occurred in  these  two
                                   62
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Fig. 8.   Diversity values (H )  for each  treatment and control during the
         three sampling periods.   Treatment  designations are as follows:
         G=grass control; P=pen;  C=cage;  P0= pen + open cage; PC=pen + cage;
         0=open, topless cage;  C  =new cage set; PC =new cage + pen set.
                                    66
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treatments probably caused  some  of  this  reduction as  well  as from the
overall reduction of  light  from  the netting of  the cages.   The
reduction in  -_he density  of the  grass  in the newer cages was not as
dramatic since  these  cages  were  in  the field for a shorter length of
time.  The pen  had the  highest density of epifauna where the grass
density inside  the pen  was  apparently  not impacted by the  presence of
the pen.

     Crepidula  maintained  the dominant ranking  in all treatments
through November (Table 10).  The very large numbers  in the pen (P)
treatment caused the  large  numerical density for this treatment
discussed above.  In  addition, Crepidula densities in all  treatments
except the cage (C) made  up greater than 96£ of the total  sample.  In
the cage treatment where  grass density was low,  Crepidula  made up only
49% of the sample.

     Also in  the November  samples,  only three species were shared
dominants in  all treatments compared with seven in June and four in
September.

     The diversity (H1) of  each  treatment was very low again except in
the old cage  (C) because  of overwhelming dominance by Crepidula in
these treatments (Fig.  8).   The  decreased abundance of Crepidula in
the old cage  (Table 10) allowed  the other species to  assume a greater
rank which resulted in  a higher  equilability component of  H1  for the
cage treatment.

III.  Blue Crab Stomach Analysis

     The mastictory mode of feeding made the identification of gut
contents to the species level difficult.   Percent frequency of
occurrence of food items  indicated  the blue crabs feed on  both
epifaunal and infaunal  species (Fig. 9).   Zostera was found in 70% of
the stomachs  analyzed.  Generally,  live,  intact,  and  very  uniformly
cut sections  of leaf material were  present, indicating that crabs may
ingest the blades but digest only the  encrusting organisms.  Epifaunal.
molluscs, isopods and Balanus improvisus were among the major food
items in crab stomachs.  Callinectes also foraged among the rhizome
mat on infaunal molluscs.   Feeding  burrows and  infaup.al feeding were
frequently observed in  the  field.

IV.  Sediments

     Table 11 presents  the  percent  sand,  silt and clay in  the
sediments taken from the various treatments in  the sand and grass
habitats in November, 1969.  Within the  sand  habitat  and grass
habitat, there appeared to  be little difference  among the  treatments.
The percent sand in the grass control  (G)  and new cage (C1) was not
different from the sand.  However the  percent silt was higher in these
same grass  treatments compared to the  sand.
                                     69
-------
                      100-r
                       75--
                            700
Fig. 9.  Percent  frequency of occurrence of food items  in Callinectes
         sapidus  stomachs.
                                      70
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TABLE 11.  PERCENT SAND, SILT AND CLAY IN THE SEDIMENTS TAKEN FROM THE
           DIFFERENT SAHD AND GRASS TREATMENTS IN NOVEMBER, 1979 (NUMBERS
           REPRESENT MEANS OF THREE SAMPLES).
TREATMENT
SAND
CLAY
SILT
Sand (control)

Sand + Cage

Sand + Pen

Sand + Pen + Cage
94.2

93.6

Samples Lost

93.6
4.7

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Grass (Control)

Grass + Cage (old)

Grass + Cage (new)

Grass + Pen

Grass + Pen + Cage (old)

Grass + Pen + Cage (new)
92.2
88 7
92.9
90.2
90.2
90.4
5.2
7.9
4.9
6.3
6.5
6.2
2.6
3.4
2.2
3.5
3.3
3.5
                                        73
-------
     The cage  and  pen  treatments  in the  grass  (except the new cage)
had  less sand  and  more clay and  silt than the  control plot.  The old
cage had the  lowest  percentage of sand and the highest percentage of
clay, a result of  this cage being out the longest.   These alterations
in the sediment  fractions  are not of a large magnitude.   This may be
because the treatments we'e not  in the field  for a  long  enough time to
create sediment  alterations and  that the very  dynamic nature of this
a^en prevents  fine sands  and silts and clays  from accreting.  It may
also indicate  that the cages and  pens did not  have  as significant
impact on  the  sediments as originally hypothesized.

                              DISCUSSION

     Benthic  environments  colonized by submerged vegetation represent
a unique habitat with  respect to  their impact  on the associated faunal
communities.  The  most notable characteristic  of these beds are the
large densities  of animals that  are found associated with SAV compared
to the surrounding,  unvegetated  sediments (Thayer et al., 1975; Orth
1977; Reise,  1977b;  Orth  and Heck 1980;  Heck and Orth 1980a).  Recent
interest has  centered  on  the factors that cause such high densities
(e.g. increased  physical  heterogeneity,  decreased predation rates and
increased  food)  (Oi.th  1977;  Reise 1977a;  Peterson 1980;  Nelson 1979;
Stoner 1980;  Heck  and  Orth 1980b).

     The advent  of manipulative  techniques in  understanding function
as well as structure of the benthic communii-y  has clearly identified
major factors  that overwhelmingly influenced the makeup  of the
community  (Virnstein,  1977;  Woodin 1974;  Young et al. 1976; Reise 1977
a,b; Orth  1977;  Peterson  1980).   Though  there  are distinct problems in
field manipulation experiments,  especially predator  exclusion cages
(Virnstein 1978; Peterson  1980),  these manipulative  techniques have
been extremely useful  in  allowing a more thorough examination of
causative  factors  through  experimentation, rather than previously
stressed correlative information.

     Despite  some  of the  difficulties encountered in the caging and
pen experiments  (e.g.  the  breaching of the pen in June just prior to
our sampling and the crab  disturbance of  the sand cages), our results
suggest that exclusion of  predators through cages and pens has an
impact on  the macroinvertebrate  fauna inhabiting the sand and grass
habitats.

     Several trends  were evident  over the entire sampling period.
With respect to numbers of species,  the  grass  habitat always had more
infaunal species per core  than the  sand  habitat  for  all  treatments.
There were apparently  no distinct  differences  among  the  grass
treatments for species  infaunal numbers  but in the sand,  species
numbers were higher  in pen and cage treatments compared  with the
control, with the  cage  treatments  (C and  PC) in  June having the most
dramatic effect on increased numbers  of  species.  However,  once the
cages were disturbed and new ones  set to  replace the disturbed ones,
                                    74
-------
the effects were not as  dramatic  though  species  numbers  were  stil
higher compared to  the control.

     In addition, except  for  the  sand  cage  treatments  (C and  PC) in
June, there appeared to  be no difference between the pen and  cage
treatments for numbers of species  in both habitats.

     Infaunal species numbers were  lower in September  compared to  June
and November.  This could possibly  be  a  result of natural mortality
due to high summer  time  water temperatures  as well as  predation
activity in Che uncaged  and unpenned areas, and  also possible
predatory activities of  infaunal  species on other infaunal species
that could not be detected through  our study design.   Similar summer
time depressions in infauna were  observed by Virnstein (1977) for  the
York River, Virginia.

     Density of individuals in  the  grass habitat treatments was
generally higher than the sand  habitat treatments except for  the June
cage treatments.  The differences  in densities were not  as pronounced
in the September period  as in the  other  two periods.   The September
density in the grass crntrol  this month  was very similar to the sand
treatments, but by November,  the  differences in  densities had
increased between the sand and  grass habitats.

     In the sand area, densities were  always higher in the cage and
pen treatments compared  to the  control in all months while in the
grass habitat only  the September  treatment  densities were higher than
the control.

     As with the species number, densities  of infauna  decreased in the
summer both in the sand  and grass habitat.  The  large  depressions  in
the grass control suggests that predation in grass habitats can be
significant, especially  in the  summer  when  not only crabs are present
but also different benthic fish predators.  The  difference between the
highei numbers persisting in  the  treatments versus the control for
September's grass infauna possibly  relates  to that portion of the
infauna that escapes predation  via  the cages or  pens.

     The grass habitat acts as  a refugia for infauna though many
individuals are still cropped by predatory  effects during the period
when predation may be intense (e.g. summer  periods).   It is
interesting to note that the  decrease  in infaunal densities from June
to September was similar for  both the  sand  (19%)  and grass habitat
(17%).  Those grass infauna surviving  the high summer  predation levels
do so primarily because of the  presence  of  the grass.

     Species numbers and density in the  new cages in the grass  area
were lower than comparable figures  for-the  older cages.   This  suggests
that timing of the placement'of cages  has an effect on the settlement
densities as recruitment periods for different species may vary,
exposing the benthic environment to a  different  suite  of recruits  at
different periods.
                                    75
-------
     Species response  to  the  cages  and  pens  in  the  sand  and  grass
habitats was variable.  Gemma gemma,  a  sand  dominant,  persisted in the
cage and pen areas, but was almost  entirely  eliminated from  the sand
control area by predation, either by  blue  crabs  or  rays  which
frequented this habitat all summer.   The large  increase  in Mya in  the
June cage areas in the sand only suggests  a  cage effect  in  that the
reduction in current velocity inside  the cages may  have  allowed high
densities of Mya to settle preferentially.   It  is also possible that
the planktonic larvae  of  Mya, when  contacting the cage surface,
initially attached to  the cage via  byssal  threads and  then  fell into
the cage as they got older, which could explain  why they were not  as
dense in the pen treatment (P).  The  absence of  Mya in the grass
habitat cages cannot be explained except by  vagaries of  planktonic
recruitment or the grass  bed  acting as  a filter  and allowing a more
even settlement of Mya throughout the grass  area.

     Crassostrea, the American oyster,  appeared  to  be  affected by  the
older grass cages.  Like  Mya  it has planktonic  larvae  and the
conditions that favored Mya settlement  may have  been the same for  the
oyster.  Crassostrea attached to the  blades  of  grass and subsequently
grew on the bottom.  The  lack of a  hard substrate in the sand cages
(oysters do not produce byssal threads) may  be  the  reason for lack of
oyster settlement in the  bare sand  area.

     The results of our caging work present  both similarities and
dissimilarities to recent work done in  other comparable  grass
habitats.  Reise's (1978) data paralleled  our results  where  predator
exclusion experiments he  conducted  had  greater  impact  in unvegatated
habitats than vegetated habitats, which was  similar to what  we found
in our experiments.  Reise concluded  that  predation was  mitigated  by
the spatial resistance of the grass and limited  accessibility.   Orth
(1977) showed densities in caged treatments  significantly increasing
over short periods of  time in similar vegetated  habitats in  the
Chesapeake Bay.  The cause for this difference may  be  timing at which
cages were initialized, i.e.  August in Orth's study and  April and  June
in this study.  This suggests that  the  timing of the csge placement
relative to predatory activities may  be critical in understanding  the
role of predation in these habitat.                                  !

     Epifauna densities were  impacted by the cage's effect on the    \
growth of grass inside the cage.  This was particularly  evident in the
older cages where, compounded with  the dense set of oysters  and
fouling on the cages,  the density of  grass was reduced inside the
cages compared to outside the cages.  In discounting the cages  and
their effect on the grass density,  the  pens  alone,  where grass  density
was not affected, had a large effect  on epifaunal densities.   In June,
densities of BaIanus were higher in the pen  treatment  and in September
and November, the very high densities of Crepidula  inside the pen  were
responsible for the large differences inside and outside the  pen.   In
September the new cages (C1 and PC1), which had  not drastically
affected the growth of grass, also had very high densities of
Crepidula.  Some stomachs of blue crabs examined during  this  period
                                   76
-------
(1979) which had different contents  than  those  examined  in 1978,
suggest that blue crabs were  feeding on Crepidula  and  that their
exclusion from the penned area and newer  cages  caused  the  high
densities of Crepidula to persist.   Blue  crabs  do  ingest grass and
most likely whatever  is on the blades.  Being very opportunistic  they
will feed on anything that is available.  The large abundance of  food,
both in the sediment  and on the blades, allow the  blue crab an ample
food supply.

     It appears that  during the course of this  experiment, predators
(including fish) consumed large quantities  of food items from the
sediments and grass.  The large secondary production estimates
observed from additional work done in this  area (see Section IV on
Secondary Production) suggest an enormous food  supply  available for
consumption by many pred;.tor  species, both  fish and invertebrates.
Our data show large reductions of both epifauna and infauna between
June and September in open, uncaged  or unpenned areas  and  that the
presence of the pens  or cages had a  significant impact on  the
survivalship of prey  species.  The results  here support  many of the
recent works cited above on tne importance  of predation  in structuring
the benthic fauna and that this fauna provides  a significant supply of
food to those predators.
                                   77
-------
                              REFERENCES

Annala, J. H.  1974.  Foraging strategies and predation effects of
     Asterias rubens and Nucella  lapillus.  Ph.D. Thesis.  Univ. of
     New Hampshire.  233 pp.

Brooks, J. L. and S. I. Dodson.   1965.  Predation, body size and
     composition of plankton.  Science  150:28-35.

Dayton, P. K.  1971.  Competition, disturbance, and community
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Dayton, P. K., G. A. Robilliard and A.  L. DeVries.  1969.  Anchor ice
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Dayton, P. K., G. A. Robilliard,  A. L.  DeVries and R. T. Paine.  1970.
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Folk, R. L.  1961.  Petrology of  sedimentary rocks.  Hemphill's,
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Grigg, R. W. and J. E. Maragos.   1974.  Recolonization of herciatypic
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Heck, K. L., Jr. and R. J. Orth.  1980a.  Seagrass habitats: The
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Heck, K. L., Jr. and R. J. Orth.  1980b.  Structural components of
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Nelson, W. G.  1979.  Experimental studies of selective predation on
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Odum, E. P-  '971,  Fundamentals of ecology.  W.  B. Saunders Co.
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Orth, R. J.  1975.  The role of disturbance in an eelgrass,
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Orth, R. J.  1977.  The importance of sediment stability in seagrass
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Ortu, R. J. and K. L. Heck, Jr.  1980.  Structural components of
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Orth, R. J., K. A. Moore and H. H. Gordon.  1979.  Distribution and
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Patrick, R.  1970.  Benthic stream communities.  American Sci.
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Peterson, C. H.  1980.  Predation, competitive exclusion, and
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Pielou, E. C.  1975.  Ecological Diversity, Wiley Interscience, N. Y.
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Reise, K.  1977b.  Predator exclusion experiments in an intertidal
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Reise, K.  1978.  Experiments on eoibenthic predation in the Wadden
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Thayer, G. U., S. M. Adams, and M. W. LaCroix.  1975.  Structural and
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     1:518-540.                                                      ',

Stoner, A. W.  1980.  The role of seagrass biomass in the organi-
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Virnstein, R. W.  1977.  The importance of predation by crabs and
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Virnstein, R. W.  1980.  Measuring effects of predation on
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                                   80
-------
                        CHAPTER 3                                                /

PREDATOR-PREY INTERACTIONS IN A ZOSTERA MARINA (EELGRASS)
     ECOSYSTEM IN THE LOWER CHESAPEAKE BAY, VIRGINIA

                           by

                     Robert J. Orth

                           and

                  Jacques van Montfrans
                           81
-------
                                   ABSTRACT

     Experiments were conducted with artificial seagrass  in  small  wading  pools
to assess the ability of prey to survive predation at different  densities  of
grass.  Experiments using Mulinia laterialis, a bivalve,  as  prey,  and  adult
Callinectes sapidus, a crab, as predator, showed  that almost  no  >1.  lateralis
survived at 3 different densities of grass.  Experiments  with juvenile £.
sapidus as prey and adult C_. sapidus as predator  showed greatest survival  at
highest densities of grass.  The behavior of the  juvenile crabs  in relation to
its predator was different in the presence of the grass than  in  its  absence.
It was believed that the survival of a particular prey species in  a  vegetated
habitat will depend upon tne life style and life  cycle of both prey  and
predator and the density and morphology of the vegetation.
                                      82
-------
                                  INTRODUCTION

     A species presence or absence and  its  abundance  in  any  particular  habitat
is regulated by its physiological tolerances, morphological  constraints,
habitat preferences, and biological  interactions  such as competition  and
predation with other animals (Dayton, 1971; Connell,  1972; Virnstein, 1977;
Nelson, 1979a, b; Stoner, 1980a, b, c).  Predation  is a  significant biological
interaction in aquatic systems and has  been shown to  be  the  primary
structuring agent in many recent ecological studies in terrestrial, freshwater
and marine systems (Pai.ne, 1966; Dayton, 1971; Brooks and  Dodson,  1965;
Connell, 1975).  The ability of a prey  species to avoid  a  predator will depend
on the morphological and behavioral modifications that the prey  has evolved
over time in response to predation.  In addition, the ability  of the  habitat
to mediate the effect of predation can  play a significant  role in  the survival
of the prey (Stoner, 1980c).

     Seagrass systems contain a very dense  and diverse raacroinvertebrate
assemblage (Orth, 1973; Kikuchi and Peres,  1977)  in which  predation plays  an
important if not key role in structuring the associated  faunal community
(Nelson, 1979a, b; Heck aivi Orth, 1980a).   Several  studies have  also  alluded
to the idea that vegetations 1 density can have a significant affect on  the
survival of the prey (Weinst-.'in and Heck, 1979; Heck  and Orth, 1980a, b; Orth
and Heck, 1980; Stoner, 198Ub; Heck anf4 Thoman, 1981).   Our  objectives  in  this
study were to examine predator-prey interactions  in a Zostera  marina  dominated
system and to assess the prey's ability to  survive  predation at  different
vegetational densities.

                             MATERIALS  AND  METHODS

     Two prey species with very different life styles were chosen  for
predation experiments.   We used a sedentary infaunal  bivalve,  Mulinia
lateral is, in one series of experiments.  Mulinia is  ubiquitous  throughout the
Bay and inhabits a variety of sediment  types where  it  burrows  to a depth of
2-3 cm.  This species grows to a length of  approximately 2.5 cm  and undergoes
periodic population eruptions, particularly in the  spring  (Boesch, 1973,
1974).  However, the dense Mulinia populations experience  high mortalities in
the summer,  most likely due to predation (Virnstein,  1977).

     Juvenile blue crabs, Callinectes sapidus, were used as  prey in the second
set of experiments.   Callinectes is highly  motile and  juveniles  are
characteristically abundant in vegetated habitats of  the lower Bay (Heck,
1981).  This species is very important  comnercially,  being second  only  to
oysters (Crassostrea virginica) in terms of dollars contributed  to Virginia's
seafood economy.
                                      83
-------
     Large, adult male C^. sapidus  were  used  as  predators  in both sets of
experiments.  Blue crabs are  voracious  predators  and  can  be considered a
keystone species (sensu Paine, 1966)  in structuring Chesapeake Bay benthic
communities (Virnstein, 1977).

     Prey refuge experiments  were  conducted  in  three  wading pools (2.43 m
diameter x 0.45 m high).  Each pool had 10-13 cm  of sand  placed on the bottom
and was filled with estuarine water from the York River.   Predation
experiments on Mulini •» were conducted outdoors  using  a flow-through water
system whereas those on Callinectes were  set up indoors.   Water from each
indoor pool was continually pumped through a large header tank which contained
crushed oyster shell to insure adequate oxygenation and removal of any
suspended material (Fig. 1).  Each pool in both sets  of experiments received
additional aeration from two  piston air pumps.

     Artificial Zostera marina rhizomes and  leaves were used to simulate the
live system.  Three densities of rigid  plastic  netting (1 incn diameter)
measuring 1 ra x 1 m (Conwed Corp. Vexar)  were used to simulate the rhizomes.
High density mats were arbitrarily set  at the normal  plastic mesh density.
Medium and low density mats were reduced  proportionately  by cutting out cross
mesh strands.

     Extruded polypropylene ribbon 5 mm wide and  tinted green was used to
simulate the leaves.  Three densities of  grass  were chosen for the
experimental treatments: a high density count consisting  of 1600 blades/m ,  a
medium density count consisting of 800  blades/m^;  and a low density count
consisting of 400 blades/m .  The high  values were Approximated from maximum
stem density counts of eelgrass frr-m  a  Zostera  marina bed at the mouth of the
York River, Virginia.  Artificial grass was  attached  to the artificial
rhizomes for those treatments testing both rhizome and leaf effects.  This was
done by taking a single strand 30 cm  long and tying it in half around the
plastic rhizome nat yielding 2 strands,  each approximately 15 cm long.  Low
densities of grass blades were attached to low  rhizome density mats,  medium
density blades to medium density rhizomes and high density blades to high
density rhizomes based on the assumption  that in  2_. marina beds,  there is a
positive correlation between blade and  rhizome  densities.   Biomass
measurements from different j£. marina beds support this assumption (Orth and
Moore, 1982).  All rhizome mats (with or without  leaves)  were placed  1 to 2  cm
beneath the sediment surface  in each pool.

     Adult male Callinectes were collected from a  large Zostera marina bed at
the mouth of the York River using an otter trawl  and  maintained in holding
tanks.  Prior to being used in experiments,  all predators were starved for
48 hours by placing them in wire mesh cages, which prevented them from
effectively foraging in the hol'ding tank.  Care was taken to handle the crabs
as gently as possible to prevent damage  to the  crab.   Only males  ranging in
size from 10 to 15 cm (carapace width),  that had  all  appendages and no
visible, external breaks in the exoskeleton  were  used.  Any male  that was near
the   shedding phase was not used.

     The Mulinia-Callinectes experiments  initially tested the eftects of high
density rhizome mats and also the high  density  rhizome and leaf combination  on
                                      84
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the survival of Mulinia. This  was  done  because  we  assumed  that if the bivalves
received  little or no protection  from  the  high  density treatments,  there would
be no refuge at lower rhizome  and  grass  densities.   One pool with no
artificial vegetation was  also established and  each  treatment was randomly
assigned  to each pool.  Hulinia were provided  from laboratory cultured stocks
grown at  the VIMS Eastern  Shore Laboratory.  One hundred Mulinia ranging in
size from 1.5  to 2.5 cm were placed within the  boundaries  of each mat or
within a  1 m^  area at the  center of the  pool with  no artificial vegetation.
Prior to  introducing the predators the Mulinia  were  left undisturbed from 24
to 48 hrs until all individuals had burrowed into  the sediment.  Two predators
per pool  were  then introduced  between 0800 and  1000  hrs and experiments were
terminated 24  hrs later.   The  crabs were removed from each pool and the
remaining Mulinia were enumerated  after  raking  and  sieving the sediment.

     The  initial Callinectes-Callinectes experiment  tested the condition
thought to provide the greatest and least  protection, respectively, to
juvenile  crabs: high density leaves/rhizomes in one  pool and no
leaves/rhizomes in a second pool.  It was  again assumed that if no refuge
existed for juvenile Callinectes at high density leaves/rhizomes, there would
be no refuge at lower densities.   Since  the prey appeared  to derive some
protection from the high density treatment (see results),  medium and low
density treatments were also tested.  Experiments  were conducted
simultaneously in pools with sand  but without  artificial grass mats.
Treatments were randomly selected  and assigned  to  each of  the 3 pools.  Only
combinations of rhizome mats and grass were used because it was assumed that
juvenile  crabs, being motile prey, would not remain  buried when approached by
a crawling predator, and therefore the rhizome  mat alone would provide little
or no protection.  Leaves  provide  the greatest  protection  for juvenile crabs
by restricting the visual  cues  initiating  predator attack  and by presenting a
physical  barrier to successful  predation encounters.

     Juvenile male and female  blue crabs ranging in  size from 4 to 6 cm in
carapace  width were separated  from otter trawl  collections taken in a York
River Zostera marina bed and held  in e flow-through  tank for use as prey.
Only intact crabs showing  no signs of molting were used.   Ten prey  were placed
in each of three pools and left to acclimate for 24  hrs.   Four predators were
then introduced into each  pool.  Experiments were  terminated 48 hrs later and
all remaining crabs were removed, measured and  examined for any physical
damage.

     Salinity and both sediment and water  temperature were monitored twice
during each experiment.  Frequent visual observations were made of  predator
and prey behavior in each  pool  from a vantage  point  that caused minimal
disturbance to che experimental animals.   All  experiments  were conducted under
ambient light conditions.

                                    RESULTS

Muliria-Callinectes Experiments

     Data from two complete experimental series (Table 1)  indi^afe  that
Mulinia received virtually no  protection from  either the high density rhizome
                                       86
-------
TABLE  1.  NUMBER OF MULINIA  SURVIVING  IN  24  HOUR  EXPOSURES  TO CALLINECTES  AT
          HIGH RHIZOME DENSITY, HIGH LEAF/RHIZOMES  DENSITY  AND BARE SAND (EACH
          TREATMENT UTILIZED 100 M  '.INIA  and  2  CALLINECTE3).
                                             Percent Mulinia  Surviving

	Treatment                        Test  1	Test  2

High Leaf/Rhizome                         2                             1

High Rhizome                              2                             1

Bare Sand                                 2                             3




TABLE 2.  NUMBER OF JUVENILE CALLINECTES  SURVIVING  IN 48 HOUR EXPOSURES TO
          ADULT CALLINECTES AT THE DIFFERENT LEAF/RHIZOME DENSITIES.



Treatment          Percent of juvenile Callinectes  surviving  - Test  No.


                   JL   _?.   _!   JL   _JL  _A   _!  _§.   _i  *

High               70   70        80   70   60   100             77

Medium                       80                      100  100  93

Low                50   20   60   60   40   90 .  70        80  59

None               30   20   30   50   30    0   30    0   40  26
                                      87
-------
component or a combination of high density  leaves  and  rhizomes.

     Our observations indicated  that Callinectes began feeding on Mulinia soon
after they were introduced into  the pools regardless of the  experimental
treatment being tested.

Callinectes-Callinectes Experiments

     The initial test conducted  with three  adult predators  and 10 juvenile
prey in each pool yielded 90% survival  in t.ie  pool  with high density rhizomes
and leaves and only 20% surviving in sand.   This suggested  a protective value
of grass for juvenile Callinectes.  We  then proceeded  to test the effects of
other vegetational densities on  prey survival.

     Nine experiments were conducted at  four different levels of leaf density
(high, medium, low and none) (Table 2).  Only  three tests were run with the
medium grass density because of  inadequate  time and the reduced  availability
of suitably sized crabs.  Declining water temperatures caused the adult male
crabs to move into deeper water  while most  individuals of the year class that
provided the prey had grown too  large.  Lower water  temperatures  also caused
crabs in several later experiments to be lethargic  thus confusing
interpretations of vegetational  refuge  effects.  These data  were not included
here.

     Several earlier tests were  aborted  because the water flow was interrupted
due to pump breakdowns causing conditions in the pools to change.  Crabs
behaved differently in these situations, being more lethargic.  Thus the
number of replicate experiments  was lower than that originally planned.

     Experimental data were statistically treated  using a one way analysis of
variance and the Studeiit-Newraan-Keuls multiple comparison test for unequal
sample size was used for testing differences among  the means (Table 2).
Because of the nature of the data (counts),  they were  square root transformed
/ (x +.5) and then the calculations were performed.   There  was  a significant
difference (p<.01) between the vegetated and the unvegetated treatments.
However, there was no significant difference among  the three densities  of
vegetation though the trend in the means indicated  that low  density vegetation
had less of a refuge value than  the high and medium dense vegetation.   We feel
that increased replicability could have  reduced the variability  but
environmental considerations (low temperatures) did not allow for further
effective experimentation.

     Observations on both juvenile and  adult Callinectes during  the course of
the experiments provided some insight into  predator-prey behavior as affected
by the presence or absence of vegetation.   When first  placed in  the pools with
no vegetation, juvenile (3. sapidus initially moved  around the entire pool and
eventually burrowed randomly throughout  the  pool.   However,  when placed in the
pools with vegetation, juveniles initially  exhibited the same random movement
but then virtually all individuals gravitated  towards  the grass  mat and
burrowed within its boundaries.
                                      88
-------
     When predators were  introduced  after  the  juveniles  had acclimated tc the
pools for 24 hours, different  responses  were again elicited by the juveniles
dependirg upon the presence or  absence of  the  artificial vegetation.   The
introduction of predators  into  the unvegetated pools  caused the juveniles to
immediately emerge from the sand  and move  actively around the  pool in an
attempt to avoid predation.  In several  experimental  replicates without
vegetation, a juvenile would be caught and either  be  eaten or  lose several
limbs soon after the introduction of the predators.   The restless disposition
of the prey in these treatments was  evident during the  entire  experiment.  In
contrast, when the adult  crabs  were  introduced into  the  vegetated pools,
juveniles remained buried  or hidden  in the grass plot for the  duration of the
experiment.  Phis was particularly true  in treatments with highest density
vegetation.

     The adult crabs behaved in a dissimilar manner  at  the three different
densities of vegetation.   At the  high density  vegetation, they were rarely
seen foraging inside the  vegetation  zone although  some were occasionally
observed resting on the artificial blades.  It appeared  that the dense area of
blades imped€-d their ability to forage effectively.   Adult crabs were
frequently observed in the low  density vegetated area and their movements did
not appear to be as significantly impeded  by the grass  as they were in the
densely vegetated plot.   Remains  of juvenile crabs were  seen in the sparse
vegetation but where these  prey were actually  caught  was not witnessed.

                                  DISCUSSION

     Beds of submerged aquatic  vegetation  serve as both  a refuge from
predation and a feeding area.   Though a  species might utilize  a grassbed  for
multiple reasons (e.g.  both feeding and  protection),  it  is important  to
separate the relative ecological  role of each.  Furthermore, a species
utilization of a grassbed  may vary depending on its  life cycle stage.  For
example the habitat may be more important  ac a refuge to a juvenile individual
of a species but may shift  towards importance  as a feeding area in the adult
stage.  Or efforts concentrated on examining the refuge  function of eelgrass.

     The results of our experiments suggest that beds of submerged vegetation
do not have the same refuge 'value for all  species.  Mulinia received  little
protection from the presence of the vegetation.  Its  only escape responses is
to retract into its thin  shell.   Even in very  dense vegetation,  a predator,
such as the blue crab,  has easy access to  Mulinia  which  helps  to explain  why
this species is never very abundant in vegetated habitats.   Mulinia undergoes
population eruptions but  numbers  are rapidly reduced  by  predators.  Only in
those situations where  they have  been completely protected from predators
(e.g. inside predator exclusion cages) do  large numbers  of Mulinia survive for
a significant length of time (Virnstein  1977;  our  unpublished  data from the
pen experiments for this  project).

     Juvenile Callinectes, on the other  hand,  were significantly protected by
the vegetation and the  degree of  protection appeared  to  be related to
vegetational density.   Submerged  plants  visually and  physically impede a
predator thereby interfering with its search attack strategy.   It was probably
                                       89
-------
for this reason  that  the  juveniles  we  observed  remained  within the artificial
grass mat when a predator  approached.

     Our experiments  involved  only  one predator and  two  prey species.
However, when our results  are  considered  with both  those of Ken Heck of the
Philadelphia Academy  of Natural Sciences  (unpublished) who conducted similar
experiments and with  other previously  published accounts of predator-prey
interactions (Heck, 1981;  Heck and  Thoman,  1981)  a more  generalized scheme of
such relationships in grassbeds can be conceptualized.   The following  scheme
incorporates the importance  of predator and  prey  life  styles,  life cycle
stages, and the density and  morphology of the vegetation.

1.  In general, vegetated  habitats  provide  a greater refuge for mobile prey
than for more sedentary prey.  Among the mobile  epifaunal species,  those that
are highly motile (e.g. juvenile  fish,  shrimps,  and  crabs) will be less
susceptible to predation  than  those which are slower moving (i.e.  isopods and
amphipods) in a three dimensional habitat (grassbed) than  in a two dimensional
habitat (bare sand).  This  is  particularly  true if  the predator has a  search
and destroy type strategy  rather  than  an  ambush type approach.   Among  the
sedentary infaunal prey species,  the refuge  value of the grassbed  will not
differ greatly from that  of  a  bare  sand habitat.  Instead  it will  depend on
the lifestyle and biology  of the  prey  rather than as much  on the presence of
the grass.  The degree of  protection for  such species will depend  on whether
or not it is a tube dweller, if it  has  some  external means of protection such
as a shell and if so, the  nature  and quality of the  shell, and  lastly, the
vertical extent to which  the species can  burrow.  Sedentary infaunal species
that build tubes will recieve  greater  protection  than non-tube  builders in
both kinds of habitats.  However, species which are  able to burrow beneath the
rhizome layer will derive  more protection than  those which burrow  equally as
deep in unvegetaced areas.   In addition,  species  that  live in tubes that
extend well below the sediment surface  receive  adequate  protection from the
rhizome mat so that their  abundances are  not greatly affected by additional
protection from predators  (e.g. via predator exclusion cages).   These  facts
have been demonstrated for  the deep burrowing polychaetes  such  as  Heteromastus
filiformis and Spiochaetopterus oculatus  (Orth  1975, 1977;  Virnstein,  1977)
which were more abundant  in  vegetated  areas  than  adajcent  non-vegetated areas
and also which were equally  abundant inside  and outside  predator exclusion
cages in the two respective  areas.  The refuge  value for tubeless  sedentary
forms will also depend on  their position  in  the sediment horizon with  those
species living closer to  the sediment  surface being  more susceptible to
predation.  Our data show  this to be true for Mulinia which lives  just below
the sediment surface and which derives virtually  no  protection  from its thin
shell against blue crab predation.  Within a grassbed, epifaunal tube  builders
would be less protected than those  which  are infaunal since the latter could
be protected by rhizome layer.  Yet we hypothesize that  epifaunal  tube
builders have a greater chance of survival than epifaunal  non-tube builders.

2.  The refuge value of the  vegetation will  invariably be  a function of the •
different stages of the life cycle  of  a species.  For example,  a juvenile blue
crab is highly protected by  the vegetation.  However, once the  crab reaches a
certain minimum size natural predation becomes  less  of a factor in its
survival.  Therefore the grassbed is diminished in importance  as a refuge for
                                      90
-------
the  larger crab, with  the  one  important  exception  being the very vulnerable
soft crab stages during  the molting  cycle.   The  reverse case would be that
juveniles of a  species are not  protected by the  vegetation but older
individuals are.  For  example,  the soft  shell  clam,  Mya arenaria, lives near
the  sediment surface when  their  planktonic  stages  first set and the juveniles
start to grow.  With age,  this  species burrows deeper in the sediment.
Because adult Mya are  also found  outside vegetated areas,  we suggest that the
minimum depth at which older Mya  escape  predation  is less  in vegetated  areas
than in non-vegetated  areas because  the  rhizome  mat  would  provide a physical
barrier to the  digging activities of predators such  as the blue crab.

3.  The refuge  value of  vegetation will  be  directly  correlated with the
density or biomass of  the  leaves  (Heck and  Orth, 1980a, b; Stoner, 1980a, b)
for mobile species and for deeply burrowing ones it  will be correlated  with
the density of  the rhizome mat.   The inceasing coverage offered by vegetation
makes it more difficult  for predators to effectively seek  out and consume prey
items, possibly interfering with  potential  behavioral patterns in these search
strategies (Stoner, 1980b).

4.  The refuge  value of  vegetation for a prey  item will vary depending  upon
the  foraging strategy  of a particular predator.  For example the refuge value
for  the prey of an ambush  predator (i.e. one which sits and waits for prey
items to pass by) may be different than  that for a prey species fed upon by
search predator (i.e.  one which actively forages over a given area for  prey
items).

5.  Different species  of vegetation  may  have different refuge val;.es depending
on theii morphology.  We are suggesting  that at  the  same density of plants per
unit area of bottom, the more  foliose species  (i.e.  those  which are more
highly branched or having a high  ratio of surface  area to  biomass) have a
greater reguge  value than  less  foliose species.  The degree of branching is
directly related to amount of  cover  which allows prey species to effectively
escape predation.  Data  from Heck's  (1981)  experiments showed grass shrimp,
Palaemonetes spp., to have significantly higher  survival rates in Ruppia
vegetated areas than Zostera vegetated areas.  According to Heck,  Ruppia is
more foliose than Zostera which would account  for  tl>e increased survival of
grass shrimp that he observed.  In addition, field samples collected in. a
dense Ruppia bed by Heck yielded  densities  of  grass  shrimp higher than  any
samples collected from Zostera beds  (Heck and  Orth,  1980b; Heck,  1981).

     Since the  amount of branching and plant density are both critical  factors
in the survival of prey  in grassbeds  and different species of vegetation have
different surface areas  based on  their morphology, we can  hypothesize that a
low density of  a highly  foliose species  would  have the same refuge value as a
high density of a less foliose species.   Stoner's  data (1980c) is particularly
relevant in that he concluded  that blade surface area of macrophytes provides
the best estimate of habitat complexity  in  seagrasses.  This particular factor
would be a critical one  in management's  option to  replant  vegetation.  Because
of the increased coverage afforded by more  foliose species, the most viable
option for replanting would be to use these species  where  possible.
                                      91
-------
     In conclusion, beds of submerged aquatic vegetation  play  a  distinct  role
in regulating the distribution and abundance of associated  animals  by  their
ability to mediate both predator-prey and competitive  interactions.  The
degree of mediation will depend on the animal species  in  question and  the
density and morphology of the vegetation.  This degree of interaction  of  the
plant with its environment make submerged aquatic vegetation areas  one of  the
most interesting and ecologically important systems  in the  marine environment.
                                      92
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                                 LITERATURE CITED

Boesch, D. F.  1973.  Classification and community structure of macrobenthos
      in the Hampton Roads area, Virginia.  Mar. Biol. 21:226-244.

Boesch, D. F.  1974.  Diversity, stability and response to human disturbance
      in estuarine ecosystems.  Proc. 1st Intern. Cong. Ecol., The Hague,
      Netherlands, p. 109-114.

Brocks, J. L. and S. I. Dodson.  1965.  Predation, body size and composition
      of plankton.  Science 150:28-35.

Connell, J. H.  1972.  Community interactions on marine rocky intertidal
      shores. Ann. Rev. Ecol. Syst. 3:169-192.

Connell, J. H.  1975.  Some mechanisms producing structure in natural
      communities.  Pp. 460-490.  In: M. L. Cody and J. M. Diamond (eds.),
      Ecology and Evolution of Communities.  Harvard Univ. Press, Cambridge.

Dayton, P. K.  1971.  Competition, disturbance and community organization: the
      provision and subsequent utilization of space in a rocky intertidal
      community.  Ecol. Monogr. 41:351-389.

Heck, K. L., Jr.  1981.  Value of vegetated habitrats and their roles as
      nursery areas and shelter from predation.  EPA Chesapeake Bay Program.
      Final Report.  Grant No. 806151.

Heck, K. L., Jr. and R. J. Orth.  1980a.  Seagrass habitats: The role of
      habitat complexity, competition and predation in structuring associated
      fish and motile macroinvertebrate assemblages.  Pp. 449-464.  In: V. S.
     Kennedy (ed.), Estuarine Perspectives.  Academic Press, New York.

iieck, K. L., Jr. and R. J. Orth.  1980b.  Structural components of eelgrass
      (Zostfra mnrina) meadows in the lower Chesapeake Bay - Decapod Crustacea.
     Estuaries"3:289-295.                                                  \
                                                                           I
Heck, K. L., Jr. and T. A. Thoman.  1981.  Experiments on predator-prey
      interactions in vegetated aquatic habitats.   J. exp. mar.  Biol. Ecol.
     53:125-134.

Kikuchi, T. and J. M. Peres.   1977.   Consumer ecology of seagrass beds.  Pp.
      147-193.  In: C. P. McRoy and C. Helfferich (eds.), Seagrass Ecosystems.
     Marcel Dekker, Inc., New York.
                                      93
-------
Nelson, W. G.  1979a.  An analysis of structural pattern in an eelgrass
     (Zostera marina) amphipod community.  J. exp. mar. Biol. Ecol.
     39:231-264.

Nelson, W. G.  1979b.  Experimental studies of selective predation on
     amphipods: Consequences for amphipod distribution and abundance.  J. exp.
     mar. Biol. Ecol. 38:225-245.

Orth, R. J.  1973.  Benthic infauna of eelgrass (Zostera marina) beds.
     Chesapeake Science 14:258-269.

Orth, R. J.  1975.  The role of disturbance in an eelgrass (Zostera marina)
     community.  Ph.D. Dissertation.  Univ. of Maryland, College Park.
     115 pp.

Orth, R. J.  1977.  The importance of sediment stability in seagrass
     communities.  Pp. 281-300.  In: B. C. Coull (ed,), Ecology of Marine
     Benthos.  U. Scouth Carolina Press, Columbia.

Orth, R. J. and K. L. Heck, Jr.  1980.  Structural components of eelgrass
     (Zostera marina) meadows i the lower Chesapeake Bay-Fishes.  Estuaries
     3:278-288.

Orth, R. J. and K. A. Moore.  1982. The biology and propagation of Zostera
     marina, eelgrass, in the Chesapeake Bay, Virginia. Final Report.  U.S.
     E.P.A, Chesapeake Bay Program.  Grant No. R805953.  SRAMSOE No. 265,
     Virginia Institute of Marine Science, Gloucester Point.  187 pp.

Paine, R. T.  1966.  Food web complexity and species diversity.   Amer.
     Natural. 100:65-75.

Stoner, A. W.  1980a.  The role of seagrass biomass in the organization of
     benthic macrofaunal assemblages.  Bull. Mar. Sci. 30:537-551.

Stoner, A. W.  1980b.  Abundance, reproductive seasonality and habitat
     preferences of amphipod crustaceans in seagrass meadows of Apalachee Bay,
     Florida.  Cont.  Mar. Sci.  23:63-77.

Stoner, A. W.  1980c.  Perception and choice of substratum by epifaunal
     amphipods associated with seagrasses.  Mar. Ecol. 3:105-111.

Virnstein, R.. W.  1977.  The importance of predation by crabs and fishes on
     benthic infauna in Chesapeake Bay.  Ecology 58:1199-1217.

Weinstein, M. P. and K. L. Heck, Jr.  1979.  Ichthyofauna of seagrass meadows
     along the Caribbean coast of Panama and in the Gulf of Mexico:
     Competition, structure ~nd community ecology.  Mar. Biol. 50:97-107.
                                      94
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                              CHAPTER 4

   SECONDARY PRODUCTION OF SOME DOMINANT MACROINVERTEBRATE SPECIES
INHABITING A BED OF SUBMERGED VEGETATION IN THE LOWER CHESAPEAKE CAY

                                 by

                             Robert Diaz

                                 and

                            Tom Fredette
                                  95
-------
                                   ABSTRACT

     The production of the  top 9  trophically  important  species to the higher
level consumers at the Vaucluse Shores grassbed  was  40.7  g-nf^-yr"*.   This is
a higher productivity than  reported  for most  community  production studies.  If
this rate of production is  projected over  the entire 140  hectare grassbed a
total of 53 metric tons of  dry tissue was  produced and  potentially available
for consumption by other trophic  levels.   This  also  represents 6 x 10^
individuals which are born, grow,  and die  in  a  year. The average standing
stock over the year was 4.6 metric tons leaving  48.4 metric  i_ons to be
accounted for.

     The isopod E_. attenuata accounted for 43%  of the total  production for the
9 species.  The next two high ranking producers  were C_. sapidus and (3.
mucronatus, when combined with E.  attenuata accounted for 84.8% of biomass
produced by the 9 species.  Turnover ratios were highest  for G_.  mucronatus
(24.5) and lowest for the snail B. varium  (3.2).
                                      96
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                                  INTRODUCTION

     In addition to the spatial heterogeneity  provided  by  the  presence of
bu merged aquatic vegetation the  most notable  feature of these  habitats is  the
high density of animals residing  in the grass  bed.  This large  standing crop
jf animals is thought to be fundamental to  the resource value  of  submerged
aquatic vegetation beds to migratory predators (crabs,  fish, waterfowl) that
utilize the beds for protection and as a  feeding ground.   However,  one does
not get an appreciation for the flow of energy needed to support  the  large
populations of prey and predatory species by simply looking at  the  structural
complexity of the populations at  any given  time.  The amount of energy or
biomass produced within the grass bed system can only be estimated  by a
detailed look at the secondary production of the individual species in the
beds.

     From our analysis of the feeding habits of the higher level  consumers
(fish and crabs) it is obvious that benthic invertebrates  play  a  major role in
the flux of energy through the seagrass system.  The benthos then represent
the major link between primary production,  detritus, and higher trophic
levels.  Secondary production estimation  is a  very labor intensive  process  so
we chose to focus on nine top tophically  important benthic species  to higher
level consumers.

                                    METHODS

     Twelve consecutive monthly samples were taken for  secondary  production
using a suction dredge (Fig. 1).  Quantitative samples  were collected from
within a weighted plexiglass cylinder with  a diameter of 28.6  cm  (0.065 m^)
and a height of 65 cm.  The cylinder was carefully placed  over  the  grass
blades and the sample was taken from within by filtering water  through a
plastic bag with a removable 0.5 mm uesh sieve bottom.  Samples of  larger,
more motile, or widely spaced species were  collected from  within  a  weighted
fiberglass cylinder 110 era in diameter (0.95 m ) and 30 cm high equipped with
a 0.5 mm mesh screened top (Fig.  1).  All samples from  the larger fiberglass
frame were filtered through a 1 mm x 1.5 mm mesh bag.   The sampling frame was
dropped from a boat over dense vegetation.  Only drops  over 100%  vegetation
cover were sampled.  The majority of samples were taken from mixed
Zostera-Ruppia areas.  All samples were preserved in 10% buffered formalin.
Samples were sorted in the laboratory and up to 200 complete individuals for
each species per sample date were measured, dried and weighed.  Based on their
trophic importance to higher level consumers nine species  were  selected for
production estimates:
                                      97
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                   98
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    DECAPODS                                       ISOPODS

Callinectes sapidus                      Erichponella  attenuata
Crangon septemspinosa                    Edotea  criloba
Palaemonetes vulgaris                    Idotea  balthica

    MOLLUSCS                                     AMPHIPODS

Bittium varium                           Gammarus  mucronatus
Gemma gemma

     Length-weight equations calculated  for  each species were  based  on dry
weights (Table 1).  Ash  free dry weights (AFDW)  were obtained  by ashing known
quantities of each organism in a muffle  furnace  at  500°F for  3 hours.
Production for each species was determined with  the size-frequency method
(Hamilton 1969, Waters and Hokenstrom  1980,  Hynes  1980) Multiplied by  the
factor 365/CPI, where CPI is the cohort  production  interval or maturation
period (Benke 1979).  The instantaneous  growth method  (Waters  1977)  was also
used to calculate production of Callinectes  sapidus and Bittium varium, for
which cohorts were distinct.

     Measurement data were partitioned into  6-12 size  classes  for each species
and densities converted  to individuals/in^-.   With these data and. length-weight
equations (Table 1), mean size and weight were determined  for  each month and
size class (Tables 2-10  and Fig. 2-4).

     Species abundance data were analyzed graphically  to determine population
trends and sample variation.  Population size-frequency distributions  of
replicate samples were tested with the non-parametric  G-test  (Sokal  and Rholf
1969).

                     LIFE HISTORIES OF PRODUCTION  SPECIES

     Marine isopods are  generally considered  to  be  oraniverous,  scavenging a
variety of plant and animal matter (Shultz 1969).   Reproduction is by  direct
development of young within the marsupium of  the female.   The  three  isopods
discussed here all occur within the grass bed as epifauna  or epibenthos.
Edotea triloba, common in eelgrass, is also  associated with mud substrates,  i
It is distributed from Virginia to Maine.  j£. triloba  life spans  seems to be;
about six months with peak recruitment from  June to October.   After  spawning
in the spring and early  summer larger  individuals disappear from  the
population (Fig. 2).  Erichsonella attenuata  has been  reported  from  coral
habitats as well as eelgrass.  Its range extends from  Connecticut to /Jorth
Carolina (Shultz 1969).  Ovigerous E_. attenuata  were present  in the  grassbed
from May through January, peaking in July through September.   Juvenile
recruitment occurred from June to March.  Idotea balthica  has  an
amphi-Atlantic distribution occurring  in the  western Atlantic  from the Gulf  of
St. Lawrence to South America.  Strong and Daborn  (1979) indicate that life
span is slightly longer  than one year  in Canadian populations  with juvenile
recruiLment in July.  Ovigerous females were  collected i.n  the  grassbed from
May to November with juvenile recruitment occurring throughout  this  period and
into December (Fig. 2).
                                      99
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TABLE 1.  LENGTH-WEIGHT REGRESSIONS WHERE W IS THE PREDICTED DRY WEIGHT OF
          AN INDIVIDUAL WHOSE LENGTH MEASUREMENT IS L.
     Species
  Equation
                   Length
                 measurement
                      L
P. vulgaris

C^. septemspinosa

£. sapidus

J±- trilqba

E. attenuata
I. balthica
G. mucronatus
B. varium
G. gemma
w = 0.5880 1
                                2.53
w = 0.5999 1
            2.41
w = 0.0643 1
            2.74
w = 0.0070 1
            2.87
w = 0.0066 1
            2.41
w = 0.0137 1
                                2.17
w = 0.1272 1
                                3.00
w = 0.0372 1
                                1.78
w = 0.1039 1
                                1.56
43    0.92    carapace length

42    0.89    carapace length

71    0.96    carapace width

52    0.84    head to telscn

91    0.90    head to telson

42    0.94    head to telson

72    0.96    head plus 1st abdominal
              3 segments

35    0.90    shell length

13    0.91    shell height
                                       100
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                                   40  80  120 160

                                  LENGTH MEASURE (mm)
Fig. 2.  Monthly size-frequency data for Edotea  triloba, Idotea balthica
         and  Erichsonella attenuata.
                                       110
-------
     Gammarus mucronatus is  a  shallow water  amphipod distributed  froip the  Gulf
of St. Lawrence to the Gulf  of Mexico (Bousfield  1972).   It  is  eurytopic and
can be found associated with algae,  fouling  communities,  Spartina marshes  (Van
Maren 1978, Borowsky 1980),  and eelgrass  (Marsh 1973).   It has  a  generalized
diet consisting of both macro- and microphytes (Zimmerman et al.  1979).
Development of juveniles is  diract and occurs within the  marsupium of the
female.  Ovigerous females were collected  and juvenile  recruitment occurred
throughout the year.  From June to October greater  than 10%  of  the total
population was ovigercus (range 11-232) with less than  10% of the population
ovigerous in all other months  (range 3-8%).  Based  on  laboratory  study life
span is estimated to be approximately four months.

     The filter feeding bivalve, Gemma gemma is generally found in well  sorted
fine sand.  It is distributed  from Nova Scotia to Texas  (Abbott 1974).
Maximum lifespan is approximately two years.  Reproduction is ovoviviparous
(young are brooded).  Sellmer  (1967) determined the mean  size of  juvenile
length at release to be 410  u.  New  England  and mid-Atlantic populations
release juveniles from June  to August (Sellmer 1967, Green and  Hobson 1970).
However, the growth rates for  the two areas  are quite different.   One year old
individuals from New England were estimated  to be 1.6 mm  (Green and Hobson
1970) and 4.1 mm long along  the mid-Atlantic coast  (Sellmer  1967).  Continuous
reproduction and rapid growth  of G_.  gemma  at the Vaucluse Shores  site clouded
cohort separation (Fig. 3).  The actual growth rate or  life  span  of (». gemma
would not be determined.  The  model  size  throughout the year was  1.25 to
1.75 mm with no individual ever being larger than 3.25  mm.

     The prosobranch gastropod Bittium varium inhabits  eelgrass beds from
Chesapeake Bay south to Florida. Texas, and  the West Indies  (Wulff 1970).  It
is thought to be primarily a detritivore  or  algivore.   Marsh (1976) observed
egg masses present on grass  blades in May  and June  with recruitment beginning
in late June and lasting into  the fall.   Lifespan is approximately one year,
the newly recruited population overwintering close  to the bases of Zostera
marina turions or within the sediment (Wulff 1970,  Marsh  1976).   Recruitment
was observed in the fall, from October to  December  (Fig.  3).

     The grass shrimp Palaemonetes yulgaris  is distributed in shallow coastal
and estuarine waters from Massachusetts to Texas (Williams 1965).   Population
migration from Zostera marina  beds to sandy-shell bottoms has been reported
(Thorp 1976).  Palaemonetes vulgaris is an opportunistic  omnivore often
consuming large quantities of  detritus, digesting the associated  bacteria,
fungi and protozoans (Adams and Angelovic  1970).

     Knowlton and Williams (1970) reported ovigerous females  occurring from
February to October in North Carolina populations with  recruitment of
juveniles beginning in June.   These  new recruits grow quickly and  may spawn in
the late summer.  The Vaucluse Shore populations have a slightly  less
extensive breeding season from June  to September with recruitment  occurring
from July to September (Fig. 4).

     The decapod Crangon septemspinosa has a shallow water (0-35  m)
subarctic-boreal distribution.  It occurs  on the North American Atlantic Coast
from Baffin Bay to eastern Florida (Williams 1965).  C_. septemspinosa is
                                      111
-------
Fig. .3.  Monthly si^e-frequency data for Gammarus mucronatus,  Getama  gemma
         and jittium varium.
                                      112
-------
                                                                                                  I
              P vulgartt
           19 29 J9 49
                                                              C. tapidut
                                 19 29 35 49 59 «9 79 69 99 109
                                  LENGTH MEASURE  Imm)
Fig. 4.  Monthly size-frequency  data for Palaemonetes  vulgaris, Crangon
         septemspinosa and Callinectes sapidus.
                                       113
-------
eurytopic  inhabiting  a  general  salinity  range  of 10-31%.   It  undergoes
migrations  to deeper  water  in response  to  low  temperatures (Haefner 1979).   C_.
septemspinosa is a nocturnal  feeder  with an  opportunistic  feeding strategy.
It can be  a detritivore scavenger  or predator.   Predation  on  mysids accounts
for large  portions of the diet  for some  populations  (Welsh 1970).

     Regionally, £. septemspinosa  seems  to have  a single extended breeding
period from October to  June with a March-April peak  (Haefner  1979).  Eggs are
brooded by  the  females  for  a  short period  after  which development of
planktonic  larvae occurs.   Larvae  are present  in the plankton throughout the
year with  a mid winter  to early spring maximum.   Lifespan  has been estimated
at 2.5 years from studies of more  northerly  populations  (Haefner 1979).   In
the grassbed ovigerous  females  were  observed only from March  to June followed
by recruitment  in June  through  August.   Much of  the  winter population decline
may be due  to migration to  deeper  channel  areas  (Fig. 4).

     Callinectes sapidus is distributed  in shallow coastal and estuarine
waters (0-35 m) from  Nova Scotia south  to  northern Argentina  (Williams 1974).
The species is  euryhaline occurring  in  fresh water to hyperhaline lagoons.
Trophically, £. sapidus can best be  described  as an  opportunistic omnivore.
Life span  is up to 3.5  years.   Recruitment of  juveniles begins in August and
continues  into  the fall months.  Mating  occurs froji  May  to October with
spawning delayed two  to nine months  and  occurring from May to September  of  the
following  year  (Van Engel 1958).   Spawning activity  is concentrated near the
mouths of  estuarine and shallow coastal  waters (Williams 1974).  Recruitment
at the grassbed occurred in September and  continued  through November (Fig.  4).

                            RESULTS  AND  DISCUSSION

     Abundance  data were analyzed  by determining median  and semi-interquartile
range for  replicate samples (Fig.  5).  The distribution of most species  was
typically  patchy.  Most of  the  species exhibited one major annual population
maximum.  £. vulgaris,  C^. septemspinosa, £.  mucronatus and _I. balthica showed
the least month to month variability.  The other 5 species fluctuated widely
from month  to month.  This  type of variation is  indicative of spatial
heterogeneity that may  be related  to patchiness  of populations or migration.

     Analysis of size-frequency distributions  of replicate samples was done
for _E_. triloba  and (5. mucronatus.  These two species were  chosen because it
was thought that microhabitat parameters could have  greater influence on the
population structure  of species with relatively  short maturation times and
extended breeding periods.  Significant  differences  existed in four of eight
months for E^. triloba and in two of  seven  months for (5. mucronatus (Table 11).
To further  identify where variation  in size  frequency distributions was
occurring samples from  pure stands of Zostera  and Ruppia were eliminated
leaving only samples  from the mixed  grassbed habitat. Results indicated that
much of the variation in size'frequency  distribution was related to vegetation
type.  All months that  were different when all habitats were  combined were  now
not different.  However, there were  still  two months for each species where
the replicates  from the mixed grassbed were  different.  These differences were
due mainly to variation in  the number of smaller size class individuals
between replicates.
                                      114
-------
-------
TABLE 11.  RESULTS OF G-TEST COMPARISONS.  *p^.0.05, *p<..01
      All Replicates includes Zostera, Ruppia, and mixed grassbed samples
        G. mucronatus
E. triloba
G. mucronatus
                                                                   E. triloba
— 	 — 	 — 	 — 	
Apr
May
Jun
Jul
Aug
Sep
*
**
*
*
n.s.
n.s.
n.s.
n.s.
n.s.
*
—
**
Oct
Nov
Dec
Jan
Feb
Mar
n.s.
n.s.
—
—
—
~~
^_
—
—
—
—
"•—•
                         Only mixed grassbed samples
        G. mucronatus
E. triloba
G. mucronatus
E. triloba

Apr
May
Jun
Jul
Aug
Sep
N.S.
*
**
n.s.
n.s.
n.s.
n.s.
n.s.
n.s.
- Not significant
- pj<0.05
- n<0.m
n.s.
n.s.
n.s.
n.s.
**
n.s.



Oct
Nov
Dec
Jan
Feb
Mar



n.s.
n.s.
n.s.
*
**
n.s.



n.s.
**
n.s.
n.s.
n.s.
n.s.



     - Only samples from mixed grassbed habitat were collected.
                                      116
-------
     Production estimates  using  the  size-frequency method for the nine species
are based on  combined  data from  all  three habitats (Table 12).   Secondary
production by E^. attenuata was  found to  be greatest,  amounting  to 43% of the
total production for the nine  species.   The next  two  high ranking producers,
C. sapidus and G. mucronatus,  when combined with  E. attenuata accounted for
84.8% of the  biomass produced  by these nine species.   The contributions by j^.
attenuata and (5. mucronatus can  be attributed  to  their relatively high
turnover rates (P/B),  18.9 and 24.5  respectively.   While £.  sapiuus P/B ratio
was low its standing stock was high  (Table 12).

     Since C_, sapidus  could be traced as a single  cohort production was also
estimated by  the instantaneous growth method,  which yields 15.8 g-.m~^-yr~^.
C. sapidus would then  account  for 33% of the production, as  opposed to 22%
based on the  size-frequency method,  and  the top  three species would account
for 87%.  The difference between these two estimates  illustrates the
difference that can occur  by the application of different methods.   The
results for B. varium  show that  the  instantaneous  growth method (0.2
g-m~~^ •yr*) does not necessarily produce larger estimates than  the
size-frequency method, as  in this case there is close agreement.  In the case
of C_. sapidus only production  for a  small portion  of  its life span  was
calculated, which lead to  the  discrepancy in values.

     Four species that were moderately abundant in the community but were not
used for secondary production  estimates  are the amphipods Aiapithoe  longimana,
Cymadusa c ompta, Microprotopus raneyi and the  gastropod Astyris lunata.,  For
these species and the  nine production species  wet  weight bioraass was
determined from the routine baseline epifaunal and infaunal  samples.  Wet
weight biomass was converted to  dry  weight biomass using a conversion factor
of 0.17 (Waters 1977)  (Table 13).  It can be noted that for  the secondary
production species there is at times disagreement  between the mean  dry weights
obtained from ire secjniary production data and that  from the routine data.
This can probably be attributed  to the patchiness  of  distributions  and the
different amounts of information contained in  each mean.  Tha routine baseline
biomass data  were next converted to  production values by multiplying the P/B
ratios determined from this study.   For  the amphipods the P/B ratio of <3.
mucronatus was us'ed and for A^. lunata the ratio from  JJ.  varium  (Table 10).

     The total production  based  on routine sampling was 18 g-m~2.yr-l
estimated from the monthly samples excluding the decapods.  The difference is
due to the quarterly sampling not being  able to discern turnover shorter than
3 months.  All the amphipods and isopods turnover  at  much shorter intervals,
on the order  of weeks  in the summer.  Quarterly sampling does not accurately
estimate biomass and tends to yield  conservative production  values  (compare
tables 12 and 13).   For Z_.  varium which  has a  lower turnover rate quarterly
samples do accurately estimate production.   For (J.  gemma mean biomass was
twice as high in the quarterly samples yielding twice the production of
monthly samples.  G_. gemma was. simply much more abundant in  the quarterly
samples.  Its higher turnover rate (Table 12)  and  irruptive  populations  would
tend to indicate that both the quarterly and monthly  samples underestimated
its population.
                                       117
-------
TABLE 12.  PRODUCTION ESTIMATES BASED ON THE SIZE FREQUENCY METHOD (HYNES
           1980).  P = PRODUCTION IN GRAMS DRY WT; AFDW P = PRODUCTION IN
           ASH FREE DRY WEIGHT; P/B = TURNOVER RATIO; B = MEAN BIOMASS;
           CPI = COHORT PRODUCTION INTERVAL.

P.
c.
c.
E.
E.
I.
G.
B.
G.
Species
vulgaris
septemspinosa
sapidus
triloba
attenuata
balthica
raucronatus
varium
gemma
P
g.m~?yr-l
1
0
8
2
17
1
8
0
0
.8
.5
.9
.0
.6
.0
.0
.2
.7
% Total
P
4.
1.
21.
4.
43.
2.
19.
0.
1.
4
2
9
9
2
4
7
5
7
AFDW P
g.m~2yr-l
1.
0.
5.
1.
14.
0.
6.

-
54
43
11
31
86
80
89


g
0
0
1
0
0
0
"0
.0
0
B
.m-2
.520
.119
.908
.175
.938
.116
.327
.062
.125
N
P/B
3.4
4.4
4.7
11.3
18.9
8.5
24.5
3.2
5.9
CPI
days
365
365
365
182
91
203
91
365
182
                      40.7 g.nT2yr
                                     118
-------
TABLE 13.  DRY WEIGHT BIOMASS DETERMINATIONS (G.M~2) FROM THE BASELINE
           PORTION OF THE SAV STUDY.  THE PRODUCTION WAS DETERMINED FROM
           P/B RATIOS.
Species
E. triloba
I. balthica
E. attenuata
G. mucronatus
A. longimana
C. compta
M. raneyi
B. varium
A. lunata
G. gemma
4/79
0
0
0
0
0
0
0
0
0
0
.18
.02
.23
.37
.01
.01
.04
.09
.05
.15
6/79
0.14
0.05
0.28
0.15
0.00
0.03
0.04
0.15
0.13
0.51
9/79
0
0
0
0
0
0
0
0
0
0
.11
.08
.48
.05
.01
.12
.02
.02
.01
.26
11/79
0
0
0
0
0
0
0
0
0
0
.13
.12
.74
.05
.02
.13
.03
.03
.06
.16
Mean
0
0
0
0
0
0
0
0
0
0
.13
.07
.42
.15
.01
.05
.03
.07
.09
.26
Total
Production
1.
0.
7.
3.
0.
1.
0.
0.
0.
1.
18.
5
6
9
7
2
2
7
2
5
5
0
                                    119
-------
                                                                   — *?   — 1
     The nine species examined  produced  a total  of  40.7  g dry wfm~'iyr-
which is a high production value.  While there are  no other grass bed studies
for comparison these nine species were  found  to  be  more  productive then most
community production studies  from both  freshwater and marine systems (Waters
1979).  To put this high rate of production  into perspective we project the
40.7 g-~2.yr-l to the entire  140 hectare grass bed.,  making the basic
assumption that production of the nine  species would  be  uniform over the
entire bed.  A total of 53 metric tons  of dry tissue  was then produced and
potentially available for consumption by other trophic levels,  which could be
either higher level consumers or lower  level  decomposers.   This represents
approximately 60,000,000,000  individuals  which are  born, grow,  and die in  the
grass bed each year (Table 14).  Our estimates are  most  likely conservative
since they represent only 9 of  well over 75  trophically  or numerically
dominant species in the grass bed system.  Also  our monthly sampling program
tended to underestimate production of those  species (amphipods  and isopods in
particular) which turned over in less than a  month.   Gammarus mucronatus,  as
an example, in the lab grew to  reproductive  size in less than two weeks from
marsupial release at 17°C.  In  the field under more optimal growing conditions
growth may be even faster and turnover higher.

     The importance of this high secondary productivity  to fish and crab
predators is intuitive, but the portion  of the production  which goes to
predators is unknown.  On average there  are 4.6  metric tons of standing stock
over the year in the grass bed.  This leaves  48.4 metric ^ns to be accounted
for.
                                      120
-------
Table 14.  PRODUCTION ESTIMATES PROJECTED FOR THE ENTIRE VAUCLUSE SHORES
           GRASS BED (AREA = 140 HECTARES).
Biomass
Produced/Year
G.
G.
E.
I.
P.
C.
£•
B.
G.
mucronatus
triloba
attenuate
balthica
vulgaris
septemspinosa
sapldus
varium
gemma
11
2
24
1
2
0
12
0
0
.2
.8
.6
.4
.5
.74
.5
.28
.98
x 103 Kg
x 103
x 103
x 103
x 103
x 103
x 103
x 103
x 10 3
# of Individuals
Produced/Year
2
1
2
1
8
2
1
9
4
.0
.1
.7
.1
.1
.3
.5
.5
.5
xlO10
xlO10
xlO10
x 109
x 107
x 108
x 107
x 108
x 109
                   Total        53.0  x 103 Kg               6.1  x
                                     121
-------
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Abbott, R. T.  1974.  American Seashells.  Van Nostrand Reinhold Co., N. Y.
      663 p.

Adams, S. M. and J. W. Angelovic.  1970.  Assimilation of detritus and its
     associated bacteria by  three species of estuarine animaJ".  Ches. Sci.
     11:249-254.

Benke, A. C.  1979.  A modification of  the Hynes method for estimating
     secondary production with particular significance for raultivoltine
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Borowsky, B.  1980,  Reproductive patterns of three intertidal salt-marsh
     gammaridean amphipods.  Mar. Biol. 55:327-334.

Bousfield, E. L.  1972.  Shallow-Water  Gammaridean Amphipoda of New England.
     Vail-Ballou Press, Inc.  312 p.

Green, R. H. and K. D. Hobson.   1970.   Spatial and temporal structure in a
     temperate intsrtidal community, with special emphasis on Gemma gemma
     (Pelecypoda: Mollusca).  Ecology 51:999-1011.

Haefner, P. A.  1979.  Comprehensive review of the biology of North American
     caridean shrimps with empuasis on  Crangon septemspionosa.  Bull. Biol.
     Soc. Wash. 3:1-40.

Hamilton, A. L.  1969.  On estimating annual production.  Limnol. Oceanogr.
     14:771-782.

Hynes, H. B. N.  1980.  A name change in the secondary production business.
     Limnol. Oceanogr. 25:778.

Knowlton, R. E. and A. B. Williams.  1970.  The life history of Palaemonetes
     vulgaris (Say) and P. pugio Holthuis in coastal North Carolina.  J.
     Elisha Mitchell Scient. Soc. 86:185.

Marsh, G. A.  1973.  The Zostera epifaunal community in the York River,
     Virginia.  Ches. Sci. 14:87-97.

Marsh, G. A.  1976.  Ecology of the gastropod epifauna of eelgrass in a
     Virginia estuary.  Ches. Sci. 17:182-187.

Shults,  G. A.  1969.  How to Know the Marine Isopod Crustceans.  Wm. C. Brown
     Co., Dubuque, Iowa.  369 p.
                                      122
-------
Selimer, G. P.  1967.  functional morphology and ecological life history of
     the gem clam, Gemma gemma (Eulamellibranchia: Veneridae).  Malacologia
     5:137-223.

Sokal, R. R. and F. J. Rohlf.  1969.  Biometry.  W. H. Freeman and Co., San
     Francisco.  776 p.

Strong, K. W. and G. R. Daborn.  1979.  Growth and energy utilisation of the
     intertidal isopod Idotea baltica (Pallas) (Crustacea: Isopoda).  J. exp.
     mr. Biol. Ecol. 41:101-123^

Thorp, J. H.  1976.  Interference competition as a mechanism of coexistence
     between two sympatric species of the grass shrimp Palaemonetes (Decapoda:
     Palaeraonidae).  J. exp. mar. Biol. Ecol. 25:19-35.

Van Engel, W. A.  1958.  The blue crab and its fishery in Chesapeake Bay.
     Part 1 - Reproduction, early development, growth and migration.  Comm.
    Fish. Rev. 20:6-17.

Van Maren, M. J.  1978.  Distribution and ecology of Gatnmarus tigrinus Sexton.
     1939 and some other amphipod Crustacea near Beaufort (North Carolina,
     U.S.A.).  Bijdragen Tot de Dierkunde 48:45-58.

Waters, T. F.  1977.  Secondary production in inland waters.  Adv. Ecol. Res.
     10:91-164.

Waters, T. F.  1979.  Influence of benthos life history upon the estimation of
     secondary production.  J. Fish. Res. Board Can. 36:1425-1430.

Waters, T. F. and J. C. Hokenstrom.  1980.  Annual production and drift of the
     stream amphipod Gammaras pseudolimnaeus in Valley Creek, Minnesota.
     Liranol. Oceanogr. 25:7W-710.

Welsh, B. L.  1970. Some aspects of the. vertical migration and predatory
     behavior of the sand shrimp, Crangon septemspinosa.  M.S. Thesis, Univ.
     of Maryland.   66 p.

Williams, A. B.  1965.  Marine decapod crustaceans of the Carolinas.  Fish.
     Bull. 65:1-298.

Williams, A. B.  1974.  The swimming crabs of the genus Callinectes (Decapoda:
     Portunidae).   Fish. Bull. 72:685-798.

Wulff, E. M. T.  1970.  Taxonomy and distribution of westrn Atlantic Bittium
     (Gastropoda:  Mesogastropoda).   M.S. Thesis, The College of William and
     Mary, Va.  58 p.

Zimmerman, R., R.  Gibson and J. Harrington.  1979.  Herbivory and detritivory
     among gammaridean amphipods from a Florida seagrass community.  Mar.
     Biol. 54:41-47.
                                      123
-------
                                 CHAPTER 5

                          PRELIMINARY STUDIES OF
             GRAZING BY BITTIUM VARIUM ON EELGRASS PERIPHYTON1

                                    by

                           Jacques van Montfrans

                              Robert J. Orth

                                    and

                             Stephanie A. Vay
Accepted for publication in Aquatic Botany, Contribution No. 1036 from the
Virgini : Institute of Marine Science.
                                    124
-------
                                   ABSTRACT                                                ]

     The grazing activities of Bittium varium Pfeiffer on periphyton                       >
colonizing live eelgrass (Zostera marina L.) and artificial eelgrass                       ' \
(polypropylene ribbon) were investigated.  Quantitative measurements of                    <;
grazing impact on artificial substrates were determined by periphyton pigment              !.
extraction and dry weight differences between grazed and ungrazed blades.                  ,'
Significant differences occurred in phaeophytin and dry weight calculations
but chlorophyll _a measurements were not significantly different.  This
suggests that senescent diatoms constituted the bulk of the periphyton weight                 :
and were selectively removed over more actively photosynthesizing diatoms.                 j   i-'
                                                                                           ;   \
     An examinacion of scanning electron micrographs further elucidated the                .   ;
impact of grazing by Bittium varium.  Some micrographs revealed  that B_. varium             ;
removed primarily the upper layer of the periphyton crust on both artificial
substrates and living Zostera marina.  The diatom Cocconeis scutellum Ehrenb.              •   '
which attaches firmly to living Z_. marina blades was less commonly removed
than Nitzschia or Amphora.  Through its grazing activities, _B. varium may
maintain community dominance by tightly adhering diatoms such as C. scutellum.
Evidence of the complete removal of periphyton exposing the Z_. marina
epithelium, was revealed in other micrographs.

     The grazing activities of Bittium varium which removes periphyton from
seagrass blades, could have important implications for the distribution and
abundance of Zostera marina in tho Chesapeake Bay.
                                      125
                                                                                          I!
-------
                                  INTRODUCTION

      The term grazing is generally used in terrestrial and aquatic ecological
 studies  to indicate herbivory.   Grazing food webs in terrestrial ecosystems
 are more easily studied and have historically received greater attention than
 those in the aquatic realm (Crisp,  1964).   However, in recent years, an
 emphasis has been placed on understanding buch relationships in both
 freshwater and marine habitats.

      Primary producers in aquatic systems  include planktonic and benthic
 microalgae (e.g.  diatoms), macroscopic algae, and both submerged as well as
 emergent macrophytes.   The majority of energy fixed by the latter two groups
 is  generally not  directly utilized  by invertebrate herbivores and enters the
 food chain via a  detrital pathway (Teal,  1962;  Harrison and Mann, 1975;
 Tenore,  1975;  Tenore et al.,  1977).  Phytoplankton and benthic raicroalgae
 constitute the major sources  of  energy fixed by primary producers that is
 available  for  direct transfer to higher trophic levels (Steele, 1974).

      Many  of the  original investigations  on aquatic grazing dealt with
 microplanktonic crustaceans  and  their influence on phytoplankton populations
 (Marshall  and  Orr,  1955;  Porter,  1977).   Similar studies  on the effects  of
 benthic  grazers have received more  recent  attention.

      Grazing studies  of  benthic  aquatic herbivores  emphasize  their  impact on
 both  macro-  and microscopic plant communities.   Herbivorous fishes  (Stephenson
 and Searles, 1960;  Randall, 1965; Earle, 1972;  John and Pope,  1973),  sea
 urchins  (Margalef and Rivero, 1958; Leighton  et al.,  1965;  Jones  and  Kain,
 1967; Paine and Vadas,  1969;  Breen  and Mann,  1976)  and  gastropod  mollusks
 (Randall,  1964; Southward, 1964; Kain and  Svedsen,  1969;  Dayton,  1971; Lein,
 1980) have a dramatic effect  on biomass and species composition of macroalgae
 or marine vascular  plants.

     Herbivore-plant interactions involving microalgal  communities are
 somewhat less well  studied (Nicotri,  1977).   The nutritional  importance  of
benthic marine diatoms  in  the diet  of  the  marsh  snail,  Ilyanassa  obsoleta, has
 only recently been  established (Wetzel, 1977).  Castenholz  (1961) demonstrated
 the drastic reduction in biomass of intertidal  epilithic  diatoms  in the
 presence of the grazing mollusks, Littorina scultata  and  Acmaea spp.  A
 similar result was  demonstrated and selective removal of  both  the outermost
 portion of the diatom mat  and'the more  loosely  adhering species was shown to
 occur when four intertidal gastropod  species  grazed on  diatom  colonized
 artificial substrates (Nicotri, 1977).. In contrast,  studies by Kitting  (1980)
 showed that-eve;', under high experimental grazer  (Acmaea scutum, a gastropod)
 densities, the principal  algal species  preyed upon  did  not  detectably
 decrease.  Algal declines  were instead  attributed  to  physical  factors.
                                      126
-------
                             METHODS AND MATERIALS

Field Work

     Both live Zostera marina and  diatom colonized  polypropylene ribbon which
resembled the living plants  in  size but was  slightly paler in color were used
in the grazing experiments.  Living blades,  collected from a Guinea Marsh Z_.
marina bed at the mouth of  tho  York River  on the  southwestern shore of the
Chesapeake Bay, were held for not  more than  two days in  large wooden
flow-through tanks before being used.  The polypropylene strips were suspended
for 30 days from a pier at  the  mouth of the  York  River to condition the blades
before use.  The artificial  plants consisting of  six 45  cm strips of ribbon
tied to a wire staple were  then wiped clean  and anchored to the sediment in
the Guinea Marsh seagrass bed to be colonized.  Thirty days later the
artificial grass was collected  for immediate use  in grazing experiments and
scanning electron microscopy (SEM).

     Bittium varium were collected from a  Zostera marina bed at Vaucluse
Shores on the southeastern  side of the Bay.   Snails were acclimated for two
days in aquaria with York River water (23  °/oo salinity; 22°C) and live
periphyton covered Z_. marina prior to experimental  use.

Laboratory Experiments

     Periphyton-colonized artificial grass blades were used ro quantitatively
assess the effects of grazing.  The blades that were used were initially
selected based on the extent of periphyton coverage.  Twenty-six blades with
the densest growth were cut  into 12 cm lengths.   One end of each blade was
then wedged against the inside  of  a 1/2 dram glass  vial  using a cheesecloth
plug while the other end was tied  to a piece of monofilament thread.   Care was
taken to keep blades moist  at all  times and  not to  disrupt the periphyton
layer.  The blades with vials were then suspended in two 40 liter aquaria
filled with millipore filtered  seawater (23  °/oo  salinity).  Aquaria were
situated in a laboratory window with an eastern exposure and experiments were
conducted under ambient light conditions.  Temperatures  in the laboratory
fluctuated between 23° and  25°C.   Each aquarium was equipped with a corner
charcoal filter housing an  airstone.  Thirteen blades were randomly designated
as experimental blades using a  random numbers table and  forty Bittium varium
(x length = 2.1 mm) were placed in the vials of each.  Periodic counts of the
snails which crawled onto the blades were  used as an Indication of grazing
pressure.  When the experiment  was terminated after 96 hours,  the middle 8 cm
portion of each blade (3.2  cm^  total surface area per blade) was removed for
analysis.  Twelve experimental  and twelve  control blades were  used for pigment
analyses and gravimetric calculations of periphyton biomass while the
remaining two blades (one experimental and o:.e control)  were prepared for SEM
examination.  Results of the pigment extraction and cry  weight calculations
were statistically analyzed  using  a Wilcoxin two-sample  test (Sokal  and Rohlf,
1969).

     Chlorophyll a_ and chlorophyll degradation products  (phaeopigments) were
extracted from each blade and analyzed by  a  phase partition technique (Whitney
and Darley,  1979).  Three unused 8 cm long pieces of artificial grass were
                                       128
-------
similarly tre-.'ted and  the mean absorbance  values  used  as  correction  factors
for solubles present in the  polypropylene  ribbon.  Absorbance  readings were
measured using a Bausch and  Lomb Model  21  spectrophotometer.   After  pigment
extraction the periphyton adhering  to each blade  was  removed by  scraping  with
forceps, dried to a constant weight, and weighed  to the nearest  0.01  mg on a
Mettler balance (model H-51).

     A scanning electron microscope  (AMR-1000) was used to examine Bittium
varium fecal pellets, grazing trails, and  food selectivity on  periphyton
colonizing both living Zostera marina and  artificial  grass blades.
Micrographs were also used to assess the quality  and  composition of  the
periphytic communities colonizing these substrates.   When experiments were
terminated, blades and fecal pellets were  immediately fixed  in a 3%
gluteraldehyde, 0.1 M  sodium cacodylate buffer (pH 7.2) and  0.25 M_ sucrose
solution.  After two hours,  samples were rinsed three times  for  up to 30  min
each in a 0.1 M_ buffer solution containing 0.25 M_, 0.1 M_  and 0.0 M_ sucrose
solutions, respectively.  They were  then fixed in a 1% osmium  tetroxide and
0.1 M_ sodium cacodylate buffer (pH  7.2) for  2 to  24 hours, rinsed twice
thereafter in distilled water, and  dehydrated in  a graded series (25, 50,  75,
90, 95 and 100%) of ethanol  solutions.  After dehydration, samples were
critical point dried (Polaron Critical Point Drying System) with liquid CC>2,
mounted on stubs and coated with gold/palladium (40:60) in a high vacuum
evaporator.  They were then  examined and photographed  under  the  SEM.

                                    RESULTS

     Periphyton pigment analyses of grazed and ungrazed artificial grass
blades (Table 1) showed no significant differences .(p>0.05)  in chlorophyll a
concentrations whereas experimental blades exhibited  significantly (p<0.001)
lower phaeophytin a_ concentrations  than control blades.   Mean  chlorophyll  &_
concentrations of 0.1882 and 0.1287 mg/1 for grazed and ungrazed blades,
respectively, were lower than the corresponding phaeophytin a  values  of 0.8607
and 1.8436 mg/1.  Gravimetric differences  of attached  periphyton existed
between grazed and ungrazed blades  (significant at p<0.001) indicating a  mean
overall removal of 62.7% of  the periphyton from artificial blade surfaces
(Table 2).

     Scanning electron micrographs  (plates 1 to 24) enhanced the understanding
of feeding activities by Bittium varium.   Natural Zostera marina used in  this
study was coated with a thick layer of periphyton which spanned  the width  of
the older blades (plate 1).  At magnifications of 2200 and 4400X (plates  2 and
3) the complex nature of this crust is revealed.  It  consists  of at least
three genera of diatoms (Cocconeis scutellum, Nitzschia sp., Amphora  sp.),
blue-green algae,  bacteria and organic debris, some of which might be
degrading fecal pellets.  The crust i <= up  to 15 pm thick  with  as many as  3-4
layers of diatoms (plate 4).

     Bittium varium ingests  the periphyton crust  using a  taenioglossate radula
consisting of seven teeth in each transverse row.  The central tooth  is
flanked on each side by one  lateral and two similar slender marginal  teeth
(plate 5).  With the marginals laterally compressed,  the  radular band of  an
adult j^. varium is approximatly 30 pm wide.
                                      129
-------
TABLE 1.  PERIPHYTON PIGMENT ANALYSIS OF GRAZED AND UNGRAZED ARTIFICIAL  Z_.
          MARINA BLADES.  SIGNIFICANT DIFFERENCES WERE DETERMINED  BY  THE
          WILCOXON TWO-SAMPLE RANK TEST^  (a= 0.05).
Functional Chlorophyll A
Grazed
Overall 1
Rank 3
8.5
8.5
8.5
12.5
16.5
16.5
16.5
21.5
23
24
n =
X =
s .d. =
Cy(b) =
jj(c) =
-0.2514
0.0000
0.0965
0.0965
0.0965
0.1609
0.1931
0.1931
0.1931
0.2574
0.2896
0.9332
12
0.1882
0.2738
145%

(mg/1/blade) Phaeophyutin a (mg/1/blade)
Ungrazed
3
3
5
6
8.5
11
12.5
16.5
16.5
16.5
20
21.5




82 ns
0.0000
0.0000
0.0322
0.0o44
0.0965
0.1287
0.1609
0.1931
0.1931
0.1931
0.2252
0.2574
12
0.1287
0.0889
69%

Grazed
Overall 1
Rank 2
3
4
5
6
7
8
10
11
16
24





0.0840
0.0988
0.1355
0.1583
0.4180
0.4402
0.8186
0.8756
1.1848
1.4026
1.7637
2.9488
12
0.8607
0.8609
100%
125
Ungrazed
9
12
13
14
15
17
18
19
20
21
22
23




***
1.1600
1.5014
1.6053
1.6127
1.6423
1.8254
1.9072
2.0333
2.0977
2.1025
2.1816
2.4539
12
1.8436
0.3560
19*

(a)  ns = not significant at P = 0.05
   *** = significant at P<0.001
(b)  CV = coefficient of variation
(c)   U = Mann-Whitney U test statistic
                                        130
-------
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131
-------
                                                       Reproduced from
                                                       best available copy. %,s
Plate 1.  Live Zostera marina  showing complete  coverage by the periphyton
          crust (260X; size bar  = 100 pm).
                                      132
-------
Plate 2.  Detail of plate 1 showing organic debris and at least three
          species of diatoms including Cocconeis jscutellum, Amphora,sp.
          and possibly Nitzschia sp.  (2200X; size bar =  10 inn).
                                     133
-------
                                                       Reproduced (rom
                                                       best available copy.
Plate 3.  Part of a live  Zostera.marina blade that was completely  covered
          with pcriphyton reveaTing the complex nature of the crust.   The
          periphyton community included blue-green algae, diatoms, bacteria
          and organic  debris,  embedded in a mucous matrix (4AOOX;  size bar  =
          10 urn).
                                      134
-------
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135
                                                                                                  i
-------
                                                          Reproduced from
                                                          best available  copy.
Plate 5.  The radula of BiEtium varium showing its taenioglossate
          configuration with one central  tooth flanked by a lateral
          and two marginal teeth on  each  side.  Note that the central
          teeth are badly worn in  some cases.   The width of the radula
          is approximately 30pm with the  marginal teeth laterally
          compressed (2200X; size  bar = 10 ym).
                                    136
-------
     Grazing by Bittium variant on periphyton  of  live  Zostera  marina results  in
various effects (plates 6  to 8).  Some  feeding activities  remove  the upper
layer of periphyton crust, with little  damage to  the  underlying diatoms
(plates 6 to 9).  In most  cases, grazed  patches occur in which the  total
periphyton matrix is removed, leaving only  a  few  bacteria  attached  to the  Z_.
marina epithelium (plates  10 to 12).  Distinct grazing trails are evident  in
other micrographs (plates  13 to 15).  Individual  straight-run trails vary  in
width from approximately 19 to 29 urn (plates  13 and  14).   Impressions of
Cocconeis scute Hum in the T^. marina epithelium  indicate that this  tightly
adhering species is sometimes removed by J3. variuu  (plates 14 and 15).
However, examinations of fecal pellets  (plate 16) reveal that pennate diatOu.s
such as Nitzschia and Amphora are commonly  ingested,  suggesting that these
species are less tightly adhered to the  grass blades.   Closer examination  of
the diatom frustules observed in this fecal pellet  revealed the absence of
protoplasm that fills the  pores in live  diatoms which is evidence that they
are digested as well (plates 17 and 18).

     The periphyton assemblage found on  polypropylene ribbon  (plates 19 to 21)
is very different from that of living Zostera marina.   A hair-like  mat of
unidentified tubular material (possibly  diatom sheaths and bacterial
filaments) and organic debris covers the entire blade. The virtually complete
removal of the hair-like mat after grazing  (plate 22)  revealed patches of
pennate diatoms that were  evident under  higher magnification  (plates 23 and
24).  The considerable reduction in periphyton biomass as  a result  of grazing
by Bittium variura, evidenced in the gravimetric data  (Table 2), is  readily
apparent in micrographs of grazed artificial blades.

                                  DISCUSSION

     Grazing on periphyton of marine macrophytes nay  have  important
implications for the productivity, distribution and  abundance of  the colonized
plants.  Zostera marina has experienced  widespread  declines primarily on the
western shore and, to a lesser extent,  the  eastern  boundary of the  Chesapeake
Bay (Orth et al., 1979).  Reasons for the observed  declines are as  yet unclear
but factors such as climatological changes, herbicides, increased turbidity
and increased nutrient loading have been hypothesized. Epiphytic growth has
been cited as the major causative factor in similar historical declines of
freshwater macrophytes of eutrophic lakes (Phillips et al., 1978; Moss, 1979).
The macrophyte host is adversely affected by epiphytes since  the  latter absorb
much of the light that normally reaches  the plant surface  and also  act as  a
barrier to carbon uptake (Sand-Jensen,  1977).  Epiphytic fouling, if rapid and
severe enough, can eventually kill the host plant.

     Two mechanisms to control colonization on marine  plants  by epiphytes  have
evolved.  Many plants produce mucous or  periphyton  inhibiting substances
(Cariello and Zanetti, 1979; Zapata and  McMillan, 1979) while others,  such as
Zostera marina, vegetatively generate c-lean, actively  photosynthetic tissue at
a rapid rate while sloughing off fouled  blades (Sand-Jensen,  1977).   If
conditions such as nutrient enrichment  (abiotic) and/or the reduction or
elimination of grazers (biotic) enhance  periphytoa growth,  the formation of
new photosynthetic tissue may be too slow for the continued health  and
survival of the macrophyte.  Thus, the micrograzer  component  of seagrass
                                       137
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habitats may be essential  in maintaining viability of  the  seagrasses,
especially under nutrient-rich conditions.

     We utilized artificial substrates  to quantify the  impact  of  Bittium
varium grazing on periphyton.  However, the use of inert substrates  in
assessing the quantity and quality of periphyton  found  on  natural macrophytes
is a subject of much debate.  The importance of nutrient transfer and  other
metabolic interactions between macrophytes and their epiphytic colonizers  (see
Moss, 1981; Eminson and Moss, 1980 for  review) has been recognized for
freshwater systems.  External environmental factors determine  periphyton
community composition in marine systems (see Harlin, 1980  for  review).
However, a recent study (Eminson and Moss, 1980)  demonstrated  that greater
macrophyte host specificity by microalgae occurred in  oligotrophic lakes than
in lakes with moderate to high nutrient loading.  Under eutrophic conditions
host specificity was progressively less apparent  and peiiphyton communities
became similar between all submerged vascular plant species, despite
differences in macrophyte surface texture and metabolic activity.   It  is
possible that the distinct differences we observed in  periphyton  species
composition between polypropylene ribbon and live eelgrass was caused  by the
actively metabolizing, cobblestone textured leaves of  the  living  plants.

     We have demonstrated that grazing resulted in a significant  reduction of
periphyton biomass on polypropylene ribbon.  Since an  analysis of
phaeopigments (and not chlorophyll a) showed differences paralleling  the
gravimetric analyses, implications are  that the periphytic crust  utilized  by
Bittium varium in these experiments was predominantly  senescent.   Scanning
electron micrographs of the artificial blades revealed  that  the epiphytic
complement differed considerably from that found  on live Zostera  marina.
Grazing impacts of _B_. varium on periphyton of both substrates,  however,  are
comparable.  SEM photographic evidence of short term feeding activities
supports the fact that B. varium grazing reduces  or removes  the periphytic
community on natural blades.

     Bittium varium grazing sometimes affects the structure  of the diatom
population.  This is accomplished by the mechanically  selective removal  of all
loosely adhering species such as Amphora sp. and  Nitzschia sp.  which  inhabit
the upper portion of the periphyton crust.  Cocconeis  scutellum is able  to
firmly attach to the Zostera marina epithelium by a mucilagenous  secretion
thereby avoiding extensive grazing by J3. varium.  Thus, the  grazing activity
of J3. varium may facilitate community dominance by C^.  scutellum,  a species
which has been previously identified as the most  ecologically  and  numerically
important diatom on Z_. marina (Sieburth and Thomas, 1973; Jacobs  and Noten,
1980).

     The shading imposed by epiphytes on their seagrass host can  be severe
enough to either restrict their vertical distribution  (Caine,  1980) or cause
the complete disappearance of the macrophytes (Sand-Jensen,  1977;  Moss,  1981;
Eminson and Moss, 1980).  The removal of most periphyton from  artificial
blades by grazing and evidence of similar removal from  live Zostera marina
suggests that Bittium varium plays an important role in mediating  the
proliferation of epiphytic diatoms on these substrates.  The disruption  of the
periphyton grazer component (as is the case with  the elimination  of JK varium
                                       157
-------
from seagrass habitats along the western shore of the Chesapeake Bay)  could
seriously alter the steady state of periphyton abundance to  the detriment  of
the host plant.  The preliminary results of our work indicate  that  a more
detailed and quantitative study of the macrophyte-periphyton-raicrograzer
relationship is necessary to substantiate this hypothesis as a partial
explanation for demise of Z_. marina in certain areas of the  Bay.
                                      158
-------
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-------
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                                      162
-------
                             CHAPTER 6


          WATERFOWL UTILIZATION OF A SUBMERGED VEGETATION
               CZOSTERA MARINA AND RUPPIA MARITIMA) BED
                   IN THE LOWER CHESAPEAKE BAY*

                                by

                       Elizabeth W. Wilkins
A thesis presented to the Faculty of the School of Marine Science,
The College of William and Mary, Williamsburg, Virginia
                               163
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                            ACKNOWLEDGMENTS

     Special contributions of field assistance, moral support and
technical advice were made by Cary Peet, Deborah Penry, Tom Fredette,
and Anna Vascott.  Brian Meehan, Linda Schaffner, Marcia Bowen, Karl
Nilsen and Priscilla Hinde offered vital assistance and companionship
during waterfowl censuses.  Jacques van Montfrans was a supportive and
invaluable friend and consultant through all phases of the study.

     Waterfowl specimens were collected by Vernon Leitch, Buck Wright
and Curtis Jones of Northampton County, and Louise and Bart Theberge
of the Virginia Institute of Marine Science.  Rich DiGiulio, of
Virginia Polytechnic Institute, provided a number of specimens, as
well as valuable discussion and good company, while collecting in the
area for his own research.

     Martha and Vernon Leitch, Vaucluse Pt. residents, deserve special
thanks for their warmth and hospitality during the bitter cold months.
                                    164
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                               ABSTRACT

     A study of waterfowl use of a bed of submerged aquatic vegetation
was conducted over two winters in the Lower Chesapeake Bay (Virginia).
In the season of 1978-1979, Canada geese (Branta canadensis) were the
dominant waterfowl in the study area.  Goose foraging activity was
correlated with tide stage, and was greatest at low tide.  Consumption
by grazing waterfowl was calculated from bird densities, and was
approximately 25% of the standing crop of vegetation in the shallow
portion of the habitat.  In 1979-1980 diving ducks, primarily
buffleheads (Bucephala albeola), were dominant.  Abundance of
waterfowl was influenced by wind parameters, but tide, temperature and
time of day hlad little or no influence on bird numbers.
Within-habitat variation in abundance was examined, and highest
densities were associated with the deeper Zostera marina zone.

     Gizzard samples and 6*-*C analysis revealed that buffleheads fed
primarily on small gastropods and nereid worms characteristic of the
grassbed epifauna.  Consumption of important invertebrate prey items,
calculated from exclosure experiments and waterfowl densities,
amounted to nearly 502 of the fall standing crop of these species in
Zostera marina.
                                    165
                                                              	 a
                                                                                       ; J
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                             INTRODUCTION

     Submerged aquatic vegetation (SAV) is widely recognized as a
valuable food resource for wintering waterfowl populations (Bent 1923,
Cottam 1939, Stewart 1962, Bellrose 1976, Munro and Perry 1981).  The
demise of Zostera marina during the 1930"s was thought to cause the
precipitous decline of the Atlantic brant (Branta bernicla hrota)
(Cottam 1934, Addy and Aylward 1944, Cottam and Munro 1954), although                   \
coincidence of poor reproductive success may also have been important                   •
in reducing populations (Palmer 1976).  Numbers of waterfowl utilizing                  ]
the traditionally important Susquehanna Flats as a winter feeding                       \
ground in the Chesapeake Bay plummeted during the height of the                         j
eurasian water milfoil epidemic in the 1960s, but returned to former                    S
levels after native aquatics became re-established (Bayley et al.                       ;
1978).                                                                                  j
                                                                                         |
     Recent surveys indicate that submerged vegetation has declined in                   !
most areas of the Chesapeake Bay in the last 15 years (Bayley et al.                     '
1978, Anderson and Macoraber 1980, Orth and Moore 1981).  The response                    .;
by several waterfowl species has been to alter feeding habits or                         1
distribution patterns rather than sustain population losses (Munro and                   j
Perry 1981).  Canvasbacks (Aythya valisineria) once fed primarily on                     j
wild celery (Vallisneria americana), but since the early 1970's have                     !
fed mostly on bivalves (primarily Macoma balthica; Perry and Uhler                       i
1976).  Canada geese (Branta canadensis) and to a lesser extent                          3
whistling swans (Cygnus columbianus columbianus), now rely on                            <
agricultural grain as a major dietary component on the wintering                         I
grounds (Bellrose 1976).  Other species such as redheads (Aythya                         i
americana), wigeon (Anas americana) and pintails (An as a c u t a ), wh i c h                     j
indicate a continued preference for SAV, have declined in the Bay in                     j
recent years, and it is likely that their winter distribution now                        ;
coincides with areas of greater SAV abundance (Munro and Perry 1981).   ,                  j
                                                                        \                 '
     Past or current preference for submerged vegetation in the diet     '                 >
is well documented for the above species (Martin and Uhler 1951,         l                 j
Stewart 1962, Munro and Perry 1981).  With the exception of                               j
canvasbacks and redheads, all are non-divers, or dabblers, which feed                     j
in shallow water by tipping up rather than diving to obtain food.                         :
Many diving species also feed in SAV habitats on benthic                                  j
invertebrates.  Animal communities associated with grassbeds differ                       i
markedly from those in unvegetated areas, both in structural and                          !
functional aspects.  Submerged aquatic vegetation supports a dense and       •              •
diverse epifaunal assemblage not found on bare substrates (Marsh                           ,
1970), and organisms living on or within sediments are also more                           ,]
abundant due to greater sediment stability and microhabitat complexity                     }
(Thayer et al. 1975, Orth 1977).  Grassbeds should therefore attract                       ;
waterfowl which feed on invertebrates as well as those which rely on                       i
vegetation, although there is scant evidence to this effect.  Nilsson                      '
(1969) reported that in shallow water in the Oresund, Sweden, two                          ;
                                    166
                                                                                           J
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diving duck species studied fed preferentially over Zostera marina and
one fed over patchy Ruppia sp. and 2^. marina, whereas an intervening
belt of vegetation-free sand contained no fauna of trophic importance
for these species.

     In spite of the food resources available to waterfowl in SAV
habitats, Munro and Perry (1981) found few significant relationships
between the distribution and abundance of submerged vegetation and
waterfowl populations in the upper Chesapeake Bay.  Several species,
such as whistling swans, black ducks (Anas rubripes), mallards (Anas
platyrhynchos) and buffleheads (Bucephala albeola), showed positive
associations with SAV in certain areas, but results were not
consistent over all survey zones.  In the Lower Bay, tha current
relationship between waterfowl and SAV is largely unknown.  The
purpose of this research was to provide detailed information regarding
waterfowl use of a particular bed of submerged vegetation in the Lowar                 '  s
Bay.  Specific objectives were to examine short term patterns of
utilization, and to identify and estimate consumption of important
waterfowl foods within the study area.

     Waterfowl foraging studies have traditionally emphasized gizzard
analysis, but more recent research has sought to quat.cify consumption
in addition to describing food habits.  A common approach employs
average population estimates, theoretical daily ration based on body
weight, and knowledge of trophically important foods to arrive at
values for annual consumption.  These values may then be compared with
either standing crop or production of food items to determine grazing
or predation pressure.  In the saline Lake Grevelingen, The
Netherlands, Wolff et al. (1975) and Neinhuis and van lerland (1978)
reported that waterfowl consumed less than 1% of the annual production
of Zostera marina, whereas Jacobs et al. (1981) calculated that
consumption by waterfowl amounted to 50% of the standing crop of
Zostera noltii near Terschelling, The Netherlands.  Intermediate
values for grazing pressure have been obtained by other investigators
using similar methods (Sincock 1962, Steiglitz 1966, Cornelius 1977).
Another technique compares biomass samples taken before arrival and
aft^r the departure of seasonally-resident birds (Ranwell and Downing
1959, Burton 1961).  Values obtained in this way tend to overestimate
consumption during the non-growing season, as seasonal declines
related to physical factors are also included in these estimates
(Charman 1977).

     Exclosure experiments have provided additional estimates of
consumption, using differences in biomass between grazed and ungrazed
(caged) plots to quantify waterfowl feeding.  Verhoeven (1978) used
exclosures to estimate the impact of foraging by European coots
(Fulica atra) -and found a marked reduction in the biomass of Ruppia
cirrhosa outside exposures.  Jupp and Spence (1977) protected plots
of Potamogeton spp. in Loch Leven, Scotland, and reported a similar
decline in plant biomass due to waterfowl grazing.  Charman (1977) did
not estimate grazing pressure, but attributed early seasonal depletion
                                     167
                                                                                       !J
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                                                                                 .-J
of Zostera to the foraging activities of brent geese based on the                    j
results of his previous exclosure experiments.                                       ]

     Similar information for non-grazing waterfowl is almost entirely
lacking.  Nilsson (1969) calculated that diving ducks consumed less
than 102 of the standing crop of invertebrates in a Zostera and Ruppia
bed.  Sincock (1962) estimated consumption by a number of non-grazing
waterfowl but did not relate these values to standing crop.  The
diversity and patchy distribution of potential food organisms, and the
difficulties associated with gizzard analysis may account for the lack
of quantitative data.

     A technique recently employed to characterize trophic
relationships in seagrass communities involves analysis of stable
carbon isotope ratios in tissues of herbivores or higher-level
                                            1TO1O1O
consumers.  Based on differential upcake of iJC by plants, 1JC:  C
ratios (expressed in <$^C units) have been used to identify primary
sources and fluxes of organic carbon in grassbeds and other habitats.
Comparisons of animal 6  C values with known or estimated dietary
values indicate that isotope ratios are conserved through the food
chain (Haines 1976, Fry et al. 1978, Haines and Montague 1979), with
only slight variation due to effects of metabolic fractionation (De
Niro and Epstein 1978).  Seagrasses exhibit 6 ^C values of -3 to
-13°/oo which are readily distinguished froni those of phytoplankton
(-18 to -24.5 °/oo), with benthic diatoms having intermediate values
(Fry and Parker 1979).  Resolution of dietary components is thus
limited to fairly broad taxonomic or functional groups, but the
technique is much less tedious than examination of gut contents.
     Application of 6  C analysis to waterfowl trophic studies has
thus far been limited, but suggests a similar strong relationship
between isotope ratios of bird tissue and dietary values.  Patrick
Parker and James Winters (pers. comm.) have used 5^-^C values from
liver and other tissues to study redheads foraging in shoalgrass
(Halodule wrightii).  Bird 6 ^C values exhibited a positive seasonal
shift of about 8 °/oo soon after arrival of birds from the breeding
grounds, indicating rapid carbon turnover in bird tissue associated
with the new winter diet.  McConnaughey and McRoy (1979) reported a
similar seasonal shift in values for waterfowl species in the Izembek
Lagoon, Alaska.  Although turnover may be very rapid, dietary
information is time-integrated in the short term, whereas gizzard
samples represent single foraging episodes.

     Details of diet and reliable consumption estimates are needed to
assess the functional role of waterfowl in SAV habitats and to
evaluate the importance of this resource for wintering waterfowl.  In
this study, several of the above methods wera combined, as it was felt
that an integrative approach. would provide more information than the
use of z cinglc technique, and would allow for comparison of results
obtained by different methods.
                                                                                        , !
                                                                                         f
                                    168                                                  ;
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                                METHODS
The Study Area
     Vaucluse Shores is located on the Delmar/a Peninsula  in  the lower
Chesapeake Bay, just north of Hangar's Creek in Northampton County,
Virginia (37*25'N latitude, 75°39'W. longitude) (Figure 1).   The site
consists of approximately 150 hectares vegetated subtidally by Ruppia
maritima and Zostera marina (hereafter Ruppia and Zostera) which
dominate beds of submerged vegetation in meso- and polyhaline regions
of the Bay.  These opecies are distributed according  to depth, with
Ruppia dominant in shallow water  [less than 0.5 m at mean  low water
(MLW)], Zostera dominant in deeper water (greater than 1.0 m) and a
mixed vegetation zone present at  intermediate depths.  Areal  extent of
the grassbed is delimited bayward by a series of parallel offshore
sandbars oriented obliquely to the shoreline.  Six transects  (A-F)
were established in the study area in 1978 for use in mapping
vegetation at the site (Orth et al. 1979) and these provided
convenient boundaries for waterfowl censuses.

     Biomass data for Zostera at  Vaucluse Shores indicate a seasonal
maximum coinciding with seed production in June and July, averaging 85
g m~2 in 1978 (Orth et al. 1979).  A second smaller peak in biomass
takes place in the fall, followed by winter values of less than 50 g
ra~ .  Ruppia has a slightly different growth cycle, with one  major
biomass peak occurring in August  and September.  Both species may
exhibit different patterns of growth at mixed vegetation sites (P. A.
Penhale, pers. coiam.).

     Salinity at the site varies  from 14 °/oo to 24 °/oo and  water
temperatures range from -2C to 28C.  In winter months, extreme lot*
temperatures may cause ice formation in the shallow areas.

     The same site was the focus  of a broad scale interdisciplinary
study (EPA-CBP contract #R80-59-74) designed to describe the  principal
components of seagrass communities in the lower Chesapeake Bay, and to
elaborate important aspects of the functional ecology of these
systems.  This integrated program included the following investigation
of waterfowl use of the habitat.                                      !

Waterfowl Abundance Estimates                                         \

     1978-79:  A preliminary census effort was undertaken  in  1978-79
consisting of 13 census days between 6 December and 22 March, with a
variable number of censuses per day.  Waterfowl observed between
previously established transectr. A through F were identified  and
counted with the aid of a spr-tcing telescope and located oy transect
interval.  The duration el each census was 15 minutes, and all birds
present during trhac time were counted.  Feeding activity of Canada
geese was noted, and the relationship between percent feeding and tide
level was tested using the Spearman rank-correlation coefficient rs,
computed as
                                    169
-------
                           - R2)2
where R is  the variable  rank,  and n  is  the  sample  size  (Sokal  and
Rohlf 1981).  Census  times were used  to obtain approximate  tide  level
data from NOAA tide prediction tables.

     1979-80:  All observations were  made between  transects  B  and  C
(Figure 1)  in 1979-80, allowing a more  intense effort in a  smaller
area (approximately 26.5 ha) which had  been consistently utilized  by
waterfowl the previous year.   Waterfowl were  censused at intervals
averaging 11 days  from 8 November to  3  April  and on each cencus  date
birds were  counted at approximately  2-hourly  intervals  during
daylight.

     At the outset of the study, four zones were marked in  the census
area from shore to the offshore sandbar which encloses  the  grass bed:
bare sand,  patchy  Ruppia maritime, mixed Ruppia and Zostera, and pure
Zostera marina.  Although the  zones  are not highly discrete,
fluorescent stakes were  placed at transitions along transects  B  and C
such that major vegetation type was  indicated between pairs  of stakes.
The position of each  bird was  recorded  in reference Lo  these stakes.
In order to express waterfowl  numbers in terms of  vegetation type,
areal extent of each  zone was  estimated from  the results of
vegetational transect analysis reported by Wetzel  et al. (1979)  and
from personal observation of transition zones.  Density of  waterfowl
within these zones was then calculated, and differential utilization
was tested  between each  pair by the Wilcoxon  nonparametric  two-sample
rank test.  The Wilcoxon statistic is calculated for samples of  equal
size as follows:

               -               n
          C = n2 + n(n + 1) -  jR
where n is sample size and R is the variable rank.  This  statistic  is
then compared with  (n  - C) and the greater of the  two quantities is
the test statistic  Us (Sokal and Rohlf  1981).  The  bare sand zone was
excluded from analyses, as waterfowl rarely utilized that area.

     A tide gauge consisting of a stake graduated in 5 cm increments
was placed in subtidal shallow water and water level was recorded at
the time of each census.  The stake was destroyed by ice  floes and
replaced twice, but after 1 February tide data were obtained from NOAA
tables as in 1978-79.  Time and air temperature were also recorded,
and wind speed and  direction were obtained from the National Weather
Service station in  Norfolk, Virginia.   The above parameters were
related to waterfowl abundance using nonparametric  correlation
statistics as described above.  In th
-------
              R = RUPPIA

              Z = ZOSTERA
              S  SAND
              M : MIXED
              D: WATERFOWL EXCLOSURES

                = TRANSECTS
Fig. 1.  The  Vaucluse Shores study area,  showing previously  established
         transects A-F, and the location  of waterfowl exclosures within
         transect  interval B-C.
                                       171
-------
presence of a single  large  flock of  redheads which would have  obscured
major trends.

Food Habits

     Waterfowl gizzards and livers were obtained  from  birds  collected
by local hunters and  scientific personnel  in the  study area  and  in  the
mouth of Hungar's Creek between October 1979 and  March 1980.   Because
buffleheads were predominant  in the  second year of study,  the  diet  of
this species was the  focus  of food habits  studies.   Bufflehead
gizzards were analyzed for  food items, and livers of all species  were
analyzed for stable carbon  isotope ratios  (6^C).  Gizzards  were  kept
frozen before laboratory  processing, and contents were then  sieved
into two fractions  for ease of examination.  The  coarse and  fine
fractions were retained on  0.5 mm and 62y  sieves, respectively.
Material which passed through the 62p sieve was negligible and
therefore was discarded.  Both fractions were preserved in 10%
formalin.  Contents of intact esophagi were examined,  but  were sieved
on 62y mesh only.

     Identifiable species were enumerated  under a dissecting
microscope and noted  as present or absent  in the  case  of fragmented
remains.  Total contents  of individual gizzards were not weighed, as
it was felt that differential digestion would bias these quantities to
a great extent.  Instead, a representative sample of entire  specimens
of each prey species  was  obtained and dried to constant weight.
Ash-free dry weights  were estimated  using  conversion factors in
Cummins and Wuycheck  (1971) and values provided by J.  Lunz and D.
Weston (pers. comm.)  for  two  mollusc species, as  follows:

          Peracarida  0.82  x  Dry weight
          Annelida    0.82
          Decapoda    0.74
          Mollusca    0.10  (For Bittium varium and Crepidula convexa)

These weights were multiplied by abundance per gizzard in  order  to
calculate percent composition by dry weight and ash-free dry weight.
The aggregate percent method  was used to calculate mean composition,
where the proportion  of a species in each  gizzard is averaged  over  all
gizzards (Swanson et  al.  1974).  By  this method,  each  gizzard  has
equal importance despite  differences in volume of contents.  Dietary
importance was determined using the  'index of relative importance'
(IRI) (Pinkas et al.  1971):

                        IRI = (% N -f- % W)  x % F

where N is numerical  abundance, W is weight, (substituted  here for
volume) and F is frequency  of occurrence.

     Bufflehead dietary electivity was calculated within mollusc  prey
species only, as the  numerical importance  of softer-bodied forms  may
not be as accurately  reflected in gizzard  samples.   The Jacobs index
                                   172
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(L) was used to measure electivity because  the degree of departure
from zero (non-selectivity) can be statistically tested (Gabriel
1978).  L is calculated as  follows:
          L = In (0) where 0
and pi - proportion of diet comprised by a given prey  taxon
    qj =• proportion of diet comprised by all other prey  taxa
    P2 = proportion of food complex  in environment comprised by given
         taxon
    q2 = proportion of food complex  in environment comprised by all
         other taxa

Estimates of environmental abundance of prey items were  obtained  from
cores collected in January, and only gizzard samples which were
collected within two weeks of benthic sampling were used  to obtain
dietary values.

Stable Carbon Isotope Analysis

     Waterfowl livers were rinsed in distilled water,  dried at 65°C
for 96 hr and ground in a Wiley Mill to a fine powder.   These samples
were analyzed by Dr. Evelyn Haines at the University of  Georgia Marine
Science Institute and Ors. Patrick Parker and James Winters of Coastal
Science Laboratories, Inc., at Port Aransas, Texas.  Details of
further sample preparation and analyses by these labs  are described  in
Haines (1976) and Parker et al . (1972), respectively.  In general
samples are first combusted to convert organic carbon  into CC>2, which
is then isolated from other evolved gases.  Isotopic analysis of C(>2
is carried out on a marfs spectrometer, and isotope ratios are reported
relative to a carbonate standard, in <$ ^C units (parts per mil):

                 /13C/12C sample     \
                  — __ - _H  X 103
                 WC/^C standard   /

     Tissues of important waterfowl foods (invertebrates  from the
study area) were prepared and analyzed in the same manner, except
that in many cases specimens were pooled to obtain sufficient tissue
(=60 rag).  For comparison with observed biifflehead 6*  C  values, an
expected value was calculated by multiplying the mean  percent
contribution of each prey species to the diet (ash-free dry weight)  by
its 6^- C value, and summing these values over all gizzards (Fry et al.
1978).

Waterfowl Exclosure Experiments

     To investigate the impact of grazing waterfowl (primarily Canada
geese and redheads) on vegetation density at the study site, two areas
between transects B and C were chosen to locate exclosures:  a shallow
mixed Ruppia and Zostera zone and a deeper pure Zostera  zone (Figure
                                    173
       \                                           '    -
-------
1).  Between 14 and 18 October  1979,  two  caged plots were  established
in each of these zones at depths of approximately 0.5 m  and  1.2 m  at
MLW, respectively.  Cage pairs  included one cage (cage I)  to  be
sampled at two intervals during waterfowl residence and  another  (cage
II) to be sampled only if cage  I was  damaged.

     Exclosures measured 2m x 2m x 0.5m and were constructed  with  2.5
cm mesh vinyl-coated wire sides and crab  pot wire tops (2.5  cm
hexagonal mesh), hinged on two  sides  to open from the center  during
sampling.  A frame consisting of a length of shaped concrete
reinforcing rod supported the top and penetrated the sediment to 50
cm.  In addition, aim length  of reinforcing rod was attached to  each
corner and buried to 50 cm.

     Benthic samples were taken with  a 0.031 m^ plexiglass corer to a
depth of approximately 15 cm during three sampling periods:  18 October
1979, 16-19 January 1980, and 19 March 1980.  On 18 October,  six
replicate cores were taken in the vicinity of cages located  in the
Zostera and mixed vegetation zones.   Sample size was chosen  based  on
previous estimates of variability in  plant biomass in the  study area
(Orth et al. 1979).  These samples were processed for vegetation only,
which was separated into above  and below  ground fractions, then dried
in an oven at 55°C for 48 hours and weighed.

     During the second sampling period methodolgy was modified based
on the near-absence of Canada geese from  the grassbed (see results).
As the dominant species was the bufflehead, which feeds  primarily  on
invertebrates (Stewart 1962), samples were processed for- animal
abundance as well as quantity of vegetation.  Sample size  was
increased to ten cores each from caged and uncaged sites to  account
for greater patchiness of the invertebrate species.

     Cores from uncaged areas were taken  in a pattern radiating from
the center of the cage using random compass headings and distances
between 6 m and 12 m from the cage.   Within exclosures,  cores were
taken randomly from a 2m x 2m grid.   Care was taken to position and
remove the corer with the least possible  disturbance to  adjacent
bottom.  Samples were placed in muslin bags, refrigerated  and washed
the following day on a 0.5 mm sieve.  Cores collected in January were
frozen after sieving, but this  resulted in damage to soft-bodied      ,
invertebrates and thus samples  collected  in March were stored in 10%  \
formalin.                                                             1

     In the lab, samples were rinsed  and  elutriated repeatedly to
separate vegetation from the animal and sediment component, which  was
then sieved into two fractions.  The  coarse fraction (>2 mm)  was
sorted and identified in its entirety, and the fine fraction
(<2 mm >0.5 mm) was distributed evenly on the sieve by flotation and
then split into quarters.  Two  quarters were chosen randomly  for
sorting and the counts obtained were  then doubled.  Split  counts were
compared to total counts for two samples.  Total number of individuals
was 3.0% in erior for one comparison  and  3.1% for the other.  Error by
                                   174
-------
species varied, with the rarest species most affected by the
technique.  All organisms were identified to lowest taxa, with some
exceptions.  In the January samples polychaetes, oligochaetes, and
nemertea were eliminated from analysis because damage from freezing
rendered numbers unreliable.  Only two dominant epifaunal polychaetes,
Nereis succinea and Polydora lignjL, were identified to species in the
March samples.

     Sediment cores were taken Lo determine effects of exclosures on
sedimentation processes.  Three cores were taken from each treatment                   j
in January and five were taken from each treatment in March.  Percent                  i
sand and silt-clay were determined by sieving and pipette anax/sis
outlined by Folk (1961).                                                               |
     Differences between treatment means were tested using ti.e
Wilcoxon statistic, with the exception of sediment data, which were
arcsin transformed (Sokal and Rohlf 1981) and compared between
treatments using a standard t-test.

Estimates of Consumption from Waterfowl Density

     Mean waterfowl abundances, theoretical daily intake, and days in
residence were used to estimate total consumption of biomass by birds
utilizing the study area.  Methods for determining daily intake are
from Wolff et al. (1975) where standard metabolism M is multiplied by
5 to obtain consumption in kcal/day.  M is determined by the formula:

                     Log M = Log 78.3 + 0.723 logW

where W is body weight in kg.  Kcal were converted to grams ash-free
dry weight (AFDW) by multiplying by a factor of 0.2.  These values
were then used in the following formula for consumption:

                               C = I-A-R

                    where I =* daily intake in grams AFDW
                          A = mean abundance
                          R = residence (estimated as 150 days)

Consumption was calculated over the total habitat as well zs more
restricted areas, based on patterns of utilization within the habitat.
Estimates were partitioned according to predominant feeding type
(animal vs vegetation) according to Stewart (1962) and Munro and Perry
(1981).

                                RESULTS

Waterfowl Abundance

     1978-79:  The Canada goose was the most important waterfowl
species in the study area in 1978-79, and averaged 526 individuals per
100 hectares (Table 1).  The overwhelming dominance demonstrated by
                                    175
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the species is obvious when plots of total waterfowl and Canada goose
abundance are compared (Figure 2).  Second in importance was the
bufflehead, which averaged 46 birds per 100 ha and was the only
species present on every census date.  Large flocks of redheads
utilized the study area, but occurred on only 5 of the 13 census days.
It is uncertain whether this species was adequately censused, as
foraging may have been primarily nocturnal.  Redheads were most often
observed at dawn and dusk, and did not generally remain in the area
throughout the day.

     Brant occurred on only two census dates, but one flock of
approximately 1300 birds inflated the relative importance of the
species.  Whistling swans and wigeon were present regularly (in more
than 60% of censuses) but in low numbers.  Red-breasted mergansers
(Mergus serrator) occurred less frequently but in flocks with an
average density of 19 birds per 100 ha.  Although non-divers
(primarily Canada geese) were more abundant than diving ducks, both
groups were represented by nearly equal numbers of species throughout
the season.

     Abundances of most species fluctuated without respect to
seasonality in 1978-79.  However, Canada geese were most abundant in
the first few censuses, and this trend would probably have been more
pronounced had the earliest part of the season (November to early
December) been included.

     Utilization of the study area by foraging Canada geese was
influenced by tide level (Figure 3).  At the lowest water levels (2
hr. before and after low tide) the majority of geese present were
feeding, whereas geese almost never attempted to feed at higher tide
levels, and instead remained on the offshore sandbar.  A negative rank
correlation between percent feeding and departure from low tide in
hours was significant at p < 0.001.

     1979-80:  Patterns of waterfowl abundance changed dramatically in
the second year of observations.  Fewer species utilized the area
consistently (four per day average) and the proportion of non-diving
to diving species decreased to less than 0.2 per day (Figure 4).
Although large numbers of Canada geese were noted in the vicinity of
Hungar's Creek, no large flocks were censused within the study area
(Table 2).  During a number of censuses, rafts of several hundred
geese were observed directly offshore at a distance of approximately
500 m beyond the sandbar (numbers in parentheses in Table 2), but they
did not come into the grassbed.

     The bufflehead was the dominant species in 1979-80, and total
waterfowl numbers closely tracked the abundance of this diving duck
(Figure 5).  Again, they occurred on every census date, and mean
density of this species (96 birds per 100 ha) was approximately twice
as great as in 1978-79.  Redheads were also important though
infrequent the second year, primarily due to a flock of approximately
500 birds which fed in shallow Ruppia on 6 March.  In contrast to the
                                   177
                                                                                      :J
-------
4500-
4000-


3500-


3000-

2500-

2000-
1500-

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< 500-
i
g oJ

/\ Totol Waterfowl Abundance
/ \
/ \ 1978- 1979
/ \
/ \
/ I
/ \
/ \
/ \ j
/ \
/ \
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/ \ Canada Goose Abundance
/ \
/ \ 1978-1979
/ \
/ \
1 \
1 \ _
/ \
/ \

/ \
/ \
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\T X \
/ \ xx \
•^ \
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X— """*" •"-
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1 1 Illl 1 II II I 1
DECEMBER JANUARY FEBRUARY MARCH
Fig. 2.   Abundances of total waterfowl and Canada geese at Vaucluse Shores,
         1978-1979.  Points represent means and bars are standard errors of
         the mean.
                                     178
                                                                                         i.
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      lOO—i
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  0


 LOW
 TIDE
                   O  O  O    OOO
T
 4
                                                           FLOOD
                               DEPARTURE  FROM  LOW TIDE (HRS)
Fig. 3.  Relationship between tide stage  and  foraging activity in Canada
         geese at Vaucluse Shores,- 1978.-1979.   Curve fit by eye.
                                      179
-------
                             ^'  '.] DIVERS    V///\> NON-DIVERS


                                     % SPECIES
       50%-
                 NOVEMBER   DECEMBER   JANUARY    FEBRUARY    MARCH    APRIL
                                 %  INDIVIDUALS
       50%-   1978- 1979
                              ——•—-~-—y~—_—,. 11 u 111—|—       i'''	
                 NOVEMBER '  DECEMBER   JANUARY    FEBRUARY   MARCH
                                                                      APRIL
                                     % SPECIES
       =00,   1979-
       50 lo- 1980
               i— —"	1     	^TT—"—L~J—f
                 NOVEMBER   DECEMBER    JANUARY     FEBRUARY   MARCH     APRIL
                                 % INDIVIDUALS
       5O% -
1979-
1980


                NOVEMBER    DECEMBER    JANUARY    FEBRUARY     MARCH    APRIL
Fig. 4.   Numbers of  diving vs.  non-diving waterfowl,  as a percentage of
          total waterfowl during 1978-1979, compared  to 1979-1980.
                                        180
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previous year, scaup (Aythya spp.) were important and were present  in
greatest numbers (45-60 per 100 ha) in February and early March.

     In 1979-80 waterfowl abundance was independent of  tide  level,
except in the shallow Ruppia zone, where numbers oZ birdi. were
generally low but increased with higher tide levels (Figure  6).  Rank
correlation coefficients for the mixed and Zostera zones and the total
study area were not significantly different from zero (Table 3).

     Temperatures ranged from -6C to 22C but did not influence
waterfowl abundance in the study area.  Winds were predominately NNW,
but direction had some effect on waterfowl numbers.-  A  positive
correlation was found between abundance and direction (from  10-360°),
and higher numbers were associated with winds from the  NNW (p < 0.05).
Wind speed alone did not have a significant effect, but when wind
direction was held constant, wind speed had a positive  influence on
bird numbers in the case of NNW winds (p < 0.05).  When wind speed  was
held constant (in 5 knot increments) direction had a positive effect
only at 21-25k (p < 0.05).  No correlation was found between waterfowl
abundance and time of day during daylight hours.

     Within the grassbed, vegetation zone had a pronounced effect on
waterfowl use (Figure 7).  Mean densities of birds within these zones
indicated an increasing inshore to offshore trend, with ^axiirura
densities in the Zostera zone.  Numbers of birds were very low in bare
sand and Ruppia, rarely exceeding one individual per hectare.
Multiple comparisons indicated that these differences were highly
significant ror each pair considered (Table 4).

     Again, few seasonal trends were evident in waterfowl abundance.
A gradual increase in total numbers from January through March 1980
reflects primarily the occurrence of greater numbers of scaup and
redheads, while bufflehead numbers fluctuated around the overall mean
wich no sustained increases or decreases.

Food Habits:  Gizzard Analysis

     Gizzards from 32 buffleheadc were examined.  Due to the
difficulties of collecting waterfowl during active feeding, most
gullets and a number of gizzards contained very little  or no food.  Of
25 esophagi collected, 22 were empty.  Therefore, results are
presented for gizzards only, two of which were completely empty and   \
were also omitted from analysis.  All other gizzards were analyzed
regardless of fullness, in order to obtain an adequate  sample size.

     A total of 27 taxa were identified in bufflehead gizzards,
including 23 invertebrate species, three plant species  and fish
vertebrae (Table 5).  Molluscs and peracaridan crustaceans accounted
for 18 of the 23 invertebrate species and the remainder included
polychaetes, decapods, br-ozoans and barnacles.  Plant  material in  the
                                   183
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                                          WATER  LEVEL (Relative to MLW)
              Fig.  6.   Relationships between  numbers of waterfowl and  tide  levels in three
                       vegetation zones,  1979-1980.   Numbers in parentheses refer to a
                       single flock of redheads  which were not included  in  analyses.
                       "cans are indicated  by the height of blocks, and  points are
                       individual observations.
                                                   184
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1979
JANUARY
FEBRUARY | MARCH
1980
APRIL
Fig. 7.  Within-habitat variation in waterfowl density at Vaucluse
         Shores, 1979-1980.  Means and standard errors are indicated.
                                    186
-------
TABLE 4.  EFFECT OF VEGETATION ZONE ON WATERFOWL DENSITY IN THE STJDY
          AREA.  COMPARISONS TESTED BY THE WILCOXON STATISTIC UK.
                                 Mixed       Zostera      Ug
Mean density
(Birds/ha)
± Std. Error
N=76
0.43
±0.110

1.71
+0.263
~—
4.92
±0.697

Mean Ranks R/M
M/Z
Z/R
60.62
55.72
92.38
66.30
86.70
97.28
7021.0***
5038.5**
7393.5***

** p < 0.01
                                   187
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diet consisted primarily of Ruppia maritima and Zostera marina, with                    \
corn (Zea mays) present in a single gizzard.

     Crepidula convexa was the dominant ~e- y item by numerical                         ]
abundance and dry weight, with a mean abundance of 49 individuals and                  i;
mean dry weight of 43 mg per gizzard.  In terms of ash-free dry                        '•
weight, C^. convexa was less important than the polychaete, Nereis                      i
succinea, which averaged 30% of gizzard contents by ash-free weight.                   j'
However, abundance of _N. succinea was relatively low (nine individuals                 j
per gizzard).  Only chitinous jaws and setae of this polychaete were                   :
evident in gizzards due to rapid digestion of softer tissue, but                       j,
numbers of individuals (and thus reconstructed weights) were obtained                  i
by counting pairs of jaws.                                                             [

     By taxonomic group, gastropods dominated gizzard contents (Figure                 |
8).  Of the five most important prey species by the index of relative                  t
importance (IRI) four were gastropods:  Crepidula convexa,
Pyramidellidae sp., Bittium varium and Astyris lunata.  These four                     r
species accounted for nearly 60% of gut contents by dry weight (36% by                 [
AFDW) and 64% by abundance, and occurred with an average frequency of                  !
70%.                                                                                   i
                                                                                       \
     Polychaetes were represented in gizzards only by Nereis succinea,                 j
although the contribution to the diet by this group may be                             j
underestimated.  Bivalves (primarily Anadara transversa) and isopods                   t
(dominated by Erichsonella attenuata) were of roughly equal importance                 ;
averaging from 5-12% of gizzard contents by dry and ash-free dry                       |
weight.  Mysids (Neomysis americana) were abundant in several samples,
but dry weight contribution was minor.  Identifiable amphipods and                     ,
decapods were encountered rarely and in low numbers.                                   !
                                                                                       j
     The barnacle Balanus improvisus was a consistent prey species,                    1
with shell fragments found in 25 gizzards.  Exoskeletal fragments of                   j
bryozoans were also found frequently (70% occurrence).  Because                        }
numbers could not be determined for either of these groups, dietary
importance was not assigned.  Importance was not determined for plant
material as no quantitative measure of percent composition was made.
However, it appeared by visual estimate that vegetation was a minor
dietary component, taken with invertebrate prey items found among
vegetation.

     Results of electivity calculations among mollusc prey species
indicate that buffleheads may be at least partially selective (Table
6).  Crepidula convexa was eaten in proportionally low numbers
relative tu its abundance in the grassbed, resulting in a
significantly negative L value (p < 0.001) although it was still the
dominant prey item.  The gastropods Bittium varium, Pyramidellidae
spp., Astyris lunata, and the bivalves Gemma gemma and Anadara
transversa are apparently preferred (i.e. had significantly positive L
values), Tmt are found in much lower abundances in the environment
than is C. convexa.  The gastropods Triphora nigrocincta, Acteon
                                    189
-------
                                                                      to
                                                                     13
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punctostriatus and Acteocina canaliculata contributed to the diet in
close proportion to their environmental abundances.

Food Habits: Stable Carbon Isotope Ratios

     Bufflehead livers were fairly consistent in carbon isotope
composition, with an average 613C of -17.2 _+ 0.81 °/oo (Table 7).
fil-^C values were obtained directly for 11 prey species (van Montfrans
1981) and were estimated by taxonoraic group or feeding category  for
the remaining species (Table 8).  In general, values were slightly
less negative than bufflehead liver tissue and varied widely among
taxa.  The polychaete Nereis succinea (-13.3 °/oo), the gastropod
Bittium varium (-13.4 °/oo) and the isopod Erichsonella attenuate
(-13.4 °/oo) had the highest fi^C values, while the gastropods
Crepidula convexa (-20.2 °/oo), Astyris lunata (-16.4 °/oo) and  the
amphipod Cymadusa compta (-16.8 °/oo) were less 6l3c-enri.ched.   The
suspension feeding bivalves Anadara transversa and Gemma gemma were
assigned a value of -17.5 °/oo based on measured ^C: "C ratios  for
the clams Mya arenaria and Mercenaria mercenaria.  Values for other
prey species ranged from -14.0 to -15.9 °/oo.

     From these values for prey items and the percent contribution of
each species (by ash-free dry weight) to the diet, the resulting value
for bufflehead tissue should approximate -15.4 °/oo, if all prey items
are accounted for in correct porportions.  Although this assumption
was not strictly met, the observed mean was within 1.8 °/oo of the
predicted value.

     6  C values for other waterfowl species were also lower than most
potential prey species (Table 9).  With the exception of a single
wigeon liver (-12.7 °/oo), values were even further removed from those
obtained for submerged vegetation.  Ruppia and Zostera ranged in 6  C
values from -7.5 to -10.6 °/oo, and the value for associated
periphyton was -11.2 °/oo.

Waterfowl Exclosures

     By 23 January, the inshore exclosures had been removed by ice,
and results are presented for cages in pure Zostera only.  Cage  I in
Zostera was sampled in January but not in March, as the top had  been
forced open for an unknown length of time.  Instead, Cage II was
sampled, and therefore the results from the two dates are not strictly
comparable.

     Samples from both cages (i.e. both sample dates) yielded
significantly greater numbers of individuals and species than samples
from uncaged areas (Table 10).  Species abundances were significantly
greater inside cages in approximately half of the comparisons (p<0.05)
(Figures 9 and 10).  Eight species were found in significantly higher
numbers in both sets of caged samples; the gastropods Doridella
obscura, Crepidula convexa, Astyris lunata, and Bittium varium,  a
bivalve Anadara transversa, the isopods Erichsonella attenuata and
                                   192
-------
TABLE 7.  CARBON ISOTOPE COMPOSITION OF BUFFLFHEADS
          COLLECTED NEAR VAUCLUSE SHORES, 1979-1980.
613c Values
Bufflehead
Livers
°/oo
-15.8
-17.1
-16.4
-18.0
-17.2
-17.4
-18.0
-18.0
-17.8
-15.5
-16.8
-17.8
-17.0
-17.3
-16.5
-17.5
-18.4
-17.7
-17.6
-17.9
-16.4
-16.7
-17.0
-18.3
-15.3
-18.1
-15.3
-18.1
-18.5
-16.9
-18.0
- ' -16.5
-17.3
-16.8
X = -17.2 o/oo
S.D. ± 0.81
Date
Collected
12/18/79
12/18/79
12/18/79
12/18/79
12/18/79
12/19/79
12/19/79
12/19/79
12/19/79
12/24/79
12/26/79
01/02/80
01/14/80
01/14/80
01/15/80
01/16/80
01/16/80
01/16/80
01/16/80
01/16/80
01/23/80
01/23/80
01/23/80
01/23/80
01/23/80
01/23/80
01/23/80
01/23/80
01/23/80
01/23/80
02/22/80
02/22/80
02/22/80
02/23/80


                                                                            I
                                                                            i 1
                                                                            !J
                        193
-------
TABLE 8.  ISOTOPIC COMPOSITION OF BUFFLEHEAD INVERTEBRATE PREY
          SPECIES.

PREY SPECIES
Crepidula convexa
Nereis succinea
Pyrainidellidae sp.
Bittium varium
Astyris lunata
Erichsonella attenuata
Anaoara transversa
Cra igon septemspinosa
Neoniysis americana
Nassarius vibex
Triphora nigrocincta
Edotea triloba
Gemma gemma
Acteocina canaliculata
Gamrcarus mucronatus
Idotea balthica
Cymadusa compta
Epitonium rupicola
Acteon punctoscriatus
Xanthidae sp.
Paracerceis caudata

,13C
&/00
-20.2
-13.3
-14. 5a
-13.4
-16.4
-13.4
-17. 5b
-14.2
-17. 5b
-14.2
-14. 7C
-15.5
-17.5b
-14. 7C
-15,9
-14.0
-16.8
-14. 7C
-14. 7C
-14. 5a
-14.3d
PROPORTION
OF DIET
BY AFDW
0.164
0.296
0.060
0.041
O.C90
0.098
0.061
0.050
0.029
0.040
0.007
0.009
0.008
0.008
0.006
0.017
0.004
0.002
0.001
0.004
0.002
CONTRIBUTION
TO TOTAL
6l3C
-3.31
-3.94
-0.88
-0.55
-1.48
-1.31
-1.07
-0.71
-0.51
-0.57
-0.10
-0.14
-0.14
-0.12
-0.10
-0.24
-0.07
-0.03
-0.001
-0.06
-0.03

                                 Total = Expected  13g = -15.35 °/oo
aMean value for:
b      «       .
c      »       .
d      11
predator/omnivores
suspension feeders
gastropods
isopods
                                   194
-------
TABLE 9.  CARBON ISOTOPE COMPOSITION OF WATERFOWL OTHER
          THAN BUFFLEHEADS COLLECTED NEAR VAUCLUSE SHORES,
          1979-1980.
Species
Canada goose


American wigeon






Black duck

Pintail
Lesser scaup
Greater scaup
Oldsquaw

Surf scoter

Red-breasted merganser
613c Values °/oo
(Livers)
-19.6
-21.6
-19.6
-19.1
-17.6
-16.2
-16.3
-15.0
-16.2
-12.7
-18.8
-17.8
-16.9
-18.9
-19.1
-16.5
-17.7
-17.1
-18.3
-20.8
Date
Collected
12/31/79
01/05/80
01/11/80
12/17/80
12/17/79
12/17/79
01/01/80
03/14/80
03/14/80
03/14/80
01/01/80
01/02/80
01/11/80
01/23/80
12/31/79
01/16/80
01/23/80
01/01/80
01/01/80
02/23/80
                            195
-------
TABLE 10.  NUMBER OF SEPCIES AND INDIVIDUALS FROM
           CORES TAKEN IK CAGED AND INCAGED
           ZOSTERA IN JANUARY AND MARCH 1980.


DIFFERENCES WERE
STATISTIC Us.
O
TESTED BY THE WILCOXON









January
N=10











March
N=10






No. Species
Caged Uncaged
33 29
34 29
30 29
31 ?.9
32 29
31 28
29 26
33 25
29 26
29 25
X 31.1 27.5
S 1.85 1.78
Us 92.5***
45 41
38 35
34 32
38 34
39 31
33 29
41 29
31 29
43 33
42 32
X 38.4 32.5
S 4.58 3.66
Us 84 . 0**
No; Individuals
Caged Uncaged
1257 854
1615 937
1264 1000
978 1335
1343 1027
1002 941
1360 930
1153 694
1089 620
997 740
1025.8 907.8
202.62 202.00
88.0**
1179 1161
1504 1202
1987 1522
2154 1559
2015 1741
2013 1681
2098 1444
2316 1259
2218 1079
2607 1556
2069.1 1420.4
297.55 230.16
95.0***

** p <
*** p <
0.01
0.001


                      196
-------
                                                   MEAN RANK SCORE
               Acteocma canofrculota  +
                Acleon puncfostr/atus  +
                     Astyns  lunjfa  +
                  Crepidvlo con veto  -f

                 Tlyonosso obsoteta  -f
                   Pyrom.delhda* spp  +
                Tftphoro nigrocmcta  +
                  Dortdella ebscura  —
                x3 /7 p >OOI
 **  001 > p > 0001
*K*  p
-------
                                                      MEAN RANK SCORE
                                         15
                            Nemerleo sp  —
                     Acfeocma cano/'Cutafa  +
                     Acteot punctostriotus  -*•
                           Astyns lunota  +
                        Crtptdulo  COnvexa  +
                           Biffium vQrtum  +
                       Hyonossa  obsolete  —  #*|
                        Pyramideliidae spp  +
                       TttphorQ ntgroctnctQ  +
                        Do ride ila obsc wo  ~~
                        Anodofo tronivcrsa  +
                          Gemma gemma  +
                          Lyonsio ftyglino  -

                            Mytihdoe sp  —
                          Poiychoeio spp  +
                          Polydo ra iignt  ~
                         Nereis sue c me a  +
                          Oi-gochoefa spp  —
                           Osiracodo spp  —
                       Bo/anus- impfOviSuS  -+•
                        Cydaspis  if ortats  —
                       LeptQcfieim rapa*  —
                         Idotey botlhtca  +



                        Cymodasa compto  +
                        flasmopvs taevts  —
                     GamrrtQrus muc'Ofiotvs  +
                           Me/i to ritttcta  -f-
                     Microp 'otopus raneyi  —
                        Cap re HQ p e nanti $  —
                      Porocapreifo fenuis  —
                                                NO CAGE
                                                   (0
                                              CAGE  H
                                                 10
                                                          I **
                                                          • * **
                                                          • ***«•
                                                         | **•
                                                         *
HBHH Higher score, significant
r.Tm Not significantly different

  —   Abser.t from gut sompfes
                                                          * 005 > p) 0 0(
                                                         ** 001 >p>000>
                                                        ***** P< °°°l
Fig.  10.   Rank  scores for  species abundances  in  caged  vs.  uncaged  samples
             taken in March 1980,  as designated  by  the Wilcoxon 2-sample test.
             Expected score under  Ho (that treatment means are  equal)  =  10.5.
             Significance  level  of  the  U  statistic  is  indicated.
                                                  198
-------

Edotea triloba, and an amphipod Paracaprella tenuis.  With  the
exception of P_. tenuis and D^. obscura, all of these species were  found
in bufflehead gizzard samples, and most were important components of
the diet.  Other species with significantly higher abundances inside
cages which were not present in gizzard or gullet samples included  a
number of peracarid crustaceans and juvenile blue mussels (Mytilus
edulis).  Only one species, the gastropod Ilyanassa obsoleta, was
found in significantly higher numbers outside cages.

     For most bufflehead prey species, the magnitude of the observed
differences between treatments did not increase with the duration of
the experiment, as indicated by a Wilcoxon test comparing these trends
between January and March samples (Table 11).  However, abundances  of
five prey species were significantly greater inside cages in March  but
not in January, and the reverse was true for two prey species.

     Determinations of plant biomass indicated that the cage structure
may have had a negative impact on plant survival and/or growth  (Table
12).  Orth et al. (1979) reported lower biomass values for Zostera  in
winter months, and a similar decline was observed from October  to
January in uncaged cores.  However, biomass of vegetation inside cages
was reduced to a greater degree, and the difference was significant
(p<0.001) in March.  Cages were observed to be badly fouled with
macroalgae and hydrozoans at that time.

     Differences in percent sand and silt-clay were not apparent
between treatments in January or March (Table 13).  Sediments were
fine sands, with less than 15% silt-cl?y.

Consumption Rates

     Total consumption estimated from waterfowl density in  1978-79  and
1979-80 amounted to 11.67 and 1.70 g AFDW nT? respectively, over the
entire area censused (Tables 14 and 15).  In 1978-79 vegetation i*as
the predominant waterfowl food, according to the general food
preferences of abundant species.  Foraging Canada geese removed
approximately 8.26 g AFDW m~2, or 74% of the total for vegetation.
Brant, redheads, and whistling swans consumed 2.72 g, while the
remaining grazers ate an estimated 0.18 g AFDW m~2.  If only the
vegetated shallows are considered (approximately half the total area)
the adjusted estimate for consumption of vegetation becomes 21.44 g '
m~2.  Of the total for animal material consumed by waterfowl in 1979;
buffleheads and red-breasted mergansers consumed 92%, or 0.28 and 0.21
g AFDW m~2, respectively.

     In 1979-80, plant and animal foods were consumed in roughly equal
proportions, although total consumption was an order of magnitude
lower than in the previous year, reflecting primarily the absence of
Canada geese.  Redheads were the only important grazing species,
removing 0.76 of the 0.88 g AFDW m~^ vegetation consumed over the
entire area.  Buffleheads and scaup were the only other abundant
waterfowl, and together consumed 0.76 g of animal material per m  .
                                   199
-------
                                                                                    "1
TABLE 11.  ABUNDANCES OF PREY SPECIES WHICH SHOWED SIGNIFICANT
           DIFFERENCES BETWEEN TREATMENTS IN JANUARY OR MARCH 1980
           (INDICATED BY *).  Ug COMPARES THE MAGNITUDE OF THESE
           DIFFERENCES OVER ALL SPECIES ACROSS SAMPLE DATES.
           MEANS AND STANDARD ERRORS OF THE MEAN.
VALUES ARE

JANUARY
NO CAGE CAGE
Crepidula convexa
Pyranidellidae
Bittium varium
Astyris lunata
Erichsonella attenuata
Anadara transverse
Edotea triloba
Acteocina canaliculate
Gammarus mucronatus
Idotea balthica
Acteon punctostriatus
Balanus improvisus
Paracerceis caudata

22690
±1937.7
200
±81.0
255
±47.9
92
±21.5
370
±31.9
169
±33.9
427
±99.3
57
±27.6
866
±266.8
373
±30.0
70
±19.5
99
±28.3
204
±19.1
28254*
±1643.7
825*
±254.1
519**
±70.6
631***
±166.7
796**
t!74.1
306**
±30.5
936**
±113.7
121 n.s.
±51.5
573 n.s.
±70.3
675**
±82.6
102 n.s.
±22.2
213 n.s.
±52.2
201 n.s.
±35.2
Uq = 88. C
MARCH
NO CAGE CAGE
12230
±1171.6
328
±88.3
150
±60.5
51
±20.3
382
±71.7
80
±15.2
946
±83.9
22
±13.5
940
±101.4
248
±46.9
54
±25.1
48
±13.6
89
±20.6
n.s.
21540***
±1761.0
468 n.s.
±88.4
271*
±48.5
541***
±147.2
573*
±87.8
188
±36.6
1306*
±179.2
121
±34.5 '
1436*
±175.6
338 n.s.
±53.3
194**
±34.4 '
140*
±36.8
201*
±44.3
                                   200
-------
    w
%
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                                                                               CM 
                                                                              O rH
                                                                               O en
                                                                              CTi OO
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                                                  O cs| f)
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                                                           201
-------
TABLE 13.  COhPOSITION OF SEDIMENTS SAMPLED IN JANUARY AND
           MARCH 1980, FROM CAGED AND UNCAGED ZOSTERA.
           DIFFERENCES WERE TESTED BY A T-TEST, ON ARCSIN
           TRANSFORMED PERCENTAGES.
                        Sand
                Uncaged
  % Silt and Clay
Uncaged      Cagec

January
N=3






X
S
t
91.36
91.64
89.24
90.75
1.315
1.76
92.09
92.68
92.31
92.36
0.297
n.s.
8.64
8.35
10.76
9.25
2.329

7.91
7.32
7.69
7.64
0.638

                                                                                n
March
  N=5
             X
             S
             t
93.68
89.76
88.83
89.33
92.45
90.81
2.129
2.00
89.81
88.23
86.04
89.25
90.03
88.67
1.630
n.s.
  6.32
 10.24
 11.17
 10.67
  7.55

  9.19
  2.259
10.19
11.77
13.96
10.75
 9.97

11.32
 2.981
                            202
-------
TABLE 14.  ESTIMATES OF CONSUMPTION BY WATERFOWL AT VAUCLUSE  SHORES,
           1978-1979, BY PREDOMINANT FOOD TYPE.

Canada goose
Brant
Redhead
Whistling swan
American wigeon
Pintail
Black duck
Mallard
Daily
Consumption
g AFDW ind"1
193.6
120.6
83.3
308.1
62.1
73.0
85.8
85.8
Vegetation (over
Mean
Abundance
100 ha"1
(total habitat)
284.3*
46.1
44.2
29.9
12.0
2.7
2.2
0.3
total habitat)
(over vegetated shallows)
Bufflehead
Red-breasted merganser
Common goldeneye
Scaup spp.
Surf scoter
40.6
73.0
73.0
73.0
46.1
18.9
2.2
0.9
75.6 0.4
Invertebrates/Fish (over total
habitat)
Annual
Consumption
g AFDW m~2
8.26
0.83
0.55
1.34
0.11
0.03
0.03
<0.01
11.15 g
21.44 g
0.28
0.21
0.02
0.01
<0.01
0.52 g
* Foraging geese only.
                                 203
                                                                                     I ,
-------
TABLE 15. ESTIMATES OF CONSUMPTION BY WATERFOWL AT VAUCLUSE SHORES,
1979-1980, BY
PREDOMINANT FOOD TYPE.




Redhead
Brant
American wigeon
Whistling swan
Canada goose
Pintail
Black duck

Bufflehead
Scaup
Red-breasted merganser
Surf scoter
Horned grebe
Oldsquaw
Common goldeneye
Common loon


Mean
Daily Abundance
Consumption 100 ha"-*-
(g AFDW ind"1) (total habitat)
83.3 60.1
120.6 1.8
62.1 1.6
308.1 1.5
195.6 0.4
73.0 0.3
85.8 0.1
Vegetation (over total habitat)
(over vegetated habitat)
40.6 96.1
73.0 15.0
73.0 3.1
75.6 2.2
0.9
59.3 0.3
73.0 0.3
<0.1
Invertebrates/Fish (over total
habitat)
(over vegetated habitat)
(over Zostera only)

Annual
Consumption
(g AFDW nT2)
0.76
0.03
0.01
0.07
0.01
<0.01
<0.01
0.88 g
1.19 g
0.59
0.17
0.03
0,03
<0.01
<0.01
<0.01
<0.01
0.82 g
1.09 g
3.32 g
204
-------
Consumption by all other species totalled only 0.18 g AFDW a"* over
all habitat zones.  Because the distribution of birds within these
zones was recorded consumption of plant and animal foods was also
calculated over the vegetated area (for all species) and the Zostera
zone (for non-grazers).  Utilization of the bare sand area was
negligible and thus consumption rates are higher per -or of vegetation
than when averaged over the entire habitat.  Consumption of animal
foods in the Zostera zone was approximately three times the rate
averaged over all zones, reflecting higher bird densities associated
with Zostera.

     The results of the two methods used to estimate consumption of
invertebrates in Zostera marina in 1980 are compared in Table 16.  The
disparity between measures was greatest in January, whereas in March
the difference was negligible.  Total consumption of six important
prey species amounted to approximately 1.46 g and 1.43 g AFDW m~^ in
January and March respectively by the exclosure method.  Based on
calculations from bird density, buffleheads, scaup and surf scoters
removed 0.59 and 1.42 g of these prey species in January and March
respectively, assuming a similar diet within this habitat for all
three waterfowl species.  Degree of agreement varied for individual
prey speceis, and was generally poorer than between combined values.

     Consumption estimates calculated for March are cumulative, and
should approximate total annual consumption per unit area, for
comparison with the fall standing crop of the same species (Table 16).
Combined ash-free dry weight biomass was approximately 3.1 g in
Zostera in October/November 1979 (data from van Montfrans 1981), or
about twice the amount consumed by waterfowl.

                              DISCUSSION

Patterns of Waterfowl Abundance

     Short term fluctuations in waterfowl abundance are difficult to
interpret, and may relate to changes in conditions on the breeding or
wintering grounds.  Absence of Canada geese from the grassbed in the
second year of this study, following high abundances in 1978-79, did
not simply reflect local changes in wintering populations, as aerial
surveys conducted by U.S. Fish and Wildlife Service and the Virginia
Commission of Game and Inland Fisheries indicated similar abundances
of this species in the Eastern Shore survey zone in both years (F.
Settle, pers. comm.).  Large flocks of geet'e rafting directly offshore
from the study area in 1979-80 also indicaced the presence of a
comparable wintering population.

     The intense foraging activity exhibited by Canada geese at
Vaucluse Shores in 1978-79 is presumably atypical, as the species is
primarily field feeding in the Chesapeake Bay (Stewart 1962, Munro and
Perry 1981).  Factors which influence such short term use of submerged
vegetation are not clear, but possibly reflect the availability and
accessibility of SAV in a given year.  It is likely that when aquatic
                                   205
-------
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                                                                                      1
vegetation is abundant in a localized area, geese may switch from or
supplement field feeding.  Grain fields on the Eastern Shore of
Virginia are often adjacent or very close to beds of submerged
vegetation, and thus a temporary transition would not involve a
redistribution of the population.  This is especially important  for
Canada geese, as wintering flocks are highly organized socially, and
members remain strongly attached to specific feeding and resting
sites (Raveling 1979).                                                                 i

     Goose foraging may have had a negative impact on SAV in the
shallows in 1978-79, discouraging utilization the following year.
However, several authors report comparable or more extensive depletion
of SAV by waterfowl, yet do not infer a significant impact on
vegetation (Ki^rboe 1980, Jacobs et al. 1981).  Alternatively, Ruppia
may have been less abundant in 1980 for reasons unrelated to waterfowl
grazing.  Comparable biomass data are not available for both years,
but researchers in the area noted a visible decline in cover of Ruppia
in the shallows, and low abundance of this species was also reported
in other areas of the Bay in 1980 (R. J. Orth, pers. comm.).  The
decrease in numbers and species of non-diving waterfowl as a group in
1979-80 may also reflect depleted SAV resources in the area, as
non-divers are restricted to very shallow water for feeding and as a
general rule, vegetation is the principle dietary component.

     The importance of the bufflehead at Vaucluse Shores in both years                   <
of this study is consistent with the findings of Perry et al. (1981)                     j
that populations of this diving duck wintering in the Chesapeake Bay                     -t
appear to be stable over the short term, and have shown a long term                      ;
increase in proportion to increases in the flyway as a whole.                          "j
Vegetation comprises a minor portion of the diet of buffleheads, and                     1
declines in SAV have not greatly affected its abundance or                             ,  |
distribution (Perry et al. 1981).  An invertebrate diet increases the                    j
range of suitable foraging habitats available to buffleheads, and this                   |
flexibility may partially account for the relative stability of                        \  j
wintering populations.                                                                 ;  j

                                                                                       !  1
     Species historically more dependent on submerged vegetation, such                 >  >
as brant and redheads, were infrequently observed at Vaucluse Shores                   :  j
but were occasionally very abundant.  Brant.are more typically found                  , '  \
in coastal bays rather than estuaries, and now feed primarily on sea                     f
lettuce (Ulva latuca).  Within the Chesapeake Bay, brant are abundant                    j
only where large areas of Zostera still exist (Stewart 1962).                            I
Redheads still rely on submerged vegetation, and therefore have                          j
declined in the Bay in response to declines in SAV.  As with brant,                      |
they are concentrated only in areas with considerable coverage of SAV,                   j
such as Tangier Sound (Perry et al. 1981).  Sporadic use of the study                    j
area exhibited by these two species thus  reflects a currently patchy                     i
distribution throughout the Bay.-  Whistling swans and wigeon were                      i  \
relatively important in 1978-79 but the following year were nearly                     '  j
absent.  Both species are primarily herbivorous, but whistling swans                   i  I
have recently begun field-feeding and include some animal material in                  '  j
                                    207
-------
the diet, whereas wigeon have not greatly altered food habits (Munro
and Perry 1981).

     In 1978-79 water depth was found to be important in determining
the periodicity (via tide stage) of foraging by Canada geese.  This
relationship undoubtedly results from the behavior of up-ending rather
than diving to obtain food, whereby foraging is restricted to very
shallow water.  Palmer (1976) states that timing of feeding  in brant
is governed by tide stage, food being more accessible at low tide.
Jacobs et al. (1981) also found a relationship between low tide and
numbers of waterfowl foraging in a Zostera noltii bed in the Dutch
Wadden Sea.  The area available to non-diving waterfowl for  feeding is
greatly increased at low tide, especially where the depth gradient is
gradual, as is characteristic of seagrass meadows.

     Tide level had little effect on foraging by waterfowl in the
second season of study, as the most abundant species were diving
ducks, notably buffleheads, redheads and scaup.  Buffleheads will feed
at all stages of the tide in areas where the preferred feeding depth
of 2 to 3 m is not greatly exceeded at high tide (Erskine 1971).
Redheads usually feed at depths less than 2 m, including extremely
shallow water where they will feed as dabbling ducks if they cannot
dive (Palmer 1976).  Scaup forage at comparable depths, and  are
affected by tide level only when feeding grounds are completely
exposed at low tide, in which case they cannot feed (Cronan  1957).  In
the present study the only significant effect of tide on-waterfowl
numbers in 1979-80 occurred in the inshore Ruppia zone, due  to the
fact that the area was often exposed at low tide or covered  by only a
few cm of water, which effectively excluded all waterfowl.   The
maximum depth in the study area at high tide was approximately 2 m,
which is well within the preferred range of the above species.

     The range of temperatures observed had no effect on waterfowl
abundance, as ice formed rarely at the study site.  Open water always
remained in deeper areas and therefore birds could feed throughout
freezing conditions.  Time of day was not an important factor
influencing numbers of birds present in the study area..  Buffleheads
moved in and out of the study area in small groups throughout the day,
and did not exhibit obvious morning flights to the feeding area
typical of many waterfowl species.  Johnsgard (1975) notes that, while
data are few, local movements of buffleheads on the wintering grounds \
are probably limited.                                                  \
                                                                       \
     Waterfowl generally seek shelter from severe winds, which may
account for the observed correlations between wind parameters and
waterfowl numbers.  At most stages of the tide, the sandbar  which
encloses the grassbed acts as a buffer to wave action, especially when
winds fetch across or down the bay.  Shoaling is more extensive at the
extensive at the northern end and thus the sandbar offers more
protection from NNW winds than from winds with a more westerly
component.  When winds are from the east or northeast, the entire
western shore of the peninsula is equally protected and the  study area
                                    208
-------
offers no additional shelter.  The presence of greater numbers of
birds during strong NNW winds therefore reflects the orientation of
the study area and the configuration of the protective sandbar.

     Variation in bird density within the habitat in 1979-80 may be
related to several factors.  Densities were greatest in the Zostera
zone, which approximates the preferred feeding depth of buffleheads
(Erskine 1971) and is also the vegetated area farthest from shore.
Avoidance of the irr,hore sand and Rupfia zones can be partially
explained in similar terms in that these areas are very shallow and
close to shore.  Availability of food may be a more important factor.
Abundances of epifaunal invertebrates were much lower in Ruppia than
in the mixed and Zostera zones (van Mont trans 1981), possibly due to
the shorter growth form and narrower blade width of Ruppia, and also
its patchy distribution within the grassbed.  The bare sand zone
contained even lower numbers of invertebrates, with very few species
of importance to foraging waterfowl.  Nilsson (1969) also  found that
diving ducks in the Oresund fed over dense Zostera marina  in
preference to mixed areas with patchy cover, and that food resources
were less abundant in the latter zones.

Bufflehead Food Habits

     The importance of invertebrates in the diet of buffleheads is
well documented, and small molluscs and crustaceans are tne dominant
prey in salt water habitats.  Weimeyer (1967) found that buffleheads
in the Humboldt Biy region fed primarily on bivalves, crustaceans,
fish and gastropods and that the relative contribution of  these groups
varied between habitats.  Erskine (1971) also emphasized the
importance of crustaceans (mostly decapods and isopods) and molluscs
as bufflehead foods on the wintering grounds.  Nereid worms and
bryozoans were cited as minor components of the diet.  In  these and
other general accounts of bufflehead food habits (Cottam 1939, Stewart
1962, Munro and Perry 1981), diversity of food items is high, whereas
Stott and Olson (1973) found that on the New Hampshire coast, sand
shrimp (Crangon septemspinosa) comprised 75% of the diet of
buffleheads,

     Bufflehead gizzard contents analyzed in this study were dominated
by species which are also abundant members of the epifaunal
communities associated with Ruppia and Zostera, such as Crepidula
convexa and Nereis succinia, suggesting that buffleheads rely heavily
on commonly encountered animals.  This agrees with the findings of
Madsen (1954), who maintained that the diet of most diving duck
species reflects the availability of prey.  Stott and Olson (1973)
also reported a close relationship between foods utilized  by sea ducks
and the abundance of these foods in preferred habitats.  However,
buffleheads in this study exhibited a degree of apparent electivity,
with several species eaten in numbers disproportionate to  their
relative environmental abundances.  Foraging behavior in buffleheads
is probably similar to the closely related goldeneye (Bucephala
clangula), which takes food items singly with a forceps action of the
                                   209
-------
bill  (Pehrsson 1976).   Prey selection is enhanced by such a strategy
and is  limited only  by bill morphology,  visual  acuity,  and energy
.cost.   A major difficulty in demonstrating  electivity is  that the
relationship  between numerical  abundance and  ecological availability
is often unknown.  Madsen (1954)  stated  further that among available
(i.e. abundant) food items, the most  easily obtainable  within size
limits  are  preferred.   Thus positive  selection  may indicate real
preference  or degrees  of availability, and  for  this reason tha term
apparent electivity  is used.

      Crepidula convexa was the  only species which was apparently
selected against by  foraging buffleheads, although it was still the
dominant prey item.   This Hirk-shelled species  lives attached to
vegetation  or hard substrates which,  combined with the  extremely  small
size  of overwintering  individuals (less  than  2  mm average), may make
it difficult  to collect.   Alternatively, some gastropods  may move into
the rhizome layer  in th~  winter when  above-ground vegetation is
reduced (Marsh 1976),  and may be  encountered  infrequently rather  than
avoided by  diving  ducks.

      The gastropod Bittium varium is  also dark  in color,  but is not
firmly  attached to vegetation and is  conical  in shape.  It should
therefore be  more  easily removed  from blades  by predators, although
size  in winter is  comparable to Crepidula convexa individuals. The
dove  shell  Astyris lunata and the bivalve Anadara transv'ersa arc
larger  (3-5 mm) and  therefore more visible, which could explain the
greater importance of  these soecies in the  diet relative  to
environmental abundances.   Selection  of  pyramidellid gastropods is
difficult to  reconcile with the minute size of  individuals (1.6 mm
average) and  the translucent nature of che  shell.   However, species of
the genus Odostomia  are reported  to b^ ectoparasitic on other
invertebrates,  notably^,  varium  (Hyraan  1967),  and this association
should  increase availability.

      Electivity studies inherently assume that  the predator has fed in
the same area where  samples of  prey abundance are taken.   Because
waterfowl are highly mobile, this may not always be true.  In the
present study,  the presence of  Ruppia and Zostera fragments in gizzard
samples, as well as  epifauna characteristic of  the habitat, suggest  (
that  birds  had fed either in the  study area or  in similar vegetated  \
habitats.                                                             !

      Carbon isotope  analysis also indicated the importance of
SAV-associated invertebrates in the bufflehead  diet.   The difference
between the mean 6^c  value for bufflehead  liver tissue and that
predicted from mean  composition of gizzard  contents and prey 6^C
values  was  within  the  1-2 °/oo  variation typically reported for such
comparisons.   However,  the departure  was in the negative  direction
whereas the shift  is usually positive, resulting from metabolic
processes which conserve  '^C (De  Niro and Epstein 1978).   It is likely
that  gizzard  data  used in this  study  to  predict g^C values did not
accurately  reflect the diet, due  to inadequate  sample size or
                                    210
-------
differential digestion of  prey  items.  Gizzard analyses appear  to  have
underestimated the nutritional  contribution of species with more
negative 6^^C values  (primarily suspension feeders) rather than the
softer-bodied polychaetes  and crustaceans which had higher 6*-'C
values.  Barnacles and bryozoans may account  for most of the
discrepancy, as these filter feeders were frequently eaten, but
because only shell fragments remained  in the  gizzard, proportional
contribution to total 6^C could not be calculated.
     Intraspecific variability  in bufflehead 6C values  (3.2  °/oo
range) exceeded that suggested  by Fry et al. (1978)  for animals having
the same diet «1.6 °/oo).  However, the low standard deviation
obtained suggests that  individuals did not vary widely in food habits,
at least with respect  to broad  trophic groups.  The  greater
variability in S^-^C values of food items and species composition of
gizzard contents emphasizes the value of time-integrated  data  when
describing food habits  of species with highly mixed  diets.
          analysis confirmed the minor role of submerged vegetation  in
the diet of buffleheads and most other waterfowl sampled.  With few
exceptions, waterfowl values were several paits per mil lower  than
those for Zostera and Ruppia, with considerable overlap between
species having known preferences for vegetation (Canada geese, wigeon,
pintails, black ducks) and the remaining species which rely more on
animal foods.  It is likely that terrestrial sources (especially
agricultural grains such as corn and wheat) provide a large portion  of
vegetation eaten by Canada geese and possibly black ducks, as  these
plants are highly negative in £13C values (De Niro and Epstein 1978).
Slightly more positive values exhibited by wigeon and pintails suggest
a more substantial contribution by aquatic vegetation.  Values for
species with predominately animal diets were generally more negative
than those for buffleheads, implying greater importance of suspension
feeders or planktivorous fish.

Waterfowl Consumption Estimates

     Submerged vegetation was an important resource for wintering
waterfowl (primarily Canada geese) at Vaucluse Shores in 1978-79.  If
80 g AFDW m~2 is considered a maximum early winter biomass value for
Ruppia and stands of mixed Rupfja and Zostera, (R. J. Orth, unpubl.
data) then waterfowl removed 2i>% of the standing crop in shallow water
at the study site.  A comparison of this estimate with those from
other studies is attempted in Table 17, by standardizing all reported
values to percentages of standing crop biomass, and restricting
examples to studies conducted in the non-growing season.  From these
data, it is evident that the impact of waterfowl grazing varies widely
among habitats and with waterfowl species composition and density.  At
Vaucluse Shores, grazing pressure was moderate in 1978-79 and minimal
the following year, relative to previous estimates.

     Apart from variable research conditions, a major difficulty with
such comparisons is that consumption is often averaged over a  large
                                   211
-------
TABLE 17.  REPORTED OR CALCULATED ESTIMATES OF WATERFOWL GRAZING PRESSURE
           (% OF STANDING CROP CONSUMED) IN SAV HABITATS.
      References
Habitat and Location
   Estimated
Grazing Pressure
Ranwell and Downing (1959)    Zostera nana
Sincock (1962)
Steiglitz (1966)
Zostera hornemanniana           30-75%
Scolt Head Is.,  England

Submerged Aquatics              20%
Back Bay, VA and Currituck
Sound, NC

Halodule wrightii               32%
Cornelius (1977)
Jupp and Spence (1977)
Verhoeven (1978)
Ki«*rboe (1980)
Jacobs et al. (1981)
Wilkins (1982)
(This study)
Ruppia maritima
Apalachee Bay, FL
Halodule beaudettei
Laguna Madre, TX
Potamogeton spp.
Loch Leven, Scotland
Ruppia cirrhosa
Texel, Netherlands
Submerged Aquatics
Ringkfibing Fjord, Denmark
Zostera noltii
Dutch Wadden Sea
Ruppia maritima
Zostera marina
Chesapeake Bay, VA
4%
13%
21%
50%
50%
25%
                                    212
-------
area,  ignoring within-habitat variations  in resource  use.   Jacobs  ct
al. (1981) found  that grazing pressure by geese and wigeon  was  not
uniform  in Zostera noltii, and was directly proportional to initial
percent  cover of  vegetation.  In the present study, bird densities,
and therefore consumption rates, were much higner  in  the vegetated
area than in the  total habitat.  Foraging by Canada geese was
restricted to the shallows,  further  increasing consumption  estimates
in those areas.   Variable consumption rates within a  given  habitat
have also been reported  for  wading birds  (Wolff et al. 1975) and
diving durks (Nilsson 1969), emphasizing  the need  to  partition
consumption within a habitat before  attempting to  estimate  impact  on
benthic  communities.

     The results  of exclosure experiments carried  out  in 1979-80
suggest  that waterfowl had a significant  effect on the abundances  of a
number of invertebrate species in the Zostera zone.   By 19  March, when
exclosures were removed; both estimates indicated a consumption of
nearly 50% of the combined ash-free  dry weight standing crop of six
important bufflehead prey species.   Qualitative agreement was obtained
between  the results of caging experiments and bufflehead gizzard
analyses, in that species most affected were also  important prey
items.   However,  caging  results obtained  in January are diffcult to
interpret on the  basis of waterfowl  foraging alone, with respect to
these dominant prey species.  Consumption calculated  from exclosure
samples was much  higher  than that based on bird density, and was
within 0.03 g of  the estimate for March.  Waterfowl densities were
comparable over the two  intervals, and one would expect an  increased
difference between treatments in proportion to the number of days
between sampling  periods.

     In  studies where cages  are used to exclude predators,  the
possibility of an artificial cage effect  must always be considered.
Larval settlement is enhanced by the current-baffling  effect of the
cage structure, and has  been a major problem in previous caging
experiments in soft-bottom habitats  (Virnstein 1981).  This effect was
not demonstrated  by sediment analyses in  this study, although pipette
analysis may not  have detected slight changes in the silt and clay
fractions.  Increased sedimentation would have been expected from the
degree of fouling that reduced the effective mesh size of the cages.1
In this habitat,  however, few invertebrates which were significantly :
more abundant inside exclosures have free-swimming larval stages, and1
recruitment should not be affected by current velocity.  Crepidula
convexa exhibits  direct  development  of larvae, with individuals
hatched as juvenile snails (Ament 1979).  The same is probably  true
for the gastropod Astyris lunata, and peracarid crustaceans are known
brooders (Barnes  1980).

    • The prosobranch gastropod Bittium varium has a planktonic  veliger
larva, as does the bivalve Anadara transversa, but it is unclear
whether reproduction continues into  the fall.  Marsh (1970) reported
egg masses of j^.  varium  in May and June in a Zostera bed in the lower
York River, with  juveniles predominant through the late summer  and
                                    213
-------
fall.  Newly set individuals (0.5-0.7 mm) were not found in field
collections at Vaucluse Shores in September 1979 (J. Lunz, pers.
comm.) although bufflehead gizzard samples contained some individuals
less than 1.0 mm.  Information on the reproductive cycle of A^
transversa was not available, but Marsh (1970) reported peak densities
in August poss;bly indicating larval settlement.  High densities of
these two species may be related to the effect of the cage structure,
but only if recruitment occurred after mid-October when exclosure
experiments began.  The high abundances of Mytilus juveniles in caged
samples in March was almost certainly induced by the cage structure,
as planktonic larvae are produced in early spring in the Chesapeake
Bay, and Mytilus was not recorded in gizzard contents.  The reverse
trend for Ilyanassa obsoleta (higher numbers outside cages) may also
be an artifact of the experiment, as I_. obsoleta are attracted to
artificial structures in order to deposit egg capsules and would
therefore be found at the edges of the cages rather than in the                        i
sampled area (R. Orth pers. comm.).                                                    S

     The above comparisons between estimates of waterfowl consumption                  ;
are made with caution, as confidence intervals on each estimate are                    )
very broad and many assumptions are involved in calculations.                          j
However, 1979-80 data suggest a range of values for annual consumption
of invertebrates of approximately 2-3 g ash-free dry weight m   in
Zostera marina, with lower values for the total habitat.

     Few previous studies provide comparable estimates of the impact
of waterfowl on invertebrates.  Nilsson (1969) calculated that diving
ducks consumed 9% of the total standing crop of invertebrates, or 22 g
fresh weight m~^j in the most heavily utilized part of the habitat.
If this quantity is converted to ash-free dry weight and only the
standing crop of prey species considered, the resulting values would
probably be within the range obtained in this study.

     Consumption by waterfowl at Vaucluse Shores was undoubtedly low
relative to total standing crop biomass and annual production of
invertebrates, but it was shown that significant cropping of dominant
prey species occurred.  Given the predominance of very small food
items in the diet of buffleheads, this habitat represents an optimal
feeding ground for the species, as the density and diversity of
invertebrates are higher than in unvegetated areas.  This research
suggests that the interaction between waterfowl and the benthic fauna
of SAV ecosystems is of greater trophic importance than has been
previously recognized.  Further long-term studies are required to more
clearly define the role of non-grazing waterfowl in SAV habitats, and
to determine and interpret patterns of direct utilization of submerged
vegetation by grazing species.

                                SUMMARY

1.  Canada geese were the dominant waterfowl at Vaucluse Shores in
    1978-79, averaging 526 birds per 100 ha.  Foraging by this species
    was influenced by tide level, with greatest activity around low
                                   214
-------
    tide.  An estimated 21.4 g AFDW ra~^ of vegetation was removed by
    grazing waterfowl during the season, if bird density calculations
    are based on shallow vegetated areas.  This represents
    approximately 25% of the estimated fall standing crop of
    vegetation.

2.  The following year (1979-80), the waterfowl community in the study
    area was dominated by diving ducks, primarily buffleheads.  Canada
    geese and other non-diving species were nearly absent, although
    local wintering populations were much the same size as in the
    previous year.  Reasons for this marked contrast are unclear, but
    intense grazing in 1978-79 may have reduced the availability of
    vegetation in the shallows, or a decline in Ruppia biomass
    unrelated to waterfowl activity may have discouraged foraging in
    the study area in 1979-80.

3.  In 1979-80, daily patterns of waterfowl abundance were influenced
    by wind parameters, whereas tide level, temperature, and tine of
    day had little or no effect.

4.  Differential waterfowl use of areas within the SAV habitat vas
    found to occur in the 1979-80.  Bird densities were greatest in
    the Zostera and mixed vegetation zones, and minimal in RujpLf. and
    bare sand areas.  The latter areas are very shallow and contain
    lower densities of invertebrates, and would therefore be less
    attractive to foraging buffleheads.

5.  Bufflehead gizzard analyses indicated the importance of small
    gastropods such as Crepidula convex';, peracaridan crustaceans such
    as Erichsonella attenuata and the polychaete Nereis succinea in
    the diet of this diving duck.  Predominant food items were also
    abundant members of the grassbed epifauna, although some evidence
    for selectivity was found.  Carbon isotope analysis generally
    supported conclusions regarding bufflehead diet.  Variability in
    bufflehead S  C values was low compared to the range obtained for
    food items, indicating a similar diet among individuals.  These
    analyses confirmed the minor role of submerged vegetation as a
    direct food source for buffleheads and other waterfowl in the area
    in 1979-80.

6.  Exclosure experiments yielded estimates of consumption of
    invertebrates which compared well with calculations based on bird
    density in March, and annual consumption in Zostera was estimated
    at 2-3 g AFDW m~ .  Approximately 50% of the fall standing crop cf
    six important prey species was removed by foraging waterfowl in
    1979-80.

7.  These data suggest that waterfowl foraging may be an important, if
    unpredictable, component'of energy flow in SAV habitats in winter
    months, both from direct consumption of vegetation and predation
    on associated epifaunal invertebrates.
                                    215
-------
                           LITERATURE CITED

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Ameni, A. S.  1979.  Geographic variation in relation to life
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Anderson, R. R. and R. T. Macomber.  1980.  Distribution of submerged
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Barnes, R. D.  1980.  Invertebrate Zoology.  Saunders College/Holt,
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Bayley, S., V. D. Stotts, P.  F. Springer and J. Steenis.' 1978.
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Bent, A. C.  1923.  Life histories of North American Wildfowl.
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Charman, K.  1977.  The grazing of Zostera by wildfowl in Britain.
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Cornelius, S. E.  1977.  Food and resource utilization by wintering
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Cottam, C.  1934.  The eelgrass shortage in relation to waterowl.
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Cottam, C.  1939.  Food habits of North American diving ducks.  U.S.
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                                   216
-------
Cottarn, C. and D. A. Munro.  1954.  Eelgrass status and environmental
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Cronan, J. M.  1957.  Food and feeding habits of scaup in Connecticut
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Cummins, K. W. and J. C. Wuycheck.  1971.  Caloric equivalents for
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Erskine, A. J.  1971.  Buffleheads.  Canadian Wildl. Serv. Mongr.
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Gabriel, W. L.  1978.  Statistics of selectivity.  In:  S. J. Lipovsky
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Raines, E. B.  1976.  Stable carbon isotope ratios in the biota,
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Haines, E. B. and C. L. Montague.  1979.  Food sources of estuarine
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Hyman, L. H.  1967.  The Invertebrates:  Vol. VI.  Mollusca I. McGraw-
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Jacobs, R. P. W. M., C. den Hartog, B. F. Braster, and F. C.
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Johnsgard, P. A.  1975.  Waterfowl of North America.  Indiana
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                                   217
-------
Jupp, B. P. and D. H. Spence.  1977.  Limitations of macrophytes  in a
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Ki^rboe, T.  1980.  Distribution and production of submerged
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Marsh, G. A.  1970.  A seasonal study of  Zostera epibiota in the
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Marsh, G. A.  1976.  Ecology  of the gastropod epifauna of eelgrass in
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Munro, R. E. and M. C. Perry.  1981.  Distribution and abundance  of
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Nienhuis, P. H. and E. T. van lerland.  1978.  Consumption of
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Nilsson, L.  1969.  Food consumption of diving ducks wintering at
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                                   21P
-------
Parker, P. L., E. Wra. Behrens, J. A. Calder and D.  Shultz.
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Perry, M. C., R. E. Munro and  G. Michael Haramis.   1981.  Twenty-
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Perry, M. C. and F. M. Uhler.  1976.  Availability  and utilization of
     canvasback food organisms in r.he Chesapeake  Bay.  Spring Mtg.
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Raveling, D.  1979.  Traditional use of migration and winter roost
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Stieglitz, W. 0.  1966.  Utilization of available foods  by diving
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Stott, R. S. and D. P. Olson.  1973.  Food-habitat  relations of sea
     ducks on the New Hampshire coastline.  Ecology 54(5):996-1007.

Swanson, G. A., G. L. Krapu, J. C. Bartonek, J. R.  Serie, and D. H.
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Thayer, G. U., S. M. Adams and M. W. LaCroix.  1975.  Structural
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                                   219
-------
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Van Montfrans, J.  1981.  Structural analysis of benthic communities
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Verhoeven, J. T. A.  1978.  Natural regulation of plant biomass in a
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Virnstein, R.  1980.  Measuring effects of predation on bfinthic
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Weimeyer, S. N.  1967.  Bufflehead food habits, parasites and biology
     in northern California.  M. S. Thesis.  Humboldt State Coll.
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Wetzel, R. L., K. L. Webb, P. A. Penhale, R. J. Orth, D. F. Boesch,
     G. W. Boehlert, and J. V. Merriner.  1979.  The functional
     ecology of submerged aquatic vegetation in the lower Chesapeake
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Wolff, W. J., A. M. M. van Haperen, A. J. J. Sander, H. J. M. Baptist
     and H. L. F. Saejis.  1975.  The trophic role of birds in the
   ,  Grevelingen estuary, The Netherlands, as compared to their role
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                                   220
-------
                     CHAPTER 7

TROPHIC RELATIONSHIPS IN A SUBMERGED MACROPHYTE BED
               BASED ON 6l3c VALUES


                        by

               Jacques van Montfrans

                        and


                  Robert J. Orth
                                                                             j

                                                                             M
                                                                             ;f
                         221                                                !   i
                                                                              J
-------
                                   ABSTRACT

     Trophic relationships in a lower eastern  shore Chesapeake  Bay (Vaucluse
Shores at the mouth of Hungars Creek) seagrass bed were  investigated  by
examining time integrated stable carbon  isotope  ratios  C^C/^c)  in primary
producer and consumer populations.  The  periphyton grazing  snail,  Bittium
varium exhibited close ties to the microalgae  found on Zostera  marina leaves.
Dominant isopods (Erichsonella attenuata and Idotea baitica) were  more closely
linked to the seagrasses themselves.  In several  other  invertebrate and
vertebrate species trophic relationships were more obscure  although these  will
be more closely examined in a forthcoming publication.   Overall,  carbon
isotope analysis appears promising as a method for elucidating  general trophic
relationships in seagrass communities.
                                      222
                                                                                           ,J
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                                  INTRODUCTION

     The natural proportions  of  two  stable  carbon isotopes, ^C (1.1  °/o) and
*-*G (98.9 °/o) are  fractionated  differentially by the  various  functional
groups of primary producers depending  in pav t  on their photosynthetic pathway
(Thayer et al., 1978).  Vascular  plants  with  the C^ metabolic  pathway tend to
incorporate the *3C isotope t.o a  greater degree than  those having a €3 pathway
(Hatch and Slack, 1970; Black, 1971; Welkie and Caldwell,  1970).   For the
purposes of comparing  carbon  isotope ratios in plant  and animal tissues and
those of inorganic  substances, the 5 (delta)  13C index is  used and is defined
as:
         .„               /    (13C/12C) in sample  	     \
        61JC(°/oo) «      {-rr:——	    -1    X 1000
                          \UJC/12C) £n  standard (Chicago  PDB)     /

     Carbon isotope ratios fixed  by  plants  remain relatively constant in both
living and decomposing plant  tissue  (De  Niro  and Epstein,  1978; Haines and
Montague, 1979).  This ratio  (i.e. 8^  C  value) is maintained in a near
one-to-one correspondence when transferred  to  herbivores specialized  for
feeding on a particular plant source and subsequently  to higher trophic levels
through carnivory or omnivory (Fry et  al.,  1978;  De Niro and Epstein, 1978).
Because 6  C values can remain relatively unchanged  throughout various trophic
levels, consumer tissue S  C values  reflect the organisms  time-integrated
dietary history.  Thus, herbivores and their  predators should  reflect a narrow
range of 6  C values characteristic  of the  original  plant  substrate fed upon
whereas species with a general feeding habit will have a broader  range of
values.

     The primary producers which  supply  organic carbon for utilization by
marine organisms such  as those found in  Chesapeake Bay grass beds include:
seagrasses and fringing C^ marsh  plants  such  as Spartina alterniflora
with 613C values from  -9 to -13 °/oo (Thayer et al.,  1978; Haines, 1976);
benthic microalgae, mostly diatoms,  with values from -16 to -18 °/oo  (Haines,
1976); phytoplankton with ratios  of  -20  to  -26 J/oo  (McConnaughey and McRoy,
1979; Haines and Montague, 1979); C$ photosynthetic plants showing values of
-24 to -29 °/oo (Haines and Montague,  1979); and algae (no distinction between
macro- and microalgae) with ratios ranging  from -12  to -23 °/oo (Haines,
1976).  Clues to the origin of those organic carbon sources should appear in
tissue 6*3C values of  the major grass  bed utilizers and therefore shed light
on the trophic structure of the grass  bed community.   The  purpose of  this
section is to report on the preliminary  results of trophic interactions based
on 5*3C values found in primary producers as well as  secondary resident and
migratory consumers of the Vaucluse  Shore grass bed.   A more complete analysis
and presentation of these data will  be forthcoming.
                                        223
                                                                                          .—J
-------
                              METHODS  AND  MATERIALS
     Tissue samples for   C/C  carbon  ratio  (6C)  analyses  were collected
throughout the summer of  1978  and  1979.   Additional  samples  of waterfowl were
collected during the winter  of 1979-80.   Plant  material  was  carefully checked
for epiphytes or epifauna which  were  removed  by scraping or  brushing prior to
drying.  Both the macrophyte and attached material were  saved for analysis.
Benthos and fish had their guts  removed  or were held in  screened containers in
aquaria for 24 hr to permit  the  voiding  of gut  contents. Specimens of resident
consumers and predators were grouped  by  size.   A special effort was made to
examine changes in 6  C values with growth.   Shelled animals  were treated with
10 °/o HC1 prior to analysis to  remove  carbonate shell  fragments.  Waterfowl
liver tissue was collected from  freshly  killed  birds in  the  grass bed.   All
tissues collected were then  dried, ground to  a  fine  powder with a mortar and
pestel or Wiley mill and distributed  to  consultants  for  further analyses.
Some tissue samples were analyzed  for S^C values by Dr. Evelyn Hair.es  of the
University of Georgia Institute  of Ecology, Athens,  Ga.   The  majority were
analyzed by Drs . James Winters and Patrick Parker of Coastal  Science
Laboratories, Inc., Port Aransas, Texas.

                             RESULTS AND  DISCUSSION

     A wide variety of grass bed associated species  representing different
trophic levels and several feeding modes  were analysed  for <5^C values  (Table
1).  The primary producers in  the  system had  a  wide  range of  values.
Macroalgae exhibited 6^C values between  -16  and -19 °/oo, epiphytic
microalgae around -11.2 °/oo and submerged macrophytes had higher 6  C values
of between -7 and -10.6 °/oo.  Additional  food  sources sampled in the system
which include or originate from  primary  producers are periphyton (i.e.
microalgae and associated microfauna  and  detritus) found growing on the plants
with 6l-*c values of -18.3 and  particulate detritus having 6"c values of -16.3
°/oo .  Three sources of primary  production with potential carbon input  that
were not sampled in this study bu1: for which  literature  values exist include
benthic algae, (mostly diatoms,  with  carbon isotope  ratios between -16  and -18
°/oo, Raines, 1976), phytoplankton (-20  to -26  °/oo,  McConnaughey and McRoy,
1979) and the fringing marsh vegetation  consisting of Spartina alternif lora
(-9 to -13 °/oo, Haines, 1976).  Some of  these  carbon sources can be detected
in the invertebrates and vertebrates  which inhabit or feed in the grass bed.
For example a hydrozoan feeding  on zooplankton  had a value (-20.5 °/oo) very
near that reported for phytoplankton  based food webs  (-20 to  -26 °/oo,    i
McConnaughey and McRoy, 1979).   Similar  plankton based food sources were
indicated for other filter feeders such  as adult epifaunal Crepidula convexa
(-20.2 °/oo) and My a arenaria  from the York River (-20.2 °/oo) as well  as
red-breasted mergansers (-20.8 °/oo)  which consume primarily  planktivorous
fishes.  Interestingly, several  infaunal  bivalve filter  feeders showed
somewhat higher than expected  6^C values  ( -15.5 °/oo)  indicating perhaps the
incorporation of some SAV detrital carbon.
     The grazing gastropod, Bittium varium had  6^-^C values  (-13.4  °/oo)
approximating those of microepiphytes  (-11.2 °/oo) which  confirm  the
utilization of eelgrass associated diatoms by 15. varium.   Species  which  show
close trophic ties with SAV (S^C of -7.1 to -10.6 °/oo)  include  the  isopods
                                       224
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Erichsone 11 a attenuate-;  (-7.9  to -12.2  °/oo)  and  Icotea baltica (-8.7 to
-14.0 °/oo).

     Many of the remaining  invertebrates  showed  intermedite  ^/C values
ranging from between -11  to -17 °/oo.   A  complete  interpretation of trophic
relationships among these species must  await a more  detailed analysis of the
data although at first  glance  these values  imply considerable utilization of
submerged macrophyte derived  carbon.

     Trophically important  fishes in the  grass bed included pipefish
(Syngnathus fuscus), silver perch (Bairdiella chrysura)  and spot (Leiostomus
xanth'Tus) with 6  C values from -13.1  to -17.0  °/oo.   Other migratory
consumers utilizing the grass  bed included  numerous  species of waterfowl with
a wide range of 6*->C values between species.   Lowest 6^C ratios were seen in
the American coot (-25.0 °/oo), Canada  goose (-21.6  °/oo in one individual)
and red-breasted merganser  (-20.8 °/oo).  The low  values observed for the
former two species can  possibly be explained by  feeding  on agricultural
grains, primarily corn, which  is readily  available in  nearby fields.  The
latter species, as already  explained,  feeds  on planktivorous fish reflecting a
plankton based food source.  Buffleheads, for which  a  large number of liver
saftples were obtained,  exhibited a fairly narrow range of vaiaes (-15.8 to
-18.5 °/oo).  These values  closely correspond with those for the invertebrates
which were important food items in bufflehead gizzards.

     Although there was inadequate time to  fully discuss the implications of
our observed values, we expect to publish these  results  foil-owing a more
complete evaluation and detailed comparison  with other pertinent studies on
this topic .
                                      230
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                                     232
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