PD89-134548
Structural and Functional Aspects of the
Ecology of Submerged Aquatic Macrophyte
Communities in the Lower Chesapeake Bay
Volume 1. Studies on Structure and
Function of a Temperate, Estuarine
Seagrass Community: Vaucluse Shores
Lower Chesapeake Bay, Virginia, USA
Virginia Inst. of Marine Science
Gloucester Point
Prepared for
Environmental Protection Agency, Annapolis, MD
U.S. Environmental Protection Agency
Region III information Resourca
Center (3PM52)
841 Chestnut Street
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Aug 82
J
w»V» I^^MrflV l^^^^^W ^W
EPA Report Collection
Information Resource Center
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TECHNICAL REPORT DATA
iPtease rtatl /mrnic com on Ihr rrrmt bt/orr completing!
1 REPORT NO.
^PA/fcOO/3-88/05la
4 TITLE ANosusT.TLESTRUCTUAL~& FUNCTIONAL ASPECTS OF THE
ECOLOGY OF SUBMERGED AQUATIC MACROPHYTE COMMUNITIES IN
THE LOWER CHESAPEAKE BAY. Vol. I: Studies on Structure &
Function of a Temperate, Estuarine Seagrass Community:
3 RECIPIENT'S ACCESSION NO
PB89 184548/AS
8 RCPCHT DATE
August 1982
6 PERFORMING ORGANIZATION CODE
7 AUTMORISI Vane 'use Shores. Lower Chesapeake Bay,
Virginia, USA
». PERFORMING ORGANIZATION REPORT NO
F. Tin., and
» PERFORMING ORGANIZATION NAME AND ADDRESS
1.. Ut.t?»1 , PHn.
10. PROGRAM ELEMENT NO.
Virginia Institute of Marine Science and School of
Marine Science, College of William and Mary,
Gloucester Point, VA 23062
11. CONTRACT/GRANT NO.
R80597* and XOOJ245-01
12. SPONSORING AGENCY NAME AND AODRtSS
EPA, Chesapeake Bay Program
2083 West Street
Annapolis, MD 21401
13. TYPE OF REPORT AND PERIOD COVERED
14. SPONSORING AGENCY CODE
EPA/600/05
is SUPPLEMENTARY NOTES
8. ABSTRACT
There is increasing evidence which suggests that primary production in
both marine and estuarine systems is generally nitrogen limited. Orth (1977)
demonstrated a positive growth response in Zostera marina communities in the
Bay to added commercial fertilizer treatments. These data together with the
consistent and predictable pattern for the depth distribution of seagrasses in
the Lower Chesapeake Bay suggests that light (and/or factors influencing the
quality and quantity of the light regime) and nutrients are principal factors
governing the distribution and metabolism of the submerged aquatic plant
communities.
The overall objectives of the studies reported in the following chapters
of this volume were: 1) to describe structural chracteristics of the plant
community and environmental regimes of a natural, unperturbed SAV community in
the Lower Chesapeake Bay, 2) to derive estimates of community productivity and
metabolism for diel, seasonal and annual periods, and 3) to evaluate potential
mechanisms controlling productiviity and community dynamics. Various studies
and experimental designs were initiated and completed during the period July,
1978 through November, 1981 to accomplish these overall objectives.
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NOTICE
This document has been reviewed in accordance with
U.S. Environmental Protection Agency policy and
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Final Report
STRUCTURAL AND FUNCTIONAL ASPECTS OF THE ECOLOGY
OF SUBMERGED AQUATIC MACROPHYTE COMMUNITIES
IN THE LOWER CHESAPEAKE BAY1
Volume I
Studies on Structure and Function of a Temperate, Eatuarine
Seagraas Community: Vaucluae Shores, Lower Chesapeake Bay,
Virginia, USA
Richard L. Wetzel, Editor
Virginia Institute of Marine Science
and School of Marine Science
College of William and Mary
Gloucester Point, VA 23062
Contract Nos. R805974 and XO03245-01
Project Officer
Dr. David Flemer
U.S. Environmental Protection Agency
2083 West Street
Annapolis, MD 21401
1. Special Report Number 267 in Applied Marine Science and Ocean
Engineering.
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TABLE OF CONTENTS
Page
ACKNOWLEDGEMENTS iv
PREFACE 1
Program Overview 2
Study Site 4
Literature Cited 7
CHAPTER 1. Plant Community Structure, Elemental Composition
and Sediment Characteristics of a Temperate, Eatuarine
Seagrass Ecosystem; Vaucluse Shores, Lower Chesapeake
Bay, Virginia by R. L. Wetzel, and P. A. Penhale and /
K. L. Webb 9 /
/
Introduction 10 /
Methods and Materials 10 A
Results and Discussion 11
Literature Cited 45
CHAPTER 2. Photosynthesis, Light Response and Metabolism of
Submerged Macrophyte Communities in the Lower
Chesapeake Bay by R. L. Wetzel and P. A. Penhale 50
Introduction 51
Methods and Materials 54
Results and Discussion -S
Summary 98
Literature Cited 103
CHAPTER 3. Oxygen Metabolism of a Temperate Seagrass (Zoatera
marina L.) Community: Plant-Epiphyte, Plankton -~~r"
and Benthic Microalgae Productivity and Respiration
by L. Murray and R. L. Wetzel 108
Abstract 109 \ '
Introduction
Study Site
Methods and Materials
Results
Discussion 120
Summary 124
Acknowledgements 126
Literature Cited 127
ii
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TABLE OF CONTENTS (continued)
CHAPTER 4. Preliminary Observations on Nutrient Enrichment and
Light Reduction Effects on Zostcra marina L.
Epiphytic Growth by L. Murray and R. L. Wetzel 130
Abstract 131
Introduction. 132
Experimental Design 133
Results 134
Discussion 144
Acknowledgements 148
Literature Cited 149
CHAPTER 5. Fungi and Bacteria in or on leaves of Eelgrass
(Zoatera marina L.) from Chesapeake Bay by S. Y.
Newell 152
Abstract 153
Introduction 154
Methods and Materials 154
Results 156
Discussion 158
Literature Cited 162
CHAPTER 6. Preliminary Studies on Community Metabolism in a T pical
Seagraas Ecosystem: Laguna de Tenainos, Campeche,
Mexico by R. L. Wetzel, L. Murray, R. F. van Tine,
J. W. Day, Jr. and C. J. Madden 165
Abstract 166
Acknowledgements 185
Introduction 167
Study Area 167
Methods and Materials 169
Results 170
Discussion 181
Literature Cited 186
111
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ACKNOWLEDGEMENTS
For a research program of this size, duration and scope it would be -'
impossible to acknowledge the individual support of all persons associated at
various times with the project. Collectively, I wish to thank the many ;'
colleagues, graduate students and technical staff of the Institute who have <
contributed to the studies and the overall success of the program. Specific ;
thanks are due our Eastern Shore colleagues, Michael Castagna, John Kraeuter •'
and Jim Moor*, without whose efforts and active involvement, the program would ^
not have succeeded. Also, without the local support of many Eastern Shore
residents, particularly Lloyd and Buddy Otten, Jack Watts, Donald Hammon and
Harold Rehm who contributed both the use of personal property and their time,
we would certainly have fallen short in accomplishing many of our goals. -
At various times throughout the program, we have used and relied on
outside program reviews and visiting scientists. Especial thanks are due
Richard G. Wiegert and Steven Y. Newell, University of Georgia, Robert R.
Christian, East Carolina University, Douglas G. Capone, State University of
New York at Stony Brook and Steven Smith and John Harrison, University of
Hawaii, for their participation in the research program. To Gordon fhayer,
National Marine Fisheries Service-Beaufort, John W. Day, III, Louisiana State
University, Scott W. Nixon, University of Rhode Island and, again, Doug
Capone, SUNY at Stony Brook, we offer our thanks tor interim program reviews
and constructive criticisms. I also wish to express my thanks and -
appreciation to Mr. William Cook, U.S. E.P.A. and former project officer of
this program, for his many efforts to see the program successfully completed
and effectively reducing administrative burdens. His support and enthusiasm "
certaily made management of the program much easier.
From a more personal standpoint, I want to acknowledge the unfailing ^
support and dedication of my graduate students, Laura Murray, Robin van Tine,
and Rick Hoffman in these efforts. To Bob Middleton and Bill Rizzo, thanks /
for allowing yourselves to be pressed into service at critical times. Also, , ,
to my colleagues of the "other" Chesapeake Bay, Michael Kemp, Walt Boynton and •/
J. Court Stevenson, University of Maryland, thanks for the many stimulating "'.
discussions, comparison of results and reviews.
Especial thanks are due Ms. Carole Knox and Nancy White for their
secretarial and editorial skills. Their concerted effort in typing, compiling
and editing many drafts of the manuscripts contained herein while maintaining
some sense of organization as well as humor is greatly appreciated. Also, to
our Photographic, Graphic Arts and Word Processing Service Center, thanks to
all for your excellent and quality work. j"
R. L. Wetzel
Program Manager >
-,,v
iv
A
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PREFACE
Richard L. Wetzel
Virginia Institute of Marine Science
and School of Marine Science
College of William and Mary
Gloucester, Point, VA 23062
r
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Program Overview
Various autocrophic populations contribute to total prl=«rv production in
estuarine and coastal marine environments. The distribution, abundance and
relative contribution of each is determined in large part by geomorphology and
physical-chemical regimes of a particular area (Mann, 1975). In open water,
phytoplankton dominate the autotrophic input of organic matter while in
shoal-benthic habitats, algae, seagrasses and marshes contribute to primary
production as well. Unit area comparison of annual biomass production
generally results in the ordering: salt marshes > seagrasses > macro-algae >
phytoplankton for many Gulf of Mexico and U.S. Atlantic Coast estuaries.
However, areal comparisons and ranking vary considerably from system to
system. Coastal, temperate estuaries along the U.S. Atlantic Coast are
generally characterized by inputs from all. The Chesapeake Bay is a good
example.
Partitioning primary production among these various components in the
Chesapeake Bay for comparative purposes is difficult due in large part to the
extreme biological complexity and physical characteristics of the Bay. At
least three mejor subdivisions of the Bay can be identified based on
hydrodynamic regime. These are; I) bay stem, the major open water area, 2)
shoal-benthic areas within the major water body and grossly delineated by the
basin shoreline and extending to a mean aubtidal depth of 2 to 3 meters, and
3) sub-estuarine water bodies consisting of the principal tributaries entering
the Chesapeake Bay. The bay stem is a major open water area and is dominated
by phytoplankton production. Shoal-benthic areas and the tributaries however,
make up a significant part of the estuary and are autotrophically dominated by
benthic inicroalgae and macroalgae, submerged aquatic vegetation, and the
wetland, tidal marshes. Based on gross areal approximations for the Virginia
portion of Chesapeake Bay and estimates of annual net primary production for
each of these components, the shoal-benthic habitat and the tidal marshes
contribute 30-40% of the annual net primary production for the lower
Chesapeake Bay (R. L. Wetzel, unpublished data).
On a smaller spatial scale within shoal-benthic areas, the relative
contribution of submerged aquatic vegetation (SAV) primary production varies
considerably. Orth et al. (1979) has estimated that Virginia SAV occupied a
bottom area of approximately 85.4 km? in 19""3. Using their figure of a 5300
km shoreline length and assuming an average lateral extent of 300m, the
shoal-benthic habitat area would be approximately 1600 km^ or approximately 5Z
of the bottom area. However, their figures included shoreline areas of the
major tributaries where the major portion of the habitat is oligohaline or
tidal freshwater and the two dominant species ot SAV for the lower Bay,
Zostera marina and Ruppia maritima do not exist. Using the present relative
abundance and distribution data for these two species (Orth et al., 1979) and
limiting the estimated shoal-benthic habitats to brackish water and mesohaline
areas, the percentage of shoal-benthic habitat occupied by Zostera-Ruppia
communities would significantly increase.
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The productivity of SAV communities in temperate, U.S. Atlantic coast
estuaries is high (Dillon 1971; Thayer et al. 1975; Penhale 1977). Within SAV
communities typical of the lower Chesapeake Bay, total autotrophic production
is partitioned among several autotrophs: Zostera marina, Ruppia maritima,
epiphytic algae (those attached to seagrass leaves), benthic microscopic and
macroscopic algae, and phytoplankton. A comparison of the productivity
estimates for the major primary producers in an estuarine system near
Beaufort, North Carolina, indicates the importance of aeagraases in shallow
coastal systems and the relative contribution made by other autotrophs. For
this system net annual production (g C m~2 yr~l) were; 66 for phytoplankton
(Thayer 1971), 249 for Spartina alterniflora (Williams 1973), 330 for Zoatera
marina and 73 for epiphytes of Zostera (Penhale 1977). Therefore, at least at
the local level, SAV production is comparatively high and for some systems may
dominate total estuarine autotrophic production (Thayer et al. 1975; Mann
1975). For certain areas of the Lower Chesapeake Bay, e.g. Bayside Eastern
Shore, Mobjack Bay and shoal benthic areas in and around the York River mouth,
SAV primary production is a significant source of fixed energy and organic
matter available to either directly or indirectly support heterotrophic,
secondary production of ecologically and/or economically important species.
Nevertheless, simply comparing production of the various estuarine
primary producers oversimplifies and perhaps underestimates the role and value
of SAV communities in the Lower Chesapeake Bay system as a whole. The SAV
communities add structural complexity and diversity to the shoal-benthic
environment creating habitat for epifaunal, infaunal and motile species and
refuge areas for prey species. In addition to these non-trophic
characteristics, SAV also function in the stabilization of sub-tidal sediments
and probably influence shoreline erosion processes by dissipating both tidal
and wind generated wave energy. SAV communities in the Lower Chesapeake Bay
therefore have both trophic and non-trophic significance for the ecology of
the overall system.
Historical studies of seagrass distribution and relative abundance of SAV
communities in the Lower Chesapeake Bay show periods of major decline followed
by periods of recovery. Eelgrass (Zostera marina) has undergone major changes
in abundance in 1854 and during the period 1889-1894 (Cottarn 1935 a,b). More
recently, major declines in Chesapeake Bay SAV were observed during the
wasting disease of Che 1930's (Cottam and Munro 1954) and during a decline
which began in 1973 and continued until 1978. This recent decline resulted in
the lowest population levels in 40 years (Orth et al. 1979). Despite
documentation of these events, these fluctuations in distribution and
abundance remain largely unexplained.
Geographically, the dominant lower Bay species, Zostera marina, has a
world wide distribution in temperate and subartic regions of the Northern
Hemisphere (den Hartog 1970). The southern range limit along the U.S. East
Coast is North Carolina. This limit is ascribed to high temperature stress
effects. Thus, Z. marina in the lower Chesapeake Say exists very near its
geographical distribution limit.
Ruppia maritime has broader temperature and salinity tolerances
(Richardson 1980 and references therein) which is reflected in the
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geographical distribution of the species. Within Chesapeake Bay, Zo^tera
marina and Ri'ppia maritima are limited tc shoal benthic habitats less than two
meters mean depth (Orth et al. 1979).
Factors vhich regulate productivity of seagrasses have been ascribed to
various environmental parameters. The influence of light, temperature and
salinity have received the major research affort (Biebl and McRoy 1971;
Bachman and Barilotti 1976; Penhale 1977; Congdon and MeComb 19/9; and
references therein). It is generally accepted that the local light regime
limits the subtidal distribution of Zoatera, while light, temperature and
nutrient regimes (primarily nitrogen) interact to control specific rates of
productivity over an annual cycle.
There is increasing evidence which suggests that primary production in
both marine and estuarine systems is generally nitrogen limited (Postgate
1971; Ryther and Dunstan 1971; Valiela et al. 1973; Gallagher 1975; Pomeroy
1975; Orth 1977; Nixon 1980). Orth (1977) recently demonstrated a positive
growth response in Zostera marina communities in the Bay to added commercial
fertilizer treatments. These data together with the consistent and
predictable pattern for the depth distribution of seagrasses in the Lower
Chesapeake Bay suggests that light (and/or factors influencing the quality and
quantity of the light regime) and nutrients are principal factors governing
the distribution and metabolism of the submerged aquatic plant communities.
The overall objectives of the studies reported in the following chapters
of this volume were: 1) to describe structural characteristics of the plant
community and environmental regimes of a natural, unperturbed SAV community in
the Lower Chesapeake Bay, 2) to derive estimates of community productivity and
metabolism for diel, seasonal and annual periods, and 3) to evaluate potential
mechanisms controlling productivity and community dynamics. Various studies
and experimental designs were initiated and completd during the period July,
1978 through November, 1981 to accomplish these overall objectives.
Study Site
Selection of the principal study site was decided by consensus of the
five original principal investigators which was composed of members associated
with different aspects of SAV research within the overall program. A seagfass
bed approximately 140 hectares in size located on the southeastern shore of
Chesapeake Bay was chosen as the principal study area. This area is known
locally as Vaucluse Shores and is situated north of Hungar'a Creek at
approximately 37*25'N latitude, 75'59'W longitude. Criteria used for site
selection were:
1. the site had been previously studied and some background
information was available,
2. the ted is well established and historically stable,
3. the area is relatively remote and unperturbed,
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4. vegetationally, the bed contained the two dominant lower Bay
species, Zoatera marina and Ruppia maritima, and,
5. the bed was large enough to simultaneously acconodate varied
•tudies and sampling regimes.
The bed is roughly triangular in shape with the apex to the north and
bordered on the east by the Vaucluse Shoreline, to the south by the Hungar's
Creek channel and to the west by an off-shore sandbar (Figure 1). The area
was a site for an intensive vegetational mapping program that was completed by
the initiation of our studies in July 1978. During this study permanent
transects were established and are represented in Figure 1 as transect lines
labeled A through F proceeding south to north. These were used in our studies
for selection and identification of within-site sampling stations. The figure
also illustrates the submerged plant distribution and zonal dominance by
species. From these data, five habitat types were indicated and have
subsequently been u.'ed for within habitat sample site selection. These areas
are:
1. Ruppia maritima dominated community
2. Mixed vegetation areas
3. Sand Patches or Unvegetated bottom within the bed
4. Zostera marina dominated community, and
5. Sand Bar.
Sampling sites were selected for each of the areas between transects B and C
and permanently marked with buoys to identify stations for routine sampling
and experimental studies.
The contents of this volume are divided into six chapters. Each is
presented separately with regard to introduction, methods, results and
discussion. The format was adopted to more closely align with our previously
stated overall objectives. The final chapter presents the results of ntudies
in a tropical, Thalassia testudinum seagrass community that was in part funded
through the SAV Chesapeake Bay Program and paralleled in many respects our
studies in the Chesapeake Bay. Techniques and experimental designs used in
the study were developed in the Chesapeake Bay seagrass research program.
This volume constitutes one of three major efforts on the functional ecology
of SAV in the lower Chesapeake Bay (see Brooks, et al., 1981 and Orth, et al.,
1982).
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LITERATURE CITED
Backman, R. W. and D. C. Barilotti. 1976. Irradiance reduction: effects on
crop* of the eelgrass Zostera marina in a coastal lagoon. Mar. Biol.
34:33-40.
Biebl, R. and C. P. MeRoy. 1971. Plasmatic resistance and rate of
respiration and photosynthesis of Zostera marina at different salinities
and temperatures. Mar. Biol. 8:48-56.
Brooks, H. A. et al. 1981. Higher level consumer interactions. Draft final
report, grant no. R805974, U.S. Environmental Protection Agency, Chesapeake
Bay Program, Annapolis, MD. 196 pp.
Congdom, R. A. and A. J. MeComb. 1979. Productivity of Ruppia: seasonal
changes and dependence on light in an Australian estuary. Aquatic Bot.
6:121-132.
Cottarn, C. 1935a. Further notes on past periods of eelgrass scarity.
Rhodora 37:269-271.
Cottarn, C. 1935b. Wasting disease of Zostera marina. Nature 135:306.
den Hartog, C. 1970. The Sea-Grasses of the World. North-Holland,
Amsterdam. 275 pp.
Dillon, C. R. 1971. A comparative study of the primary productivity of
estuarine phytoplankton and macrobenthic plants. Ph.D Dissertation,
University of North Carolina, Chapel Hill. 112 pp.
Gallagher, J. L. 1975. Effect of an ammonium nitrate pulse on the growth
and elemental composition of natural stands of Spartina alterniflora and
Juncua roemerianus in a Gerogia salt marsh. Amer. J. Bot. 62:644-648.
Mann, K. H. 1975. Relationship between morphometry and biological
functioning in three coastal inlets of Nova Scotia, pp. 634-644. In, L.
E. Cronin (ed), Estuarine Research, V.I, Academic Press, N.Y. 738 pp.
Nixon, 5. W. 1980. Between coastal marshes and coastal waters; a review of
twenty years of speculation and research on the role of salt marshes in
estuarine productivity and water chemistry, pp. 437-525. In, P. Hamilton
and K. B. MacDonald (eds.), Estuarine and Wetland Processes, Plenum Publ.
Co., N.Y.
Orth, R. J. 1977. Effect of nutrient enrichment on growth of the eelgraas
Zostera marina in Chesapeake Bay, Virginia. Mar. Biol. 44:187-194.
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Orth, R. J., K. A. Moore and H. H. Gordon. 1979. Distribution and abundance
of submerged aquatic vegetation in the lower Chesapeake Bay. U.S.
Environmental Protection Agency, Report no. 600/8-79-029/SAV1, Chesapeake
Bay Program, Annapolis, MD.
Orth, R. J. and K. A. Moore. 1982. The biology and propagation of eelgrass,
Zostera marina, in the Chesapeake Bay, Virginia. Final Grant Report, U.S.
Environmental Protection Agency, Grant no. R805953, Chesapeake Bay Program,
Annapolis, MD. and VIMS SRAMSOE 265, Gloucester Point, VA. 187 pp.
Penhale, P. A. 1977. Macrophyte-epiphtye biomass and productivity in an
eelgrass (Zostera marina L.) community. J. exp. mar. biol. Ecol.
426:211-234.
Pomeroy, L. R. 1975. Mineral cycling in marine ecosystems, pp. 209-223. In,
F. G. Howell, J. B. Gentry and M. H. Smith (eds.), Mineral Cycling in
Southeastern Ecosystems, NTIS, Conf-74013, Springfield, VA.
Postgate, J. R. 1971. Relevant aspects of the physiological chemistry of
nitrogen fixation, pp. 287-307. In, D. E. Hughes and A. H. Rose (eds.),
Microbes and Biological Productivity, Symp. Soc. Gen. Microbiol., V. 21,
Cambridge Univ. Press, London.
Ryther, J. H. and U. M. Dunstan. 1971. Nitrogen, phosphorus and
eutrophication in the coastal marine environment. Science 171:1008-1012.
Richardson, F. D. 1980. Ecology of Ruppia maritime L. in Mew Hampshire
'U.S.A.) tidal marshes. Rhodora 82:403-439.
Thayer, G. W. 1971. Phytoplankton production and the distribution of
nutrients in a shallow unatratified estuarine system near Beaufort, N.C.
Chesapeake Sci. 12:240-253.
Thayer, G. W., S. M. Adams, and M. W. LaCroix. 1975. Structural and
functional aspects of a recently established Zostera marina community, pp.
518-546. In, L. E. Cronin (ed.), Eatuarine Research, V. 1, Academic Press, /'
N.Y. 738 pp.
Valiela, I., J. M. Teal, and W. Sass. 1973. Nutrient retention in salt marsh
plots experimentally fertilized with sewage sludge. Est. Coastal Mar. Sci.
1:261-269.
Williams, R. B. 1973. Nutrient levels and phytoplankton productivity in the
estuary, pp. 59-89. In, R. H. Chabreck (ed.), Proc. 2nd Symp., Coastal
Marsh and Estuary Management, Louisiana State University, Baton Rouge.
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Chapter 1
PLANT COMMUNITY STRUCTURE, ELEMENTAL COMPOSITION AND SEDIMENT
CHARACTERISTICS OF A TEMPERATE, ESTUARINE SEAGRASS ECOSYSTEM;
VAUCLUSE SHORES, LOWER CHESAPEAKE BAY, VIRGINIA
Richard L. Wetzel, Polly A. Penhale and
Kenneth L. Webb
Virginia Institute of Marine Science
and School of Marine Science
College of William and Mary
Gloucester Point, Virginia 23062
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INTRODUCTION
In order to determine structural properties of the SAV community and
physical-chemical regimes characteristic of the study area, routine studies
were carried out to investigate plant species distribution and relative
abundance, plant canopy structure and chemical characteristics, and various _,
sediment, chemical properties. All sampling was carried out at the Vaucluse .-<
study site between transects B and r (see Figure 1, Preface).
METHODS AND MATERIALS
/
Plant Distribution, Relative Abundance and Biomass /
Plant distribution and relative abundance was determined along transects /
A, B, and C in July, 1979 and along transect B in May, July and August, 1980 ' ;
to determine areal coverage by species and distribtion with water depth. A
line-intersect method was employed using two divers and is described in detail
by Orth, et al. (1979). Briefly, the transects illustrated in Figure 1
(Preface) were followed from the offshore sandbar beginning at low tide and
progressing toward the shore. A 100 m line marked at 10 m intervals was
employed along the transect line to locate point-intersections for determining
species composition and estimating percent cover. At each 10 m intersection,
a 0.5m* frame was randomly dropped and species composition determined and
percent coverage estimated by a diver. This procedure was repeated twice at _--—-
each sampling point. During each transect study, time and water depth were
recorded at each station and later compared to a continuous, relative tide
height record kept near-shore. These data were used to calculate bottom depth
relative to mean low water at each sampling point following the methods of - ~~
Orth, et al, (1979). These data also provided direct comparisons with the
previous intensive mapping effort (July, 1978) by Orth, et al. (1979) for
identifying any major differences in distribution between years and providing
relative abundance and distribution information for the bed as a whole.
Plant biomass was determined at monthly intervals for the period April,
1979 to April, 1980 in the II. maritima, mixed, and Z_, marina dominated
communities. These data were collected by Orth and details of sample ^-
collection and processing are given elsewhere (Orth, et al. 1981). However, .
for comparison to information given in this section, the results of these
studies are presented for continuity.
Plant canopy structure was determined in each vegetation zone at
approximately monthly intervals beginning in April, 1979 and continued through '.
August, 1980. Canopy structure was investigated by estimating leaf area index '» - -
(LAI), the ratio of leaf area to substrate area (Evans 1972). Plant samples "\
of each species from the II. mar it in* Z.marina and mixed vegetation area were \
collected by hand, returned to the laboratory and rinaed free of sediment. i '
Leaves were then removed and 3 replicate samples of 10 shoots each were •' >/i
sectioned at 5 cm intervals from the base to apex of the leaves. Leaf area /
(one-sided) was determined using a LI-COR Model LI13100/1+1 Leaf Area Meter. / -
10 N " ?'
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***
// .
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LAI results are reported as m2 leaf surface (one-sided) per m2 sediment
surface for 5 cm intervals for each species at each site as well as on a site
basis.
Rooting depth was determined during July 1979 in the three major
vegetation areas by hand coring using a 10.2 cm diameter (0.033 m2 surface
area) acrylic corer. Four replicate cores were taken per area and sectioned
into 0-2, 2-5, 5-10 and 10-15 cm horizontal sections. Each section was washed
free of sediment through a 1.0 mm screen and roots and rhizomes sorted by
hand. The replicate samples were dried at 60*C to constant weight (48 hours)
and percent contribution to total weight determined for each section.
Chemical analyses of plant tissue consisted of wet weight:dry weight,
organic matter (OM) and carbon:nitrogen (C:N) determinations. Samples were
collected by hand with the corer (described above) and separated by species
into above and belowground fractions at the R.. aaritima and Z, marina site.
At the mixed vegetation area, belowground tissue was not separated by species.
Leaf litter (aboveground dead, unattached material) was also collected.
Methods of analyses are as described in the following section.
Chemical Characteristics of Plant Tissue and Sediments
Routine samples for sediment analyses in the five habitat types were
taken in July and October 1978, April, 1979 and monthly for the period June
through October, 1979 and February through May, 1980. Analyses performed in
relation to habitat type were, adenosine triphosphate (ATP), water content
(WC) organic matter (OM), particulate organic carbon and nitrogen (POC and
PON). Replicate sediment samples were taken by hand-coring to a depth of
approximately 30 cm using a 5 cm diameter acrylic core tube. The cores were
capped underwater and sealed with tape for transport to the laboratory. The
cores were then extruded, divided vertically and horizontally sectioned into
0-2, 2-5, 5-10, 10-15, 15-20 and 20-30 cm intervals. For each core,
processing consisted of: duplicate 1 cc plugs extracted using boiling, 0.1 M
sodium bicarbonate for ATP analysis (Bancroft, et al. 1974); the remaining
sediment fraction was frozen for later water content, organic matter and
POC-PON analyses.
Water content and organic matter content was determined gravimeterically
on frozen sediments by drying at 60*C to constant weight for water content and
by weight loss or ignition at 550*C for organic matter determination. Wet
weight:dry weight and organic matter content for plant tissue samples were
determined in a similar manner. All weights were determined to the nearest
0.01 mg. POC and PON analyses for both plant tissue and sediments were
performed using a Perkin-Elmer Model 240B Elemental Analyzer.
RESULTS AND DISCUSSION
Sampling and analyses to characterize plant community structure and
general physical and chemical characteristics at the Vaucluse study site were
conducted over various intervals beginning in July 1978 »nd ending in July
1980. The sampling was scheduled in such a manner to evaluate both seasonal
and annual cycles of the various parameters and to create a data base for
11
-------
-V "K
-•' ' N
longer term monitoring efforts. More specifically, these data have been used
for designing studies around documented temporal events as well as for
designing sampling strategies and experimental approaches. These data have
been used to partition other data sets for both spatial and temporal
correlation with other measurements.
The results and discussion of these efforts are divided into two
principal areas; 1) Plant Community Structural Characteristics and Dynamics,
and 2) Chemical Characteristic* of Vegetated and Non-vegetated Sediments.
Plant Community Structural Characteristics and Dynamics
Relative Abundance and Distribution
Plant distribution and percent cover (as an estimate of species relative
abundance) was determined along transects A, B, and C in July, 1979. Figure 1
illustrates the distribution and percent cover by species relative to bottom
depth at mean low water (MLU) along the transects. In terms of areal coverage
and relative abundance, the most significant stands of "L, marina are located
south of transect C and occupy the deeper (>50 cm below MLW) areas of the
grass bed extending shoreward of the sand bar. Significant stands of R..
ma^itiiaa are located along all transects but are confined to the shallow «50
cm below MLW) areas at this time of year. There is a major zone of overlap
for the two species on all the transects that ranges laterally from ca. 250 m
on transects A and B to 100 m on transect C. The depth range of the overlap
or mixed vegetation areas is ca. 60 to 100 cm below MLW bottom depth on
transects A and B. The mixed area on transect C is confined to below 60 cm
and the only monoapecific Z. marina stands are adjacent to the sand bar.
These results for both relative abundance and distribution follow closely the
results obtained by Orth, et al. (1979) in July, 1978 for the same transects.
This general pattern of depth distribution and relative abundance is
characteristic of SAV communities in the lower Chesapeake Bay (Orth et al.
1979). R. maritima in the dominant species in shoal benthic habitats less
than SO OB deep (MLW) while monospecific stands of Z_. marina are found in
habitats MOO cm deep. The mixed Ruppia-Zostera vegetation zones are also
characteristic; the depth ranges (ca. 60-90 cm below MLW) observed at the
Vaucluse site are typical of the lower Bay.
The generalized distribution pattern for lower Bay aeagrasses raises
important questions and suggests hypotheses relative to factors governing
plant distribution and abundance. Apparent among these are the physiological
characteristics of each species and their response the environmental factor*
light, temperature, nutrients and desiccation. Direct competition between
species for available substrate does not appear a natural determinate;
however, few data exist to evaluate the role this potential interaction might
play.
The dynamic nature of the distribution and abundance of the plant
community was investigated by repeating the relative abundance and
distribution surveys along transect B in May and August, 1980 (Figure 2).
Based on the results of plant biomass studies (Orih and Moore 1982) these
sampling times represented months of maximum growth (May through July) and
12 , ,/.
-------
>
TRANSECT B
TTpnTVp
20
0
Inii
:;: 4-;'
y.:ji1,,,
i
700 770
S4MO BdR
ll:"!
o
ISO
100
80,
•oil
40
H
Ji.id
TRANSECT C
., r
n -r h ' ~ - r. *
' : Z manna
\ - ft marilima
100 ;oo
400
Figure 1. Submerged macrophyte diatribution and relative abundance along
tranaects A, B, and C, in July 1979, at Vaucluae Shores, Chesapeake
Bay, Virginia, illustrating the Hepth-dependent conation pattern
typical of lower Chesapeake Bay beds. The horizontal axis is in
aeten from shore.
13
-------
TRANSECT B
2
2
o
a.
UJ
a
o
o
50-
100-
150-
I Z marina
montima
ALGAE
JULY, 1979
a:
UJ
o
^o
a:
UJ
>
O
o
IOO-,
80-
60-
40-
20-
0
r|I jh- p
mi n
n
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rrfflMl
iJ
1 1 K'lii
lilil
i
i L
1
1
L
r"
1
'(T
i
i
t
nil
XUGUST, I98C
100
200 300 400 500 600 700 800
Figure 2. Submerged microphyte distribution and relative abundance along
transect B, for various times of year illustrating the dynamic
nature of te vegetated zones. Horizontal scale is in meters frc
shore.
1A
-------
die-back (August) for Z_. marina and jl. maritima at th« mixed site. II. ;'
maritima in the shallow area exhibits a late summer-fall period of maxima* -
growth. '/
Species distribution along the depth gradient is similar for the three
sampling periods; however, the relative abundance (Z cover) is dramatically
different (Figure 2). During the late spring (May) Z_. marina is uniformly •/..
distributed in the deeper areas of the grass bed and has maximum relative
abundance. By mid-summer (July), both species are relatively uniformly /''
distributed within their zonal limits although Z. marina is reduced in
relative abundance. There is some indication that II. maritima has invaded the <
deeper, Zostera-dominated areas of the bed during this period but confirmation
of this must aw*it longer term monitoring and more detailed studies. During y
the later part of the summer (August), Z. marina is significantly reduced in , /
relative abundance at the deeper stations indicating natural plant mortality,
and jl. maritima is more patch!ly distributed and generally occupies the deeper
areas within its range. These results suggest not only the dynamic nature of
the bed as a whole, but, also the complementary nature of the growth and
dynamics of the two species. We observed that species dominance and abundance
shift during the growing season and these parameters are out of phase for the
two seagrass species. This indicates that, as suggested previously,
physiological and morphological differences between species rather than direct
competitive interactions control species distribution. Consequently, from an
applied standpoint, criteria developed for management of lower Bay seagrases
may not apply universally to all species and for all locations. ''
Biomass Relationships and Rooting Depth
Above-ground plant biomass (vegetative shoots) in the three vegetated /'
zones was also determined to provide information on growth, production and
community dynamics. A summary of these data (from Orth, et al. 1981) are ^
provided in Figure 3. Over an annual period, II. maritima exhibits a unimodal /' ,
cycle in aboveground biomass. At the monospecific Jl. maritima site, peak ''
biomass is in the Fall (September-October) while peak biomass "occurs earlier ','
(July-August) at the mixed site. Z_. marina exhibits a bimodal cycle of annual .,
growth with periods of maximum biomass occurring both in the spring to early / •
summer and in the fall to early winter; these periods of maximum biomass are ;
followed by periods of shoot die-back. The re-growth of Z_. im rina is not as /
pronounced at the mixed site as in the monospecific Z. marina sit*. The data
also indicate to some extent the year to year variation in standing stock. ' />
For example, there was approximately a 50-60X increase in peak above-ground
biomass for JR. maritima between 1979 and 1980 and a corresponding 20-252 ^-
increase for Z_. marina. Unfortunately, the data are not extensive enough to ./
assign level of significance to the apparent differences. /
The above-ground biomass pattern observed for Z_. marina is simil'.r to /'.'
that observed in other temperate climates and probably reflects a negative ',
growth response to increased summer water temperatures. In North Carolina,
Penhale (1977) reported peak Z_. marina shoot biomasn ir. March followed by a
general biomass decline throughout the rest of the year; the decline occurred
as the water temperatures increased during the summer. Also in North ,'
Carolina, Thayer et al. (1975) observed a dramatic biomass decline following '/
15
i
I
-------
I
<
r '
O
O
CM
1_ 1
1 "§ 1 C
16
-------
\
Ruppia
i 1 1 1 1 1 r 1—T
MIXED BED
i 1 1 r~i 1 r——i 1 1 1 1
M
i i I
MAM
1980
Figure 4. Mean root:shoot dry weight ratio of R. maritima (•) and Z. marina
(0) at the three vegetated study sites. Calculated frooTthc data
of Orth and Moore, 1982.
17
-------
peak summer water temperatures in August. Biebl and McRoy (1971) suggested
that prolonged or frequent, temperatures rises above 30*C could result in _Z.
marina mortality. A temperature response threshold is also reflected in the
geographical distribution of JZ. marina which reaches its southernmost limit on
the eastern ".S. coast at Cape Fear, North Carolina (Thayer et al. 1975).
In general, R. maritime appears more tolerant of higher temperatures than
JZ. marina (Richardson, 1980 anU references therein). The geographic range of
JR. maritime extends south to the Gulf Coast of the United States.
Nevertheless, the annual growth cycle of JR. maritima at Vaucluse Shores nay
involve a negative response to high summer temperatures as well as to other
parameters characteristic of the habitat. At the monospecific JR. maritima
site, the shallow water allows for greater light penetration than in the mixed
site; in fact, JR. uaritima at the shallower habitat may be photo-inhibited at
times. In addition, plants in this zone are frequently exposed during extreme
low tides. The low July-August biomaas of jl. maritima here may be due to the
combination of high temperatures, high light, and desiccation damage; a more
favorable light and temperature regime is probably present during its fall
biomass peak. At the deeper, mixed site where the light and temperature
regime is less extreme, JR. maritima biomass exhibits a peak in July followed
by a decline during the warm August period. JR. maritima at the mixed site is
shaded to some extent by JZ. marina which has longer leaves; the lower annual
—• "uritima biomass here may reflect lower productivity rates due to less
light.
Root:shoot dry weight ratios (root here includes rhizome biomass) were
calculated from the mean biomass data of Orth and Moore (1981) and are
presented in Figure 4. Root:shoot ratios reflect the plant's metabolic
expenditure in terms of non-photosynthetic and photosynthetic tissue.
Root:shoot ratios less than 1 were noted during much of the year for both
species at the mixed site and for JZ. marina at the monospecific site. In
contrast, El. maritima at the monospecific site exhibited a ratio consistently
above 1, which suggests a different strategy for the plants. The below-ground
portion of the plant is considered an important site of nutrient uptake for
submerged angiosperms (Penhale and Thayer 1980 and references therein). It is
possible that lower interstitial nutrient concentrations may characterize the
jl. marina site; thus, the plant may expend more energy toward underground
growth. At th' deeper mixed and Zoatera sites characterized by lower light
penetration, root:shoot ratios less than 1 may reflect an additional
expenditure toward more photosynthetic tissue.
Rooting depth analyses of the seagrasses in the three vegetated habitats
indicate that greater than 98Z of the root-rhizome system is located in the
upper 10 cm of the sediment (Table 1). At the JR. maritima site, a greater
proportion of below-ground biomass was located in the upper 2 cm than at the
other two sites. The mean total root-rhizome biomass was highest at the mixed
site, with considerably lower values at the JZ. marina site and lowest values
at the JR. marit ima site.
The rhizosphere is the portion of the sediment under the imuediate
influence of the plant roots (Rovira and Davey 1974). Seagrass roots may
release oxygen to the sediments (Oremland and Taylor 1977; lizumi, et al.
18
-------
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1980). Also, dissolved organic carbon compounds may be released by seagras*
roots (Wetzel and Penhala 1980; Wood and Hayasaka 1981). In Che highly
anaerobic sediments of plant communities with submerged roots such as seagrass
systems, plant activity results in a profound influence on physical, chemcal,
and biological characteristics of this zone. In the Vaucluse Shores system,
the rooting depth data suggest that this dynamic zone of activity is
concentrated in the upper 5 cm of the sediment. In addition, plant influence
on sediment processes may be greatest at the mixed site due to its greater
biomasa of root-rhizome material.
Canopy Structure
Plant community growth involves the response of individual plants to the
environment as well as to other individuals in the community. LAI is a
fundamental parameter in community analysis since community growth is not
simply determined by the photosynthetic capacity of individual leaves. That
is, community growth is the product of the net assimilation per leaf area and
the LAI. An increase in plant density affords a greater potential for
community production but mutual shading reOuces the available light to a point
where net production decrases. Many factors such as light intensity, leaf
morphology, and leaf orientation (i.e., a* influenced by tides and currents
for aeagrasses), play a role in determining the optimal LAI of a community.
The results of our leaf area index (LAI) studies showed differences among
vegetated sites (Figure 5). Analysis of variance of one-sided LAI by date and
site indicated significant differences among sites (p • 0.016). The maximum
one-sided LAI values for the three sites in this study (II. maritima • 3.4,
mixed • 4.2, Z^ marina « 3.2) were within the range of maximum values reported
for other seagrasses communities. For example, maximum one-sided LAI values
reported for seagrass systems include \6.8 (Dennison 1979); 9 (Jacobs 1979),
and 3.3 (Aioi 1980) for £. marina; 9.3 for Thalassia teatudinum (Gessner
1971), and 8.3 and 1.4 for Posfdonia oceanica and Cymodocea nodosa,
respectively (Drew 1978). Few data exist for canopy structure in submerged
angiosperm communities compared to terrestrial communities. LAI values for
terrestrial communities generally range from 1 to 12 and high LAI values tend
to be positively correlated with high annual productivity (Leopold and
Kreideman 1975).
In a detailed community structure analysis of a monoapecific "L. marina
community across a depth gradient, Dennison (1979) concluded that changing
leaf area was a major adaptive mechanism to decreasing light regimes. He
observed that the LAI increased with increasing depth to the deepest portion
of the bed where the LAI decreased. At these stations with the lowest light
penetration, further increases in LAI probably could not be maintained due to
the lowered net photosynthesis with less available light. This is similar to
the results obtained in this study in which LAI values generally increase from
the Hupp!a to the mixed site and then decline at the deep Z_. marina site.
The seasonal pattern of LAI reflects various trends within the three
sites (Figure 5). At the It. maritima site, the decrease in LAI during
fall-winter, 1979 paralleled a decrease in shoot density, leaf length and
shoot biomass data. The summer increase in LAI corresponded to increasing
20
-------
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\
\
n i r r
<*• m
I I
2 O
GO
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a
z
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X3QNI V3dV-dV3"!
21
./
-------
insolation and increasing density, biomasa and leaf length. At the nixed
site, which generally exhibited the highest LAI values, the steady decrease in
LAI from August, 1979 to February, 1980 was a result of decreasing biomass,
length and density of II. n>aritima. Although the density of Z. marina
increased during this period of new shoot growth, leaf length and biomass
decreased. The rapid LAI increase in spring, 1980 reflected rapid increases
in biomass and length of "L_, marina and in density, length and biomass of R.
maritima. At the Z_. marina site, the steady LAI decline from April to October
paralleled a decrease in leaf length. The sharp decline in LAI at all three
sites from April to June, 1980 reflected an unexplained decrea.-e in shoot
density.
Interrelationships of these community parameters have been reported in
other studies. In a Z. marina community in France, Jacobs (1979) observed a
seasonal trend in which increases in LAI paralleled increases in shoot density
and the number of leaves per shoot. Aioi (1980) observed that both the LAI
and shoot biomass exhibited a similar unimodal seasonal pattern with a maximum
in May; he related this pattern to day length. Center (1981) observed a
continuous trend toward maximum LAI of a freshwater Eichhornia crassipes
community in which inceases in plant density in response to available space
were followed by increases in plant size in response to competition for light.
The results of our canopy structure studies also showed differences in
LAI between species. Using Duncan's multiple range test, the analysis
indicated that the mean LAI values of II. maritima and Z_. marina were
significantly different (p«0.05). The data were further analysed using a
least squares test for differences among means by species for the various
sites (Table 2). For Z_. marina, the mean LAI was not significantly different
than the mixed site. The mean LAI for II. maritima was significantly different
than the mixed sites; both of these were significantly different from Z.
marina.
One-sided LAI values were calculated for 5 cm vertical sections of leaf
material in order to obtain information on the vertical stratification of leaf
area at the three vegetated sites (Figures 6, 7, 8, 9). Scatter plots of the
data collected over the 17-month study show that for both II. maritima and Z_.
marina, maximum leaf area was concentrated in the lower portion of the canopy.
The 1R. maritima canopy exhibited the greatest concentration of leaf area from
0 to 5 cm above the substrate. Analysis of variance of the vertical
distribution of LAI showed highly significant differences between species (p •
0.0001).
Examples of the vertical stratification of leaf area of jl. maritima and
Z. marina at the three sites are presented in Figures 10, 11, and 12.
Although the LAI changes during the year, the relative distribution of leaf
area does not change. Over the yearly cycle, the LAI for II. maritima and for
Z. marina in the monospecific stands was generally higher than that for each
Tpecies at the mixed site. The three examples illustrate a period of major
biomass decline of Z. marina (August, 1979; Fig. 10), a period of low biomass
of both species (January, 1980; Fig. 11), and a period of generally high
biomass and long leaf length of both species (July, 1980; Fig. 12).
22
-------
30-
25 -
1
o
Z
O
p 20-
o
CO
u.
0
Q.
0
*~ 15 -
10 -
5-
1
(
Ruppia maritime
R up pi a BED
•
...
•••• •••••• •
) 0.5 1.0 1.5 2.0
LEAF AREA INDEX
Figure 6. Scatter plot of the vertical distribution of LAI for II. maritiaa at
the monospecific site at Vaucluse Shores.
23
-------
a
O
30-
u.
o
25-
E
o
Z
g
u 20-
15 -
10 -
5-»»
i
0
Ruppia maritime
MIXED BED
•• • •• •
05
i
1.0
1.5
2.0
LEAF AREA INDEX
Figure 7. Scatter plot of the vertical distribution of LAI for R. taaritima
the mixed bed site, at Vaucluse Shores.
-------
50 -
40 -
30 -
z
o
o
UJ
Q.
O
20 -
10 -
0 -1
l
0
Zoster a manna
MIXED BED
T
04
r
0.8
T
1.2
LEAF AREA INDEX
Figure 8. Scatter plot of the vertical distribution of LAI for Z,. marina at
the mixed bed site, at Vaucluse Shores.
x"
25
-------
'•*<
50H
_. 40H
E
u
a 30
CO
a.
o
o-1
Zostera marina
Zoster a BED
04 08 12
LEAF AREA INDEX
1.6
Figure 9. Scatter plot of the vertical distribution of LAI for £. marina at
the monoapecific lite, at Vaucluae Shorea.
26
-------
AUGUST 1979
50-
40-
30-
20-
Ruppio BED
Ruppia mar it i ma
10
LU
* 0
(D
tn
O
cr
en
LJ
o
50 -
40 -
30 H
20-
10-
Zostero BED
Zostera manna
MIXED BED
Ruppia maritima
"1
MIXED BED
Zostero marina
H
j
2 0
LEAF AREA INDEX
~
2
Figure 10. Vertical distribution of LAI for R. maritima and Z. marina at the
three vegetated sites for August, 1979 at Vaucluse Shores.
27
-------
JANUARY 1980
tn
m
O
IT
U.
in
cr
UJ
»-
UJ
Z
z
Ul
u
Figure 11.
50-
40-
30-
20-
10-
RuppiO BED
Ruppia maritime
50-
40-
30-
20
10-
Zostero BED
Zoster a marina
MIXED BED
Ruppia maritima
MIXED BED
Zoster a marina
:i
r
LEAF AREA INDEX
i
2
Vertical distribution of LAI for R. maritima and £. marina at the
three vegetated iitet for January, 1980 at Vaucluae Shore*.
-------
JULY 1980
CD
3
in
2
O
(T
50-
40-
30-
20 H
10-
R-jppia BED
Ruppia maritima
MIXED BED
Ruppia maritima
V)
tr
50-1
z
oj 40-
30-
20-
10-
Zostera BED
Zostera marina
MIXED BED
Zostera manna
i I ~i r
10 20 0 10
LEAF AREA INDEX
20
Figure 12. Vertical distribution of LAI for II. mar it inn and Z_. marina at the
three vegetated sites for July, 1980 at Vaucluse Shores.
29
-------
f ,•
TABLE 2. LEAST SQUARES TEST FOR DIFFERENCES AMONG MEAN LAI BY SPECIES FOR
THE THREE VEGETATED STUDY SITES AT VAUCLUSE SHORES.
Group
1
2
3
4
Species
Ruppia
Zoatera
Ruppia
Zostera
Site I/J
Mixed
Mixed
Ruppia
Zostera
1
*
0.0001
0.0031
0.001
Group Number
2 3
0.
0.
0.
001
*
0288
2412
0.
0.
0.
0031
0288
*
001
4
0.0001
0.2412
0.001
*
Prob>lTl HO: x (I) - x (J)
30
-------
The canopy structure at the three sites reflects adaptations to the
specific light regimes. In the shallow, high light environment at the It.
maritima site, the upper portion of the canopy may be photoinhibited during
portions of the year; a greater leaf area near the substrate where light is
reduced may allow for a greater net canopy photosynthetic rate. For both
species at the mixed site and for "L. marina at the monospecific site,
concentration of photosynthetic area in the lower canopy would compensate for
the decreased light with increasing mean depth. Species differences in the
vertical stratification of leaf area would be particularly important at the
mixed site where It. maritima is shaded by "L. marina; these differences
probably contribute to successful co-existence.
Although other factors are involved, competition for light in submerged
communities is probably the most important factor in determining community
structure. Haller and Sutton (1975) characterized the community structure of
Hydrilla, an introduced species which often displaces native species in
Florida. They proposed that Hydrilla communities, by forming a dense canopy
at the water surface (which can reduce light penetration by 95Z in 0.3 m) have
effectively replaced native Vallisneria americana communities which are less
dense. In a study of Myriophyllum spicatum and Vallisneria americana, Titus
and Adams (1979) observed that the former had 68Z of its foliage within 30 cm
of the surface while the latter had 62Z of its foliage within 30 cm of the
bottom. Like Hydrilla, Myriophyllum spicatum has been successful in
establishing dominance in competitive situations.
Wet:Dry Weight Ratios of Plant Tissue
Wet:dry weight ratios of II. maritima and Z_. marina were calculated for
samples collected in May, 1981. Ratios for II. maritima were 6.31 _+ 0.76 (n-5)
for leaves and 8.72 _+ 0.47 (n-5) for roots and rhizomes. Ratios £. marina
were 7.20 _* 0.65 (n-4) for leaves and 8.69 _+ 0.73 (n»4) for roots and
rhizomes. These values are similar to ratios reported by Neinhuis and DeBree
(1977) for Zosters marina (leaves, 6.7 to 11.1; roots and rhizomes, 5.5-16.7).
Elemental carbon and nitrogen analysis
Elemental analysis of plant materials is useful for a number of putposes.
Tissues analysis may provide estimates of nutrient availability for growth
(Gerloff and Krombholr 1966) or give an indication of possible food quality
for other organisms (Godshalk and Wetzel 1978). Carbon:nitrogen atomic ratios
in plankton are considered to be about 6.6 according to the Redfield Model
(Redfield 1958). Goldman et al. (1979) have suggested that phytoplankton of
composition with greater ratios are deficient in nitrogen, and grow slower
than maximally. Angiosperms contain more structural carbohydrates, such as
cellulose, than phytoplankton (e.g. Almazan and Boyd 1978), and thus these
considerations are not as straight forward as in the phytoplankton although
similar analogies have been used. Carbon:nitrogen ratios seldom aproach the
Redfield Model value of 6.6 in seagrasses and reported values range from 10 to
80 with values of 10 to 20 being usual for Zosters marina (Table 3).
Seasonal changes might be expected in the nitrogen and carbon content of
seagrasses since these plants, at least in temperate climates, undergo
31
-------
TABLE 3. SUMMARY OF THE AVAILABLE LITERATURE ON WEIGHT PERCENT C AND N AND
ATOMIC C:N RATIOS IN SEAGRASSES.
Species
Araph ibol is griffithii
Cymodocea nodosa
C. serrulata
u
it
Enhalus acoroides
u
n
u
Halodule uninervis
n
Halophilia decipiens
H. hawaiiana
H. oval is
H. ovata
H. spinulosa
*•» ^^H^^_^a«M
Phyllospadix scouleri
Posidonia oceanica
P. ostenfeldic
P. s inuosa
Svrins;odium isoet i f ol iutn
Thalassia hemprichii
T. testndinum
Zostera capricorni
it
Zostera marina
n
if
M
u
n
n
Plant Part
leaves
stems
leaves
leaves
leaves
rhizomes
leaves
rhizomes
leaves
leaves
rh i zomes
leaves
rhizomes
leaves
leaves
leaves
roots
rh i zomes
leaves
root s/rh i zomes
leaves
root s
leaves
rhizomes
leaves
leaves
rh i zomes
leaves
rhizomes
leaves
leaves
leaves
leaf-sheath
leaf-blade
rhizomes
root
dead- leaves
Locat ion
W. Australia
n
Corsica
N. Queensland
u
ti
N. Queensland
1 1
*i
Palau
M
N. Queens land
if
H
it
Hawai i
W. Austral ia
N. Queensl and
it
it
it
Cal i fornia
Corsica
it
W. Austral ia
it
N. Queensland
11
Barbados
N. Queens I and
ii
Cal i fnrnia
Rhode Island
Japan
M
M
ii
Wash . , Alaska
%C
30.
30.
37.
38.
36.
39.
35.
3b.
24.
16.
29.
36.
33.
38.
35.
33.
34.
22.
21.
31 .
32.
38.
31 .
37.4
40.
38.
29.
29.1
*N
1.3
0.96
1.59
1.25
1.70
2.18
0.94
1.21
1.13
0.63
1.45
1.9S
1.57
0.76
0.93
1.04
0.73
0.80
0.55
1.63
l.ll
6.!4
2.03
2.0
2.29
1.4
1.4
1.4
C:N
26.8
36.4
27.2
21.6
35.4
67.1
19.8
82.
24.7
20.8
43.4
33.2
27.7
34.6
24.4
24.8
29.8
51.7
77.6
46.5
23.3
21.2
24.5
58.2
43.7
16.9
54.0
32.
44.9
25.5
77.5
22.2
13.9
30.'
20.5
58.1
33.6
7.2
17.8
18.
18.6
23.
18.
18.8
Ref.*
1
1
1
2
1
2
2
2
1
1
1
2
2
1
2
1
1
2
2
2
2
2
1
3
•>
2
1
1
1
4
4
4
4
5
32
-------
V X
TABLE 3. (CONTINUED)
Species Plant Part
Zoitera marina leaves-green
" leaves-brown
leaves
" leaves
" rhizomes
root
" leaves
Location
Denmark
ti
Germany
Japan
it
M
Canada
ZC
33.9
30 7
38.5
36.0
34.1
26.2
35-43
in
2.25
1.5
2.75
2.6
2.8
2.9
1.2-4.8
C:N
12.9
17.5
12.0
11.9
10.6
7.8
9-30
Ref.*
6
6
6
7
7
7
8
*References 1) Atkinson and Smith, 1981; 2) Birch, 1975; 3) Patriquin, 1972;
4) Aioi and Mukai, 1980; 5) Godshalk and Wetzel, 1978; 6) Vinogradov, 1953;
7) Seki and Yokohama, 1978; 8) Harrison and Mann, 1975.
33
-------
seasonal periodicity in growth. Vinogradov (1953), reviewing earlier work,
suggested a seasonal change in nitrogen content of Zostera marina, with the
lowest nitrogen content occuring toward fall or during winter. Harrison and
Mann (1975) report a distinct sinusoidal character with a peak leaf M
percentage of 4.8 in March and a minimum of 1.2 in September. Our seasonal
data set for percentage nitrogen in Zostera marina leaves (Table 5) is
extensive, but, as of this writing, appears inadequate to establish good
correlations with season or with biomass (Figure 3). Hopefully with expansion
of the data set or more complete analysis, a meaningful interpretation will be
available. However, it appears extremely unlikely that any textbook picture
such as is presented by Harrison and Kann (1975) will be found. An estimated
median value for percent nitrogen for Zoster* marina leaves from the
literature (Table 3) is 2.32 and apparently higher than the mean of 1.8 from
our data. Percent organic carbon in Zostera marine leaves appear to be quite
similar for our data (mean of 36Z) and the literature (median 36Z); thus the
C:N ratio of our data (25) is almost twice the median value (13) from the
literature. These data would seem consistent with the suggestion that the
Vaucluse grass beds are nitrogen deficient (e.g. Orth 1977).
Roots and rhizomes of Zostera marina for Vaucluse clearly contain less
carbon and nitrogen than do the leaves end show a higher C:N ratio (Table 4).
This relationship seems to generally be borne out by the literature as well
(Table 3) with the data set of Seki and Yokohama (1978) a posssible exception.
Our data are for combined root and rhizome tissue and the data of Seki and
Yokohama (1978) suggest that it is specifically the root tissue that may
contain lower C concentration.
Since seagrasses are consumed primarily through the detrital food chain,
there has been considerable interest in decomposition of the plant material.
Nitrogen and carbon percentages and atomic C:N ratios in dead Zostera marina
leaf material are presented in Table 6. The means are all lower than those
from fresh leaf material. Harrison and Mann (1975) reported nitrogen
percentages from 1 to 1.5 in contrast to our values of 1 to 2 and carbon
percentages from 26 to 32 in contrast to our values of 20 to 40 for dead
leaves of Zostera marina. Although no seasonal cycle is apparent for either C
or N percentage.!, the atomic C:N ratios show a minimum of 15 in March and a
maximum of 27 in September (Harrison and Mann (1975), in contrast to our data
suggesting maxima in September and February of 31 and 41 respectively and a
minimum of 20 in October. The comparison of C and N pecentages of live vs
dead leaves of Zostera marina is in agreement with the general idea that
soluble C leaches from or is translocated from living leaves before leaf death
to a greater extent than soluble N. However, consideration of changes in
epibiota during senescense and death of leaves may alter this interpretation.
A consideration of M and C values from Zostera marina in comparison to
Ruppia maritima from the monospecific stands as well as the mixed bed may
increase our understandir.g of species response to environmental conditions as
well as factors which influence their distribution with depth. Unfortunately,
we have decided that there are technical reasons which cast doubt on the
validity of the mixed bed data and thus it is not reported here. Our present
small data base from Ruppia maritima is given in Table 7. Carbon content of
Zostera marina and Ruppia maritima are essentially the same, especially in the
34
-------
TABLE 4. PERCENTAGE CARBON AND NITROGEN OF THE TOTAL ROOT AMD RHIZOME
MATERIAL OF ZOSTERA MARINA AND THE ATOMIC RATIO OF CARBON:
NITROGEN FROM THE PURE STAND PLANT MATERIAL. NUMBERS ARE THE
MEAN, NUMBER OF OBSERVATIONS AND THE STANDARD DEVIATION.
DATE
FEB
MAR
JUN
AUG
AUG
SEP
OCT
MEAN
OPJAHIC CARBON
PERCENTAGE
80
80
80
79
80
79
79
33.6
29.1
30.1
27.5
31.3
28.5
34.8
30.6
(2) 0
(1)
(4) 2
(5) 7
(8) 3
(3) 3
(3) 0
.67
.43
.11
.46
.38
.435
ORGANIC NITROGEN
PERCENTAGE
1
2
1
1
1
1
1
1
.59
.01
.08
.27
.31
.35
.18
.26
(2) 0
(1)
(4) 0
(5) 0
(8) 0
(3) 0
(3) 0
.075
.217
.456
.194
.0411
.0705
CARBON NITROGEN
ATOMIC RATIO
24
16
33
26
28
24
34
28
.6 (2)
.9 (1)
.7 (4)
.1 (5)
.8 (8)
.6 (3)
.5 (3)
.4
0.66
9.20
3.35
5.7
3.32
1.69
-------
TABLE 5. PERCENTAGE CARBON AND NITROGEN OF TUE TOTAL LEAF MATERIAL OF
ZOSTERA MARINA AND THE ATOMIC RATIO OF CARBON:NITROGEN FROM
THE PURE STAND PLANT MATERIALS. NUMBERS ARE THE MEAN, NUMBER
OF OBSERVATIONS AND THE STANDARD DEVIATION.
DATE
MEAN
ORGANIC CARBON
PERCENTAGE
ORGANIC NITROGEN
PERCENTAGE
36.4
1.81
CARBON NITROGEN
ATOMIC RATIO
FEB
JUN
AUG
AUG
SEP
OCT
80
80
79
80
79
79
35.9
34.1
37.2
37.5
34.5
38.6
(1)
(6) 4
(10) 1
(5) 0
(3) 0
(2) 3
.22
.74
.67
.242
.41
3
I
2
1
1
2
.03 (1)
.54 (6)
.18(10)
.42 (5)
.50 (3)
.01 (2)
0
0
0
0
0
.417
.336
.135
.129
.0449
13
27
20
31
26
22
.8 (1)
.0 (6)
.3(10)
.2 (5)
.9 (3)
.4 (2)
5.09
2.49
3.38
2.38
2.48
24.9
-------
TABLE 6. PERCENTAGE CARBON AND NITROGEN OF THE TOTAL DETRITAL LEAF
MATERIAL OF ZOSTERA MARINA AND THE ATOMIC RATIO OF CARBON:
NITROGEN FROM THE PURE STAND PLANT MATERIAL. NUMBERS ARE
THE MEAN, NUMBER OF OBSERVATIONS AND THE STANDARD DEVIATION.
DATE
MEAN
ORGANIC CARBON
PERCENTAGE
27.3
ORGANIC NITROGEN
PERCENTAGE
1.20
CARBON NITROGEN
ATOMIC RATIO
FEB
JUN
AUG
SEP
OCT
80
80
80
80
79
36.4
28.3
28.4
30.8
35.2
(3) 1
(4) 4
(5) 3
(6) 1
(3) 1
.43
.06
.9
.57
.95
1
1
1
1
2
.07
.39
.31
.20
.06
(3) 0
(4) 0
(5) 0
(6) 0
(3) 0
.231
.233
.186
.207
.079
40
23
25
30
20
.9 (3)
.8 (4)
.9 (5)
.7 (6)
.0 (3)
9.54
2.08
'j 9
5.7
1.85
22.5
37
-------
TABLE 7. PERCENTAGE CARBON AND NITROGEN OF THE TOTAL MATERIAL OF RUPPIA
MARITIMA AND THE ATOMIC RATIO OF CARBON:NITROGEN. NUMBERS ARE
THE MEAN, NUMBER OF OBSERVATIONS AND THE STANDARD DEVIATION.
DATE
ORGANIC CARBON
PERCENTAGE
ORGANIC NITROGEN
PERCENTAGE
CARBON NITROGEN
ATOMIC RATIO
LEAF MATERIAL FROM RUPPIA SITE
36.5 (4) 3.53
LEAF DETRITUS
28.8 (1)
ROOTS/RHIZOMES FROM RUPPIA SITE
22.0 (2) 5.23
ROOTS/RHIZOMES FROM MIXED SITE
34.3 (2) 3.04
2.48 (4) 0.118
1.91 (1)
0.85 (2) 0.078
1.37 (2) 0.177
17.2 (4) 0.876
17.6 (1)
30.1 (2) 4.6
29.7 (2) 6.5
38
-------
. '''• X -r~-S
' V - .
-^
leaves. Ruppia maritima leaves appear to be richer in nitroge' Chan leaves of
Zostera marina. Until we expand the data base, no interpretations of these
data are possible.
Chemical Characteristics of Vegetated and Non-vegetated Sediments
For our studies, chemical characteristics of vegetated and non-vegetated
sediment were investigated by determination of organic matter and adenosine
triphosphate (ATP) concentration of sectioned 30 cm cores taken from the five
study sites (habitats) at Vaucluse Shores. Orth and van Montfrans (1982)
report other sediment properties.
Organic Matter Content
Overall, the organic matter content of the sediment was low at all sites;
most values were less than 1Z of the sediment dry weight and reflect the sandy
nature of the substrate. The organic content of sectioned cores at four sites
during July, 1979 is presented in Figure 13. Several factors contribute to
the organic matter pool in the sediments: living and dead plant and animal
tissue, microbial autotrophs and heterotrophi, dissolved organic matter, etc.
The influence of the plant community is seen in the total amount and depth
distribution of organic matter content at the four sites. The organic content
is lowest in the non-vegetated sand patch;'in addition, the depth distribution
of organic content is relatively uniform in the sand patch. In contrast, the
sediment organic matter content was generally higher at the three vegetated
sites. Here, organic matter was concentrated in the upper 10 cm of the
sediment corresponding to the zone of 982 of the root-rhizome biomass (Table
1). The organic matter content in the upper 10 cm at the _R. maritima site was
lower than at the mixed and Z_. marina sites. At these latter two sites,
root-rhizome biomass in July was 3-4 times greaer than at the II. maritima
site.
ATP Content
The results of the sediment ATP analyses did not clearly reflect the
influence of the seagrass rhizosphere. ATP concentrations are generally used
as an estimator of microbial biomass, although the ATP from any viable cell is
to some extent included in the values. Higher percentages of total ATP in the
upper 5 cm of the 30 cm cores were generally observed in the vegetated sites
compared to the non-vegetated sand patch (Table 8) although spatial and
temporal variability masked any statistically significant differences. The
vegetated sites presumably contain greater concentrations of metabolizable
substrates (such as dead plant organic matter and dissolved organic matter
secreted by the roots) than the non-vegetated sites to support microbial
growth.
The seasonal distribution of ATP concentration with depth during 1979 is
presented in Figure 14. The ATP concentration at all sites was higher in the
upper portion of the sediment; this trend wae generally more pronounced at the
three vegetated sites. The ATP concentration tended to be greater during the
summer, which corresponds to the period of highest community respiration (see
Chapter 2, this report). In addition, root-rhizome biomass tended to be high
39
-------
JULY 1979
% ORGANIC MATTER
I 2
<
a.
^
o
•
, -
•
~-4
» <
^-
1
1—4
i | f
H
^
!•
-4
t
^T^ ' '
• —^
+
•4
•. —4
V •
h*^.. <
»-f
( X rang* i n = 2)
Figure 13. Vertical distribution of «ediment organic matter (Z of Dry Weight)
during July, 1979 at the principal sampling iitea within the
Vaucluse Shores area. The mean and range of values (n-2) are given
in the figure.
40
-------
./
TABLE 8. PERCENT TOTAL ATP IN 0-5 CM AND 0-10 CM VERTICAL CORE SECTIONS
FROM THE PRINCIPAL SAMPLING SITES WITHIN THE VAUCLUSE SHORES
AREA.
Habitat
Ruppia
Mixed
Zoatera
Sand Patch
Depth
(cm)
0-5
0-10
0-5
0-10
0-5
0-10
0-5
0-10
Feb
72
81
69
79
80
88
49
64
Mar
44
59
64
75
59
72
_
-
April
45
90
51
66
59
81
43
59
June
44
61
58
73
60
74
65
90
July
63
73
49
65
40
53
48
62
Aug
36
53
41
57
_
-
_
-
Sept
45
62
37
53
85
91
_
-
Oct
23
34
37
53
36
52
34
50
X
46
64
51
65
60
73
48
65
41
-------
o_.
o
o
o
o-\
Q.
UJ
Q.
>-
<
irrrr
o
•
. ft)
« *J
u ••«
• •
IM
U M)
a e
• "<
** *§•
s §
a »
w »
• o.
• p4
« O
£ e
W •*«
hi
O O.
w •
u w
o
I -4 oo
•o o>
c 27
O 9>
•* e r-
«j — < 9>
« e
•H o •
u ••< •
u «J «
• • kl
.* U O
•O 4J X
e M
j: w
JJ O «>
o. e •
w o a
•o u <-"
o
« O. 9
JS H *
P •< >
7
Diddny
HD1XW 0 NVS
42
-------
'
in the summer. The seasonal behavior of ATP concentration is shown for the
0-2 cm portion of the sediment in Figure IS.
43
-------
s
Q.
5
o>
3
8-
4-
10-
8-
Q 6-
UJ
i «•!
X
u
I 0-
8 -
v
\
\
\
u 1 r- 1 —
1 On
-------
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Co., N.Y.
Oremland, R. S. and B. F. Taylor. 1977. Diurnal fluctuations of 0%, $%, and
CH4 in the rhizosphere of Thalassia tstudinum. Limnol. Oceanogr.
22:566-570.
Orth, R. J. 1977. Effect of nutrient enrichment on growth of the eelgrass
Zostera marina in Chesapeake Bay, Vir/.inie. Mar. Biol. 44:187-194.
Orth, R. J., K. A. Moore, and H. H. Gordon. 1979. Distribution and abundance
of submerged aquatic vegetation in the lower Chesapeake Bay. U.S.
Environmental Protection Agency Report No. 600/79-029/SAV1, Chesapeake Bay
Program, Annapolis Maryland.
Orth, R. J. and K. A. Moore. 1982. The biology and propagation of eelgrass,
Zostera marina, in the Chesapeake Bay, Virginia. Final Report, Grant no.
R805953, U.S. Environmental Protection Agency. Chesapeake Bay Program,
Annapolis, MD. and VIMS SRAMSOE 265, Gloucester Point, VA. 187 pp.
Orth, R. J. and J. van Montfrans. 1982. Structural and functional aspects
of the ecology 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.
Final Report, Grant no. R805874, U.S. Environmental Protection Agency,
Chesapeake Bay Program, Annapolis Maryland and VIMS SRAMSOE 267,
Gloucester Point, VA. 232 pp.
Patriquin, D. G. 1972. The origin of nitrogen and phosphorus for growth of
the marine angiospertn Thalasia testudinum. Mar. Biol. 15:35-46.
Penhale, P. A. 1977. Macrophyte-epiphyte biomass and productivity in an
eelgrass (Zostera marina L.) community. J. exp. mar. biol. Ecol.
26:211-224. ~
Penhale, P. A. and G. W. Thayer. 1980. Uptake and transfer of carbon and
phosphorus by eelgrass (Zostera marina L.) and its epiphytes. J. exp.
mar. biol. Ecol. 42:113-123.
Pomeroy, L. R. 1975. Mineral cycling in marine ecosystems, pp. 209-223.
In, F. G. Howell, J. B. Gentry and M. H. Smith (eds.), Mineral Cycling in
Southeastern Ecosystems, NTIS, CONF-74013, Springfield, Va.
47
J
-------
PostgaCe, J. R. 1971. Relevant aspect'. -,i the physiological chemistry of
nitrogen fixation, pp. 287-307. In, D. E. Hughes and A. H. Rose (eds.),
Microbes and Biolcgy productivity. Symp. Soc. Gen. Microbial., V. 21,
Cambridge University Press, London.
Redfield, A. C. 1958. The biological control of chemical factors in the
environment. Amer. Sci. 46:1-221.
Richardson, F. D. 1980. Ecology of Ruppia mar itima L. in New Hampshire
(U.S.A.) tidal marshes. Rhodora 82:403-439~;
Rovira, A. D. and C. B. Davey. 1974. Biology of the rhizosphere, pp.
153-2C4. In, E. W. Carson (ed.), The Plant Root and Its Environment.
Univ. Virginia Press, Charlottesville.
Ryther, J. H. and W. M. Dunstan. 1971. Nitrogen, phosphorus and
eutrophication in the coastal iiarine envi.onment. Science 171:1008-1012.
Seki, H. and Y. Yokohama. 1978. Experimental decay of eelgrass (Zostera
marina) into detrital particles. Arch. Hydrobiol. 84:109-119.
Thayer, G. W. 1971. Phytoplar.kt on production and the distribution of
nutrients in a shallow unstratified estuarine system near Beaufort, N.C.
Chesapeake Sci. 12:2*0-253.
Thayer, G. W., S. M. Adams, and M. W. LaCroix. 1975. Structural and
functional a»p,-'-ts of a recently established Zostera raarina community, pp.
518-546. lr. !. E. Cronin (ed.) Estuarine Research, v. 1. Academic
Press, New Yoi, 738 pp.
Titus, J. E. and M. S. Adams. 1979. Coexistence and the comparative light
relations of the subnerged macrophytes Myriophyllum spicatmn and
Vallisneria americana Michx. Oecologia (Berl.)4^0:273-286.
Valiela, I., J. M. Teal, and W. Sass. 1973. Nutrient retention in salt marsh
plots experimentally fertilized with sewage sludge. Est. Coastal Shelf
Sci. 1:261-269.
Vinogradov, A. P. H53. The Elementary Chemical Composition of Marine
Organisms. Translated fron Russian by Efron, J. and J. K. Set low, with
bibliography »dit»?d and enlarged hy V. W. Odum. Sears Foundation for
Marine Research, Nerf K;iv«»n, Conn. 647 pp.
Wetzel , R. G. and P. A. P^nhale. I*3?"). Transport of carbon and excretion of
dissolved organic carbon by I»av»q and roots/rhizomes in seagrasses and
their epiphytes. Aquatic Bor.. 6:149-158.
Williams, R. B. 1973. Nutrient levels and phytopl ankton productivity in the
estuary, pp. 59-89. in, R. H. Chabreck (ed.), Proceedings, 2nd,
symposium, Coastal Marsh and F.*t-:.iry Management, July 17-18, 1972.
Louisiana State Univ., Bdton kou.;e.
-------
Wood, D. C. and S. S. Hayasaka. 1981. Chemotaxis of rhizoplane bacteria to
amino acids comprising eelgrass (Zoftera marina L.) root exudate. J. exp.
mar. biol. Ecol. 50:1^)3-161.
-------
Chapter 2
PHOTOSYNTHESIS, LIGHT RESPONSE AND METABOLISM OF SUBMERGED MACROPHYTE COMMUNITIES
IN THE LOWER CHESAPEAKE BAY, VIRGINIA
R. L. Wetzel and P. A. Penhale
Virginia Institute of Marine Science
and School of Marine Science
College of William and Mary
Gloucester Point, Virginia
23062
50
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INTRODUCTION
The productivity and metabolism of submerged macrophyte communities is
comparatively high in relation to other habitats within eatuarine,
shoal-benthic environments along the U. S. Atlantic east coast. In a
significant portion of these shallow water areas, primary production by the
submerged vascular plants Zostera marina (eelgrass) and Ruppia maritime
(widgeon grass) dominates organic matter input to the habitat and provides a
substrate or surface area for additional autotrophic production by an
attached, microalgae dominated epiphytic community. These characteristics
render the habitats metabolically active and trophically important. Because
of specific diversification in autotrophic production in these communities,
i.e., vascular plants, epiphytic autotrophs, benthic micro- and macroalgae,
and phytoplankton, trophic structure is highly diverse and secondary
production is significantly greater than in adjacent, non-vegetated habitats
(Orth, et al. 1982).
Historically, studies of submerged macrophyte production in temperate and
tropical seagrass communities have focused on two species; temperate eelgrass,
Z, marina, and tropical turtlegrass, Thalassia testudinum. Estimates of
annual bioraass production reported for eelgrass range from 200 to 800 gC m~2
(Nixon and Oviatt 1972; McRoy 1974; Wetzel et al. 1979; Nienhuis 1980;
Lindeboon et al. 1982a; 1982b). For turtlegrass, the estimates range from 200
to 3000 gC m~2 (Jones 1968; Bittaker 1975; McRoy and McMillan 1977). These
values for seagrasses fall in the upper range reported for the more intensely
studied estuarine and coastal marsh communities (Keefe 1972). Also, many of
these annual values for seagrasses may prove underestimates due to the
techniques employed (Zieman and Wetzel 1980). Regardless, the estimates
certainly imply a correspondingly high nutrient demand met by either tightly
coupled remineralization processes or new nutrient sources.
Physical, chemical and biological factors which regulate seagrass
photosynthesis and organic matter production have been ascribed to
temperature, salinity, hydrodynamic properties, grazing, nutrients and light.
Complicating the analysis of these factors in temperature zones, estuarine
seagrass communities, such as those in Chesapeake Bay, undergo wide
fluctuations in all these environmental variables which have diel, seasonal
and annual periodicities.
Temperature obviously has direct effects on all enzymatically mediated
processes as well as on physical-chemical properties of the water environment.
Together, salinity and temperature govern the geographical range and
distribution of seagrasses but within the range of tolerance of a particular
species in estuarine systems, these parameters are probably of secondary
importance (e.g. McRoy and McMillan 1977) except to establish or interact
with other factors to control the rates of specific chemical and biochemical
trans formatIons.
51
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Hydrodynamic properties and specifically water circulation may limit
plant growth for some aquatic macrophytes. In tidally dominated, estuarine
systems which appear characteristically well-mixed in shallow water areas,
mixing is probably sufficient to prevent establishment of strong gradients
inhibiting gas diffusion and exchange (e.g. Conover 1967; Weatlake 1967;
1978). However, Fonseca, et al. (1982) have suggested that current velocity
within seagrass meadows may have significant influence on plant self-shading ^.
and thus plant community photosynthesis. /
Direct grazing by waterfowl and larger invertebrates has been
demonstrated important for some Z_. marina communities (Neinhuis and van
lerland, 1978). It is considered by many, however, to be of minor overall
significance and is generally regarded as episodic in character. However, any
general conclusions regarding waterfowl-plant trophic interactions, especially
in temperate zones, of the U. S. Atlantic east coast is highly tenuous due
both to a lack of detailed review and long term study (E. W. Wilkins, personal
communication, 1982).
The environmental variables considered as primary physical and chemical
controls on seagrass photosynthesis and production are light and nutrients.
Nutrient availability, particularity nitrogen, is generally considered, or at
least has become a popular opinion, a primary control. Historic evidence
supporting this view has been the general observation that estuarine and
coastal waters are generally low in nutrient concentration. Needless to say,
this observation may have little direct bearing on seagrass limitation from a
kinetic standpoint. Kinetic relationships for seagrass nutrient utilization
have only within the past decade been investigated (MeRoy and Alexander 1975;
Penhale and Thayer 1980; lizumi and Hattori 1982). The relative importance of
new sources (e.g. nitrogen fixation) versus recycled forms and root-rhizome
versus leaf uptake are currently areas of active research (e.g. Patriquii: and
Knowles 1972; Goering and Parker 1972; McRoy et al. 1973; Capone et al. 1979)
but there appears no consensus of opinion. For estuarine systems,
particularly temperate estuaries, Nixon (1981) has presented some rather
convincing agruments that nutrient remineralization in sediments dominates
estuarine nutrient cycles. There are no specific arguments that would
indicate seagrass communities operate any differently. For seagrass
communities in the lower Chesapeake Bay, Orth (1977) has demonstrated a rapid
and positive growth response by Z. marina to in situ sediment surface
application of commercial fertilizer suggesting nutrient limitation. In Rhode
Island, Harlin and Thome-Miller (1981) experimenting with water column
enrichments of ammonium, nitrate and phosphate demonstrated a variable ,
response by "L_. marina and JR. marit iroa dependant on nutrient supplied, ,/''.
potential competitive species and current velocity.
In contrast to nutrient kinetics light-photosynthesis interactions have
been extensively studied in relation to aeagrass metabolism and light alone is ""
considered the principal limiting factor in many environments (Zieman and
Wetzel 1980). Seagrasses characteristically have depth-dependent distribution
patterns that are explained as a species-specific response to ambient,
submarine light regimes (Buesa 1975; Aioi 1980; Nienhuis and DeBree 1980; Orth
et al. 1982). Williams (1977), Penhale (1977), Drew (1979), Beer and Waisel ;
(1979) and Capone «-t al. (1979) have reported on the physiological response of
52
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seagrass photosynthesis to varying light intensity. Saturating light
intensities, as photosyntheticdlly active radiation (PAR), for many seagrasses
fall in the range 400-600 E m~2 sec"'. For some coastal systems,
particularity in subtropical and tropical areas, these PAR intensities are
characteristic and light is considered not limiting except at extreme depth.
However, in more turbid estuarine conditions, light environments of these
intensities probably are not typical and submarine light regimes appear
fundamentally important for growth and survival of seagrasses (Backman and
Bertilotti 1976; Congdon and MeComb 1979). Within limits, seagrasses can
adapt both morphologically (e.g. leaf elongation and altered canopy structure)
and biochemically (pigment composition) to suboptimal light regimes (Spence
1975; Bowes et al. 1977; Wiginton and McMillan 1979). However, the range of
adaptation for a particular species is limited (Dennison 1979).
In terms of organic matter cycling and the trophic structure of seagrass
communities, the vascular plants are generally presumed to be indirectly
utilized, i.e., direct grazing or secondary, macroheterotrophic utilization of
the living plant tissue is considered minor. Mortality of the plants forms the
base of a detrital trophic structure (Mann 1973; ienchel 1977; Klug 1980).
Microbial decomposition of macrophyte tissue and microheterotrophic production
associated with the sediments probably accounts for the principal organic
matter source that goes to support secondary production indirectly from plants
per se (Fenchel 1977; Klug 1980; Newell 1981).
Heterotrophic utilization of epiphytic, benthic and planktonic algae
production on the other hand is probably mediated by direct grazing. In
contrast to other estuarine and marine communities where either detrital or
grazer pathways dominate, seagrass communities, both temperate and tropical,
appear to be characterized by both. Recent studies suggest that both detrital
and grazing pathways are potentially of equal importance in supporting
heterotrophic metabolism and secondary production (Chapter 3, this report).
Therefore, secondary production in seagrass communities results from the flux
of organic matter, energy, and nutrients via two distinct, though
interdependent pathways: 1) a detrital based trophic structure controlled by
vascular plant production and microbial degradation, and ?) a grazing trophic
structure controlled by epiphytic, benthic micro- and macroalgae,
phytoplankton and, to some extent, vascular plant primary production. The
degrae of interdependence of the two structures remains poorly understood.
The studies reported herein focused on what we a priori assumped the
principal factors controlling macrophyte production in the lower Chesapeake
Bay and were directed at: 1) studies of photosynthesis by the two dominate
vascular plant species in lower Chesapeake Bay seagrass communities, "L. marina
and JR. maritima, 2) measures of total community metabolism and, 3) evaluation
of the importance of short term light (PAR) alterations and seagrass response
at both specific and community levels. Total community metabolism studies
were employed to estimate production and consumption by the intact community
under various natural and altered light regimes as well as provide integrated
rate estimates. These data were also used for more detailed studies
partitioning production and consumption (see Chapter 3, this report). To the
extent possible, the studies were carried out contemporaneously. All sampling
and field studies were conducted in the seagrass bed at Vaucluse Shores.
53
\
\
-------
Elsewhere, we have provided a detailed description of the study site (see
Preface, Figure 1 and Chapter 1, this report).
MATERIALS AND METHODS
Vascular Plant Photosynthesis
Photosynthesis was estimated using **C radiotracer techniques for £.
marina and R_. maritima collected from the mixed bed site following the
procedures of Penhale (1977). Excised leaves of each species were collected
and incubated in separate 900 ml glass jars containing seawater collected at
the site. The jars were covered with neutral density screens which allowed for
light penetration of 2, 4, 10, 27 and 532 of ambient light. Experiments were
carried out at various times throughout the year in order to cover the range
of in situ temperatures. Samples were innoculated with 10-20 yUCi ^C- NaHCC»3
and incubated under natural light from 1000 to 1400 hours in a running
seawater system. Temperature was maintained about 1°C above ambient using
this system. After incubation, sample processing included quick freezing in
dry ice, lyophilization and combustion in a Packard Model 306 sample oxidizer.
1 C collected from sample combustion was assayed by liquid scintillation using
a Beckman Model LS 8000 counter. All radioactivity measurements were
corrected for background, recovery after combustion, and counting efficiency.
Specific rates of I^C-photosynthesis were calculated following the method of
Penhale (1977).
Total Community Metabolism
Total community metabolism (net apparent oxygen production or
consumption) within the three vegetated zones was determined using dome
enclosures (acrylic hemispheres). The dome enclosures were 1 m diameter, 0.5
m in height that enclosed a water volume of 260 liters and covered a bottom
area of 0.78 m2. Vegetated areas within "L. marina, mixed, or II. maritima
dominated communities were randomly selected in and around permanent station
markers located between transects B and C (see Preface, Figure 1). Care was
taken not to locate a specific study in an area previously used. On several
occasions, dome studies were carried out in nonvegetated, sand" patch areas
within the seagrass bed and outside the grass bed proper on the sandbar. For
all community metabolism studies, the domes were placed by diver and secured
to the substrate by forcing a 10 cm vertical flange attached to the dome
circumference into the sediment. The domes were equipped with ports for
sampling by syringe and with standard hose fittings for attachment to an
above-water pumping (recirculating) system. The pumping system was made by
modifying 12 VDC bilge pumps to accept on both intake and discharge sides
standard garden hose fittings for connection to the domes. The pump manifold
on the discharge side of each channel was constructed of standard PVC fittings
in such a aa. ner to accept 02 electrodes (Orbisphere Model 2705, Orb i sphere
Laboratories, New Jersey) and provide for both sampling of dome water and
introduction of selected solutions into the recirculated water for
manipulation of various parameters. A four-channel pumping system constructed
in this manner could be run continuously for approximately 36 hours using
three, deep-cycle, 12 VDC marine batteries connected in parallel. The rated
output of the pumps was 750 gph and we determined using Rhodamine WT dye in a
54
-------
simulated experiment that turnover in the domes was aporoximately 7 minutes.
Complete replacement of enclosed water (i.e. pump-out) could be accomplished
in 20 minutes. Figure 1 illustrates the dome enclosure experimental design.
In general, an experiment consisted of randomly placing four domes by
diver in the selected habitat, connecting each dome to a pumping channel, and
pumping out the domes for one hour following placement to insure ambient
conditions of the enclosed water. We incorporated this procedure because in
preliminary studies using these large domes, elevated NH4 concentrations were
noted which we ascribed to the effect of dome placement (sediment
disturbance). Following the pump-out period, the discharge side of the
pumping channel was connected to the corresponding dome and the incubation
period begun.
Dissolved oxygen using Clark-type, sulfide-insensitive, polarographic
electrodes (Orbisphere Laboratories, Inc.) attached to the pump manifold,
temperature and dissolved oxygen in ambient water, and photoaynthetically
active radiation (PAR) using a LI-COR 185A Quantum Radiometer equipped with
both surface and underwater PAR quantum sensors was continuously monitored in
each of the four domes and ambient water. The data were continuously recorded
by processing the instrument signals with a 10 channel, multiplexer (Dataplex
10, Hampshere Controls, Inc.) onto a two channel, portable strip chart
recorder (Soltec, Inc.). Periodic written records were also kept during the
course of an experiment as a check on recorder output and calibration. The
dissolved oxygen electrodes were calibrated at the beginning of each
experiment using water-saturated, air nomographs and again checked at the end
of the experiment. Periodically, the probes were calibrated against the
standard Winkler titrametric technique (Strickland and Parsons 1972) to insure
the accuracy of the air-water calibration procedure. Factory calibrated
temperature and PAR sensors were used throughout the study.
Prior to adoption of this technique using the domes, two other designs
were employed and when used are noted in the results. The first design
employed was placement of the domes as before but without water circulation.
Sampling for dissolved oxygen was done by removing replicate, 10 ml syringe
samples from each dome over various incubation intervals. Dissolved oxygen
was determined using standard Winkler reagents (Strickland and Parsons 1972)
and micropipet and microburet equipment (see Fraleigh 1971). Tike second
design employed a recalculating system that was integral with the dome (i.e.
all equipment was submerged) and dissolved oxygen determined by polarographic
means with the electrodes fixed to the domes and all other sampling
accomplished by syringe sampling through the dome ports.
The first design was abandoned because of the accumulating evidence which
indicates the importance of water motion in attempting measures of exchange,
whether the measures are oxygen or some other dissolved constituent (e.g.
Wheeler 1980). The second design was abandoned because of inherent
difficulties in sampling the enclosed water mass and recurring problems of
electrical failure with the pumping system. Although we were satisfied that
the pumping system maintained adequate circulation and water motion within the
domes, periodic electrical failures required termination of the experiment and
resulted in the loss of both data and time. Since adopting the first
55
-------
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41
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41 •
a
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56
-------
described technique, we have lost no experiment due to failure of the physical
design and both sampling frequency and ease were significantly improved.
At certain times of the year and for specific areas, we have employed
domes of different overall dimensions to accommodate studies when either plant
density and/or biomass was reduced (e.g., "L. marina mid-winter) or the water
depth prevented placement of the 1 m diameter domes (e.g., shallow, in shore
II. maritima areas). For these specific times aiWor locations, we used
acrylic hemispheres of the same design and construction, except they are 0.5 m
diameter, 0.25 m in height and cover a bottom area of 0.19 m^ enclosing 32
liters of water. For studies using these smaller domes, water circulation was
provided by stirrers integral to the Orbisphere oxygen probe body. Simulated
experiments using Rhodamine WT as a tracer indicated that complete mixing was
accomplished by the probe stirrtrs in a matter of a few minutes and sampling
at various "depths" within the hemisphere through the syringe ports indicated
no stratification of the enclosed water. The only experimental design change
necessitated by using the smaller domes was that we did not have the
capability of "pumping out" following placement. To reduce the potential
effects of this, we placed the small domes as gently as possible when
initiating a study and allowed the domes to "equilibrate" with ambient water
by leaving all ports open for 30 to 60 minutes before starting the incubation.
Aside from this, the designs using both large and small domes were comparable.
Total community metabolism was estimated as the net hourly rate of oxygen
concentration change over various incubation intervals throughout a study and
for experiments lasting 24 hours as the net rate for the diel (24 hour)
period. For some comparative purposes conversion of the oxygen data to carbon
equivalents was made assuming a community RQ " 1.0.
The temporal data sets were partitioned into the following intervals for
calculating net apparent 02 exchange:
Morning: Sunrise + 1 hr. 1 min. to 1000 hrs.
Noon: 1001 hr. to 1400 hrs.
Afternoon: 1401 hr. to Sunset - 1 hr.
Evening: Sunset - 59 min. to Sunset + 3 hr.
Night: Sunset * 3 hr. 1 min. to Sunrise + 1 hr.
The intervals were decided for data summary and analysis following
preliminary data reduction and inspection of oxygen concentration - time
curves. The actual intervals were established by setting the "noon" interval
to bound the period of maximum potential solar radiation (i.e. 1000 to 1400
hrs). The "morning" and "afternoon" intervals were set according to local
times for sunrise and sunset plus or minus one hour. The "evening" interval
was based on initial studies that suggested during this period oxygen
consumption was stimulated following the day light period and was often double
the night time estimate for total community respiration.
Area specific rates were calculated as:
lCi+1 - Oil
-V- A-l (1)
57
-------
where: G£ - 02 concentration in mg 1~*, i " 0,l,2...n (hour*)
t • time in hours
V
-------
30
25
o
o
LU
o:
u
a.
2
UJ
20
15
••
••
•o
••
0 ••
•0
••
•
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o
o
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o
oo
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5 -
o =1979
• =1980
90
180
DAY
270
360
Figure 2. Daily water temperature record (all observations) at Vaucluae
Shores for the period July 1979 through October, 1980.
59
-------
Figure 3 illustrates the range of variation and pattern of isolation as
PAR at the site during the study period. The interesting aspect of these data
is the extreme range in daily isolation (ca. 10 to 60 E-m~2) during the early
Z_. marina growing season, i.e. spring to early summer. Climatically, this is
the time of year the lower Chesapeake Bay area experiences increased rainfall
and cloud cover due to the passage of slow-moving frontal systems. The
theoretical maximum isolation for this latitude is illustrated by the dashed
line in the figure.
Figure 4 presents the mean and standard deviation for vertical light
(PAR) attenuation coefficients determined from light profiles taken in the
study area. The mean range over both years was -0.665 to -2.98 with extreme
values of -0.225 to -8.50. Minima occurred during the winter period and
maxima during the spring and early summer months. The highest attenuation
coefficients and their greatest variability coincided with r.hat for the
isolation pattern (Figure 3) and is due in part to the prevailing climatic
conditions during this period. The frontal systems mentioned before are
characterized by persistent and at times strong southwesterly winds. Thus the
study site, Vaucluse Shores, is the windward shore under these weather
conditions and experiences increased wave action, resuspension, and water
surface elevation. Conditions within the grassbed can become particularity
severe when these weather systems are coincident with spring tides. At these
times, the offshore sandbar provides minimum protection for damping wave
energy. For all dates and areas within the grassbed, the annual mean
attenuation coefficient and standard deviation wds -1.435 (+_ 0.511). There
were no significant differences among vegetated zones although the "L_. marina
study area was consistently lower than the R_. maritima area (annual means of
-1.323 _* 0.327 and -1.778 +_ 0.704, respectively). However, because mean water
depth at the R_. maritima site was approximately one-half that at the Z_. marina
site, PAR reaching the R_. maritima plant canopy was signilicantly greater
under all conditions.
- Photosynthesis
Figures 5 and 6 present the results of 1Z*C - photosynthesis studies with
Z. marina and II. maritima collected from the mixed-bed study site.
Photosynthesis-light relationships for both species generally follow the
rectangular, hyperbolic function. At low PAR intensities, i.e. _< 10X of
ambient, the relationship is less well defined due to both analytical and
sampling variability (i.e. plant tissues in different physiological states).
Thus, estimation of the light - photosynthesis parameters , I^ and l^ (see
Wetzel, et al. 1982) using first order reaction kinetics was not possible for
some data sets. The results indicate that maximum rates of photosynthesis
(Pmax) differ between species and are temperature or at least seasonally
related. Z. marina tends to have equal or higher maximum rates during the
earlier, cooler months of the growing season (Figure 5) and R. maritima
generally has higher maximum rates during the warmer summer and early fall
periods (Figure 6).
The data were also analyzed using Caperon's model (Caperon et al. 1971)
to estimate various photosynthesis-light paramters. Table 1 summarizes the
results of these analyses in terms of Pmax, 1^' (equivalent to the
60
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100 r
THEORETICAL
MAXIMUM
(37° N Lot, atmospheric
tronsmittance, clear sky )
o
_ L 1 L
V A
90
J
M
180
DAY
S 0
270
i I
D
360
Figure 3. Daily insolation record at Vaucluse Shores for the period July
1979 through October 1980 and theoretical maximum daily insolation
(solid line) at latitude 37*N. and assuming a clear sky and
atmospheric transmittance of 0.70.
61
-------
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62
-------
TABLE 1. CALCULATED 14C-PHOTOSYNTHESIS PARAMETERS FOR RUPPIA MARITIMA AND ZOSTERA
MARINA EXPERIMENTS.
Temp
Jan 1.0
March 8.5
May 17.5
May 17.5
Aug 28 . 0
Aug 28.0
Sept 22.0
Oct 10.0
X
S.D.
R. i
PmaxT
1.15
3.23
2.86
2.87
4.91
2.81
4.17
3.83
3.23
1.12
naritima
V"
41.8
107.
88.4
127.
152.
232.
211.
220.
147.
58.8
a3
0.014
0.015
0.016
0.011
0.016
0.006
0.010
0.009
0.012
0.004
Z. marina
Pmax
1.60
3.06
2.37
1.68
1.88
1.24
3.84
1.82
2.19
.866
Ik'
9.68
18.3
72.1
67.3
31.2
66.3
188.
23.1
59.5
57.4
X
0.083
0.083
0.016
0.012
0.030
0.009
0.010
0.039
0.035
0.031
PAR4
(pE m"2 sec'1)
766
1533
1557
1717
1827
894
1440
983
1. Pmax - mgC g(plant)"1 h"1
2. Ifc" " light (l;E m~2 sec"1) @ .5 pmax
3. a • Initial Slope, AP/AI
4. PAR - x Photosynthetically Actively Radiation; 1000-1400 hrs.
63
-------
40-i
29/30 JAN 80
o
x
Q.
40-]
2 0
2/3 MAY 80
LIGHT INTENSITY (% Ambient)
Figure 5. ^. marina (Z) and II. maritima (R) ^C-photosynthesis - Light (PAR)
response characteristics for the Winter-Spring period. Clear and
darkened symbols are replicate experiments; solid and dashed lines
connect day I and day 2 studies respectively.
64
-------
28/29 AUG 79
40-|
T"
20 40 60 80 100
LIGHT INTENSITY (% Ambient)
Figure 6. Z. marina (Z) and II. maritima (R) ^C-photosynthesis - Light (PAR)
Response characteristics for the Summer - Fall period. Clear and
darkened symbols are replicate experiments; solid and dashed lines
connect day and day 2 studies respectively.
65
-------
.
;
Michaelis-Menten half-saturation constant, K8 or I 9 0.5 Pmax), and (X, the
initial slope, or AP/AI calculated for P at 1^' and assuming the line passes
through the origin.
For £. marina and II. maritima, both Pmax and Ik' are temperature related
and the relationship is different for the two species. Pmax for £. marina was
generally equal to R_. maritima during the early growing season at colder
temperatures and declined during mid-summer to approximately one-third the
maximum estimated rate (1.24 versus 3.84 mgC g(plant)"1 hr~'). Based on these
data, temperature optimum for Z. marina photosynthesis is less than 28°C and
probably is between 22* and 28*C. Arrhenius plots (log P versus reciprocal
temperature) indicated poor correlation over all temperatures (r - -0.25) as
well as for temperatures <^ 22*C (r » -0.49) although the correlation obviously
improved. A possible explanation for the latter result is that the data for
the various temperatures at or below 22*C include two different growing
seasons, i.e. Fall and Spring.
Estimated Pmax for jl. maritima indicated a strong temperature-dependence
with maximum rates occuring at the higher, mid-summer temperatures.
Temperature optimum for R_. maritima is probably at or near 30"C and obviously
quite different than for Z_. marina. Arrhenius plots support this conclusion
wirh a significant correlation between log P and reciprocal temperature (r •
-0.89). For all data, Pmax for it. maritima was approximately 1.5 x that for
Z_. marina but differed by as much as 4 x at 28*0 (August).
Comparison of the photosynthesis - light parameters also suggests
physiological differences between the species. For £. marina, I'K is much
lower than for R_. maritina and at times by as much as an order of magnitude.
These data suggest a much lower light requirement and higher photosynthetic
efficiency of £. marina. This conclusion is supported by comparison of the
estimated values. For all data, JZ. marina light-response at low intensities
is approximately 3 x that for jt. maritina.
Overall, the results indicate that photosynthesis - light relationships
for the two species are significantly different. £. marina can be
characterized by 1) a temperature optimum in the range 22" - 28*C, 2) high
photosynthetic efficiency at low light intensity and 3) a Pmax and
light-response characteristic of 'shade' or low-light plants. R. maritima can
be chatacterized by 1) a temperature optimum at or near 30*C, 2T low
photosynthetic efficiency at low light, and, 3) a Pmax and light-response
characteristic of "sun" or high-light plants.
From these data, our beat estimate for photosynthetically saturating
light intensities for Z. marina and jl. maritima are 200-300yE m~* sec" and
600-700UE m~2 see"* respectively. From a physiological standpoint these
results fit very well as a causal explanation for the characteristic
depth-dependent distribution and zonation patterns of lower Chesapeake Bay
grassbeds (see Chapter 1, this report; Orth et al. 1982).
Total Community Oxygen Metabolism
A total of 42 community oxygen metabolism studies were carried out at the
study site during the period July 1978 through September 1980 using the dome
66
-------
enclosure*. Our major efforts focused on the mono*pecifie, Z. marina and It.
maritima dominated communities (31 or 742 of the total number of studies).
The remainder of the studies were carried out in the mixed-bed community
(five), sand patch or bare substrate areas within the grassbed (four), and on
two occasions outside the graasbed on the offshore sandbar. The only studies
carried out without stirring the domes (i.e. the first enclosure technique
tried) were conducted in July 1978. All other studies had water circulation
provided by modified bilge pumps either integral with the domes (October 1978
through July 1979) or using the surface-stationed pumps (August 1979 through
September 1980).
The Zostera marina dominated community
Table 2 summarizes the net apparent rate of oxygen production or
consumption in the Z_. marina dominated community for various intervals over
the course of the experiments. Maximum rates of net apparent oxygen
production occured during the noon intervals reflecting the strong dependence
on light. The noon rate estimates ranged from -18.1 to +663. mg02 m~* h"1
with the minimum estimate occuring in late summer (August) and the maximum
estimate occuring in spring (April-May). Water temperatures at these times of
year average about 27*C and 16*C respectively in "L_. marina dominated areas of
the grassbed. These data suggest temperature dependance during all seasons
except summer. Arrhenius plots (log rate vs. reciprocal temperature) and
simple linear regression of the transformed data indicates that net apparent
noon oxygen production is directly correlated with water temperatures
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Zostera
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1000-
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—
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E
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( 1978-1980)
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o NOON
• AFTERNOON
I 1 1 1 1
0 90 180 270 360
DAY
Figure 8. Scatter plot of mean daily noon and afternoon rates of net
apparent 02 productivity in the Z^. marina dominated community
versus calendar day.
72
-------
TABLE 3. AREAL RATE OF Z. MARINA COMMUNITY 02 PRODUCTION OR
CONSUMPTION INTEGRATED OVER THE DAILY INTERVAL FOR ALL DOME
ENCLOSURE EXPERIMENTS. ENTRIES ENCLOSED BY PARENTHESES WERE
ESTIMATED FROM POINT MEASURES.
Date
July
Oct,
May,
June
July
Aug.
Sept
Oct,
Feb.
Mar,
Apr,
June
July
Aug.
Sept
, 1978
1978
1979
, 1979
, 1979
1979
, 1979
1979
1980
1980
1980
, 1980
, 1980
1980
, 1980
Morn
570.
12.
(2600)
967.
(3383)
1492.
(1354)
(1239)
242.
(345.)
808.
(1446)
1168.
153.
(376.)
Noon
1264.
808.
2652.
468.
1228.
1444.
1456.
1332.
928.
768.
1264.
700.
424.
184.
404.
nig 02 «
AN
960.
176.
824.
838.
400.
781.
523.
490.
649.
702.
572.
-533.
421.
-420.
0.
? 1
»~* interval"1
Even Night
-628.
-68.
-2332.
-840.
-
(-1500)
-780.
532.
128.
0.
-1356.
-1428.
-1348.
-916.
-848.
-445
(-687)
-1291
-1441.
-
-2012.
-1397
-687.
-788.
-1240.
-2341.
-1337.
-1905.
-1229.
-1494.
r
1721.
241.
2453.
-8.
-
105.
1156.
2906.
1159.
575.
-1053.
-577.
-1240.
-2228.
-1562.
73
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plant tissue would be in equilibrium with the surrounding water «t the ti»« of
dome placement. If the apparent depression in the afternoon rates is a
function of internal cycling which hypothetically is concentration dependant,
then afternoon rates determined on the plant community having been exposed to
different ambient oxygen concentrations and thus different internal storage
concentrations should be different. The results of this study indicated that
there was no significant difference in afternoon rate estimates dependant on
time of dome placement although ambient oxygen <-oncentrationt ranged between
7.00 and 18.0 mg -1 depending on the time ot day the domes were set. If the
assumption that gases rapidly equilibrate between lacunae and external water
is correct, internal cycling does not appear t' explain the apparent afternoon
depression. For the second possibility we amended the domes in situ with
various combination and concentrations of NH , NO. and PO ,. Although the
data are not yet fully analysed, preliminary results suggest that the response
maj; be nutrient-related, i.e. nutrient amendments to incubation water using
N0_ and Nil, tend to increase the estimated rates. There is no indication of
what mechanism(s) may be involved as there appears to be no measurable
difference in response between single treatments using either NO- or NH
versus the combination. With regard to third possibility, we have not
attempted any studies at this time but at least for soybean Nafziger and
Koller (19/6) have demonstrated the influence of leaf starch concentration on
photosynthesis. Overall, the mean annual afternoon rate was 91.2 (_+ 35.4) mg
©2 m~2 h~' or 34.2 mgC m~* which is approximately one half the annual mean
estimate for the noon rate.
Estimates of total community respiration show a typical, temperature-
related response with a minimum of 28.1 mg 02 m~^ h~* occuring in February at
6.5*C and a maximum of 286 mg 02 m~2 h~' occuring in July at 27*C. Simple
linear regression of Arrhenius plots supports this and indicates a strong
correlation between in situ temperature and estimated night respiration (r
"-0.83). Comparison of night and early evening estimates suggests that
community respiration is stimulated at higher temperatures immediately
following the photoperiod and at these times is not a good estimate for total
community respiration. We attribute this response to the vascular plant
rather than any diel characteristic of heterotrophic metabolism occuring in
the grassbed. It would also appear that the stimulated rates are
metabolically related to the vascular plant rather than physically-chemically
controlled (i .*.. diffusion processes). The stimulated evening rates measured
at summer temperatures do not correlate with either in situ oxygen
concentration or light available during the photoperiod (discussed in greater
detail in the following section on light and community metabolism
interaction). Mean annual estimates for evening and night respiration are
-153. (+63 S.E.) and -1-.8 (^19.4 S.E.) mg 02 m~* h~l respectively and are not
significantly diffetent although the ranges are quite different. This is due
to the evening estimates including rate measurements made during part of the
photoperiod and the apparent stimulation occuring only at the higher, summer
temperatures.
Estimates for early photoperiod or morning rates, although relatively few
in comparison to the oth*r interval.!, generally tended to be lower than
corresponding afternoon intervals. Thp <>fffctive photoperiod for the morning
interval is probably less than the afternoon interval due to the approximate
-------
north-south orientation of the graasbed and the eastern border of forreated
land. The annual mean estimate for the morning interval is 62.3 ( + 39.2 S.E.)
m h~l or approximately 25Z of the annual mean afternoon rate.
Figure 9 illustrates the annual behavior for all rate measurements by
interval with the mean daily temperature over the duration of the experiments
indicated. The data suggest that net apparent community 02 productivity is
binodally distributed over the year. Productivity peaks in mid to late spring
and again in fall. Water temperatures in and around 15*C appear to be near
optimum. During mid-summer and at temperatures between 23* and 27*, net
apparent community 02 productivity is reduced and the "L. marina community as a
whole is heterotrophic (Table 3). Temperature stress is suggested as the
principal mechanism accounting for this result together with increased
heterotrophic activity.
The R. maritime Dominated Community
Table 4 summarizes the net apparent rate of oxygen production or
consumption in the shallow, R,. maritima dominated community. As for "L_.
marina, maximum rates of net apparent 02 productivity were generally
associated with the noon interval throughout the growing season. Estimates
ranged from a minimum of 90.6 (_* 108. S.E.) mg02 m~2 h~* in October (15.4*C)
to a maximum of 536 (_+ 83.4 S.E.) mg02 m~2 h~^ in July (26.3*C). Arrhenius
plots and simple linear regression of the transformed data (Figure 10)
indicate temperaturt.-dependance for noon estimates (r "-0.771). As opposed to
J5. marina, the temperature-dependance appears to hold throughout the range of
measured in situ temperatures (15.7 to 28.1"C). Although we have measured
temperatures in the shallow areas as high as 32 to 33*C, whether _R. maritima
is temperature stressed at these higher temperatures is not known. These data
suggest that temperature optimum for net apparent 02 productivity and the
tolerance range is higher for It. maritima than Z_. marina.
Morning rate estimates were equal to or greater than noon estimates in
several studies. In 1979, morning rates were equal to or greater than noon
estimates only in late summer-early fall. In 1980, the situation was
reversed, i.e. morning rates equalled or exceeded noon estimates during the
early growing season. Morning estimates ranged from a minimum of -98.7 (+_
70.3 S.E.) mg02 m~2 h'Un April (16.4*C) to a maximum of 493 C+233 S.E.) Tn
mg02 ™ h August (27.9*C). Combining all data, mean annual rates were 184
(+ 50.0 S.E.) and 266 (^48.9 S.E.) mg02 m~2 h"1 for morning and noor
intervals respectively.
..s opposed to the "L_. marina community studies, afternoon rates were
alw£/s depressed relative to morning and noon interval rates (Figure 11).
Afternoon interval net apparent 02 productivity estimates ranged from a
minimum of -514 (*^ 316) in July to a maximum of 103 (*8.30) in June. The
highest afternoon depression occurred during midsummer and indicated net
community 02 consumption under otherwise optimum temperatures and favorable
light conditions. Testing this result in the same way as for the apparent
depression in Z. marina afternoon rates during mid-summer, there was no
significant correlation between rate estimates and oxygen concentration or
time of dome placement. We must assume at this point that the apparent
75
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Figure 11. Scatter plot of mean daily noon and afternoon rates of net
apparent 02 productivity in the R. maritime dominated community
versus calendar day.
80
-------
depression which appears more characteristic for the R_. maritima community
results from either an internal, physiological or metabolic property of the
macrophyte or limitation by factor(s) not measured or included in the
experimental design. Mean annual net apparent 02 productivity for the
afternoon interval was -65.8 (*_ 56.2) mg02 m~^ h"*.
As for the Z_. marina community study, evening interval estimates for net
apparent Q£ consumption were higher on all occasions except one (July 1980)
than estimated night time community respiration. The apparent stimulation of
community 02 consumption in the early evening following the photoperiod was
much more pronounced in the R_. maritima community and was generally double the
nighttime community respiration estimate. On one occasion (August 1980), the
early evening rate exceeded nighttime respiration by a factor of six. The
highest rate or strongest depression tended to be associated with the highest
net apparent 02 noon productivity estimates. Simple linear regression of all
paired noon and evening interval rate estimates indicate a significant inverse
correlation (r • 0.871). As for the apparent afternoon depression in the _R.
maritima community, causal factors are not known although the data suggest it
is coupled to plant photosynthesis and respiration and not a heterotrophic
component of the community.
Total community respiration as estimated by the night interval
determinations (Table 4) indicate a typical temperature-dependent response.
Arrhenius plots and simple linear regression of the transformed data indicate
a significant correlation (r =-0.758) between estimated total community
respiration and temperature. Combining all data, the mean annual rate of
respiration is 131 (*_ 19.4 S.E.) mg Q£ m~2 h~^ or approximately one half the
overall mean rate of 265 (^44.1) mg )^ m~^ n"1 f°r tne evening interval.
The Co-dominated R. maritima and Z. marina Community
Table 6 summaizes net apparent 02 production or consumption at the
intermediate depth, !l. maritima and Z_. marina co-dominated community. The
studies, though fewer in comparison to the number of experiments conducted in
uonospecific stands of II. maritima or Z. marina, cover principal times in the
growing season of both species. From the discussion and presentation of
results before, it is apparent that tne behavior of 02 production or
consumption in the mixed grassbed results from the combined characteristics of
the two species. The data are too limited to determine correlative indices
for temperature or light dependance. Maximum rates of net apparent 02
productivity were associated with morning and noon intervals, the higher
morning rates probably reflecting _R. maritima influence. Depression of rates
were not as clearly indicated for either afternoon or evening intervals and
when a depression was indicated in the data, it was not as severe as for
estimates in the R. maritima or Z. *narina community alone.
Combining all data by interval, mean annual estimates of community 02
production or consumption are summarized by vegetated community type in Table
7. The results indicate the overall response of the mixed grassbed community
and suggest that in comparison to the monospecific macrophyte communities net
apparent 02 productivity extends through a longer portion of the photoperiod
as a result of species specific responses, afternoon depression in apparent
81
-------
rates is less severe and evening and night time interval estimates for
community 02 consumption are intermediate. From these studies, though limited
in comparison, the data suggest that production-consumption relationships as
estimated by 02 exchnage maybe more evenly distributed not only over a diel
period but over an annual cycle as well in these co-dominated communities.
Non-vegetated Substrates
Table 8 summarizes our relatively few studies on non-vegetated sediments
both within and adjacent to the Vaucluae Shores study area. Bare sediments
(i.e. sand patches) within the grassbed have rates of community 02 production
and consumption much lower (on most occassions by nearly an order of
magnitude) than vegetated zones. Extremely high rates were determined on one
occasion, July 1979, when an observable plankton bloom occurred in the water
column. For our nearly three continuous years of study in this area, we have
observed this on only one occasion and include it in the data to indicate the
potential though apparently infrequent range of values possible. Bare
substrates outside the grassbed (sandbar) were low and indicated net 02
consumption at high summer temperatures and low but net 02 production in
October during the day.
Interaction between Light (PAR) and Net Apparent Community 02 Productivity
A total of eighteen in situ plant community-light response studies were
conducted during 1980 in the three principal vegetation zones at the Vaucluse
Shores study site. The majority, eleven, were carried out in the £. marina
dominated community and encompassed a temperature range of 6.5*0 to 28"C which
covers the mid-winter to late summer period (February through September).
Five studies were carried out in the jl. maritima dominated community from the
late spring throughout the principal summer growing season (May through
September). Two studies were conducted in the co-dominated Z_. marina - jl.
maritima community; one at 20*0 (May) near the optimum temperature for "L,
marina and one at 27*C (J -ly) near the optimum temperature for R. maritima.
The Z. marina Dominated Community
Figure 12 illustrates the results for net apparent 0-> productivity (NAP)
versus PAR at the plant canopy top for experiments conducted during the winter
anri spring seasons. NAP during these periods shows a response to increasing
PAR typical of the light-photosynthesis interaction. For these data, we have
assumed th. t the response is best described by a rectangular hyperbola and
used the linearized form of the Michaelis-Menten function to estimate the
various descriptive parameters for the response curve using least squares.
Net apparent community productivity light-saturates between 0.5 and 1.0 E
m"^"1 or approximately 140 and 280 ^E tn"^"1 during this period of the
growing season. Calculated maximum net rates for all studies generally were
i;i the range 200 to 300 rag 02 m~2h~l. Table 9 summarizes the descriptive
parameters for the response curve in terms of the initial slope (o<) light
level at the estimated half-saturated rate (l£), and the estimated maximum net
apparent rate of 02 production (NAPmax). Estimated NAPmax is approximately
200 mg 02 nT2h~' for all studies and increases only slightly over the 6*C to
82
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TABLE 7. MEAN ANNUAL ESTIMATES OF NET APPARENT 02 PRODUCTION OR CONSUMPTION
(rag 02 tn~2 h"1 •«• S.E.) BY INTERVAL AND COMMUNITY TYPE AT VAUCLUSE
SHORES.
Interval
Community
Z . ma r i 11 a
R. marl t ima
Mixed
Morn
62.3
(39.2)
184.
(50.)
305.
(78.2)
Noon
230.
(41.0)
266.
(48.9)
128.
(3U.6)
AN
91.2
(35.4)
-65.8
(56.2)
-37.0
(40.6)
Even
-153.
(63.0)
-265
(44.1)
-178
(10.5)
Night
-148.
(19.4)
-131
(19.4)
-133.
(34.2)
84
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19-20 MAR 1980
29-30 APR i960
E m h r
3 4
AT CANOPY TOP
Figure 12. Scatter plot of net apparent rates of 02 productivity versus
available submarine light (PAR) reaching the plant canopy top in
the Z^. marina dominated community during the early growing
season.
HG
-------
"
Table 9. NET APPARENT 02 PRODUCTIVITY AND LIGHT RESPONSE CHARACTERISTICS FOR THE
^ARLY GROWING SEASON IN THE ZOSTERA MARINA DOMINATED COMMUNITY AT VAUCLUSE
SHORES, LOWER CHESAPEAKE BAY, VIRGINIA.
Date Temp
CO
20/21 Feb 1980 6.0
19/20 Mar 1980 9.5
29/30 May 1980 18.0
NAP^x1 CAP2 J Ik/4
( ".NAP/ '£)
208 278 996. 24.8
200 323 752. 35.7
241 464 2<)2 83.3
CP5
25.0
35.8
83.0
1. NAP,,,ax = Maximim rate of ostmaCo;) net apparent ('2 productivity (me 0^ m 2 n 1).
2. GAP = Estimated Gross Apparent 0-> Productivity (GAP = NAPmax + R).
3. Initial Slope or ".NAP • .'K~l.
4. Half-saturation coefficient or I 1 NAP = 0.5 NAPmax (;iE m~2 sec"1).
5. Compensating li«ht intensity (i.E m~2 sec"1) (? NAP = 0.
87
-------
:,
A
18*C temperature range encompassed by the studies. The estimated NAPmax
values compare well with the net apparent noon rate estimates derived I'M the
total community 02 exchange studies (Table 2). However, gross apparent 02
productivity (GAP), calculated as the algebraic sum of NAPmax and night
respiration, (see Table 2) increases by a factor of 1.7 in a near linear
fashion from February through May. Converting the February and May GAP
estimates to specific rates using our biomass estimates and assu- ~-'ng a
community PQ of 1.0, results in estimates of 1.10 and 1.29 mgC-g OW"'(plant)
h"1 respectively which are comparable but slightly lover than the likC
estimates of 1.60 and 1.68 for the same times of year (Table 1).
Light response characteristics of the community changed significantly.
The estimated half-saturation light level (1^) increases while the initial
slope or low level light response characteristic (£*) decreases. Both
responses are probably attributable to age and growth of the community as a
whole. In February, the plant community is dominated by "young" shoots (mean
length m 13.5 cm) while in late May-June the plant community is dominated by
"older" shoots (mean length • 24.9 cm) with obvious shoot senescence
beginning. These estimated light-response parameters .-gree very well with
those derived from the '"*C-photosynthesis studies (Table 1). Compensating
light intensities (CP^, i.e. light intensity where the net apparent rate
approaches zero, track almost exactly the T values and indicate the near
total dominance by the macrophyte in governing 02 production and consumption
during the early growing season.
Figures 13 and 14 illustrate the net apparent rate of community 02
productivity response to increasing light during the Z. marina summer studies.
The relationship indicates a linear response for all studies with no
suggestion of light saturation securing for the levels measured. It is
apparent that the community response is significantly correlated with light
but cannot be described by the typical, hvperbolic, light-photosynthesis
relationship. Therefore we have used sump'e linear regression statistics to
describe the response.
Table 10 summarises the simple linear least squares statistics for these
data. Community 1 ifc'nt response decreases over the course of the summer as
indicated by the chanp"? in slope of the regression equations. The change is
not associated with an overall increase in cor-munity respiration and thus
probably represents a phot:>synthetic response by the macrophyte. As discussed
previously, we believe this represents a temperature-induced stress.
Compensating light intensities for the community are high (x " 329 jaE m~*
sec'l) during the suntner period due to, 1) the assumed photosynthet ic
temperature stress, 2; high community respiration and 3) the apparent
simulation in afternoon r»spirat'on.
To illustrate, Figure 15 presents the results of a July studv which w»s
typical for the summer period. For this particular experiment, light levels
were high or at least sufficient to attain maximum rates based on the '^C
studies and the early growing season results. Water temperatures were near
tht- maximum in situ measures taken in the Z^. marjna community. It is apparent
that the maximum calculated rates were linearly dependant only en light
availability during the first part of the phoi period. This was followed by
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750
500-
250-
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•250-
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!750
i •
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0
-250-
-500-
-750
19 AUG. I960
t *:
23 SEPT. 1980
•• • •
•• ••
01234
E • m'-hr" AT CANOPY TOP
Figure 14. Scatter plot of net apparent rates of 02 productivity versus
available submarine light reaching the plant canopy top in the Z,
•arina dominated community during late Summer-early Fall. ~
90
-------
Table 10. NET APPARENT 02 PRODUCTIVITY AND LIGHT RESPONSE CHARACTERISTICS FOR THE
SUMMER SEASON IN THE ZOSJERA MARINA DOMINATED COMMUNITY AT VAUCLUSE
SHORES, LOWER CHESAPEAKE BAY, VIRGINIA.
Date
2 June 1980
5 June 1980
9 July 1980
16 July 1980
19 Aug 1980
23 Sep 1980
Temp
CO
26.
26.
27.
27.
25.
28.
(ANAP/AE)
307.
286.
96.5
124
89.2
108.
(mg 02 m'V1)
-472
-309
-147
-67.1
-8*. 5
-160.
CP
(HE m~2sec'1)
427.
300.
423
150
263
411
R3
(mg 02 m ' h )
-223.
-148.
-
-286.
-131.
-154.
1. Slope of the simple linear least squares fie equation.
2. Estimated y-intercept or R (mg 02 m"2 h"1) @ I - 0.
3. Estimated night time community respiration (see Table 2) (mg 02 m~ h~').
91
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9 e «
•o ••* u
« • u
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. I"
3 W k.
• I O
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-------
•n afternoon depression in apparent productivity indtpendant of light; i.e.
comparison of the four light-level response curves.
Based on these data, net apparent 02 productivity and light relationships
in the "L. marina community are governed by both in situ light regimes and
temperature. During the winter-spring period, submarine light regimes appear
to be the principal control on net apparent community 02 productivity. During
the summer period, temperature and increased respiration due to both
autotrophic (i.e. the macrophyte) and heterotrophic components controls net
apparent 02 productivity.
The R. maritima Dominated Community
Figure 16 illustrates the results typical of the light-community 02
productivity studies conducted in the shallow. Jt. maritima dominated
community. As discussed previously, the community characteristically had
depressed afternoon rates of net apparent 02 production and on many occasions
had net rates of 02 consumption with otherwise optimum light and temperature
conditions. Earlier experiments suggested the response was independant of in
situ 02 concentration but was, over the course of an entire year of study,
highly correlated with noon rates. The results of these shading studies
conducted in July indicate that the response is independent of absolute light
levels, at least those occuring during 02 productivity measurement (Figure
16). All shading treatments indicate an inflection in the rate of apparent
productivity near solar noon followed by depressed afternoon rates. Mote that
the inflection occurs prior to peak insolation regardless of treatment. We
are unable to attribute any causal mechanism(s) to this response which waa
characteristic of both the total community 02 exchange and shading studies.
Figures 17 and Id illustrate the results of all community 02 productivity
and light response studies conducted in the II. maritima area. Because of the
depressed afternoon rates, analyses of these data using the typical
light-photosynthesis response is not possible. The majority of the low rates
illustrated in the figures at otherwise high light intensities are afternoon
rate determinations. The July study results come closest to approximating a
typical light-photosynthesis response curve. Using only the morning and noon
interval rate calculations, the plant community appears to light-saturate
between 2.5 and 3 E m~2 h~* or 694 and 833 ^jE m~2 sec"1 which agree well with
our previous ^C light saturation estimates.
As a preliminary analysis of these data, Table 11 presents the simple
linear regression statistics for morning and noon intervals versus light at
the plant canopy top. Using the linear regression equations, we estimated
NAPmax at 2.34 E m~2 h"1 (650 juE m~2 sec"1), i.e. the midpoint of the
estimated ^C range, and CP at zero rate of net apparent 02 productivity. The
initial slope (OQ and y-intercept (b) are simply the terms of the least
squares best fit equation. Based on these analyses, there is no significant
change in either NAP, GAP or for the four, _R. maritima light response studies
summarized in the table. The experiments covered a rather limited temperature
range (21*C to 28*C) but did encompass the major growth period for the
macrophyte. Compared to the results from the "I. marina community shading
studies (Table 9), the estimated maximum rates of NAP and GAP are higher for
93
-------
(,-JM
(, s.
xnid
94
-------
Ruppia maritime
750
500-
250
0
-250 -I
- 500 -
C7-
E +750
500 -
n
o
o:
a
-250
-500 -i
-750
0
7 MAY 1980
I *
II JULY 1980
_u
E • m"? hr" AT CANOPY TOP
I
4
Figure 17. • Scatter plot of net aparent rates of Q£ productivity versus
submarine light (PAR) reaching the plant canopy top in the ]l.
maritima dominated community during the first half of the growing
season.
95
-------
E
O~
?
o
o
o
cr
a.
Ruppia maritima
750
500-
250-
-250
-500-
500 -
i
250-
0
-250-
-500-
-750
21 AUG 1980
-»» •-
26 SEP 1980
:*
W
E • m* hr"' AT CANOPY TOP
Figure 18. Scatter plots of net apparent.rates of 02 productivity versus
submarine light (PAR) reaching the plant canopy top in the II.
maritima dominated community during the last half of the growing
season.
96
-------
TABLE 11. NET APPARENT 02 PRODUCTIVITY (MORNING AND NOON INTERVALS ONLY) AND LIGHT
RESPONSE CHARACTERISTICS FOR THE R. MARITIMA DOMINATED COMMUNITY AT
VAUCLUSE SHORES, LOWER CHESAPEAKE BAY, VIRGINIA.1
Dace
Temp
•c
NAPmax
GAP
CP
7 May 1980 21 613
11 July 1980 28 665
21 Aug 1980 26 495
25/26 Sept
1980
24.5 531
742
867
645
722
487
346
405
306
287
133
311
167
-487 -129.
-166 -202
-453 -150.
-183 -191.
576
744
386
224
1. See Tables 9 and 10 for explanation of column headings.
97
-------
£. maritima under optimum conditions and the characteristic light response (<*)
Ts lower.Compensating light intensities in _R. maritima are comparatively
much higher than for Z_. marina. All these results are consistent with our
other findings and support the conclusion that Z_. marina and _R. maritimi are
photosynthetically quite distinct having different temperature-light optimum
and light response characteristics. Combining all light and productivity data
for the R_. maritima community since the shading studies were limited in
temporal coverage, Figure 19 indicates there may be seasonal changes in light
response characteristics and that the macrophyte is best adapted to high
light-high temperature summer conditions in the lower Chesapeake Bay.
The Co-dominated R. maritima and Z. marina Community
The results of the two shading studies carried out in Che mixed community
are illustrated in Figure 20. In May, when Z_. marina should have dominated
the community rate of 02 exchange, there was no obvious relationship to light
and overall the net apparent rates were low. It may be that in the mixed
community, the growth characteristics for the macrophyte are different than in
the deeper-water, monospecific stands. For the July study, it is apparent
that R_. maritima dominates the community. The results follow very closely
those of the study in the monospecific stand (Figure 17).
SUMMARY
Seagrass communities in both temperate and tropical environments have
received increasing research attention over the past decade (e.g. McRoy and
Helfferick 1977; Phillips and McRoy 1980). Studies have in large part focused
in four areas; 1) macrophyte productivity including both photosynthesis and
biomass production studies (e.g. Jones 1968; Zieman 1968; Dillon 1971;
Patriquin 1973; McRoy 1974; Sand-Jensen 1975; Zieman 1975; Bittaker and
Iverson 1976; Beer and Waisel 1977; McRoy and McMillan 1977; Penhale 1977;
Zieman and Wetzel 1980), 2) nutrient relationships (e.g. Goering and Parker
1972; Patriquin and Knowles 1972; McRoy and Alexander 1975; McRoy et al. 1979;
Capone et al. 1979; Penhale and Thayer 1980), 3) trophic relationships and
secondary production (e.g. Zimmerman et al. 1979; Kikuchi 1980; Ogden 1980;
Orth et al. 1982), and, 4) various aspects of total seagrass community
metabolism (e.g. Odum and Hoskin, 1958; Odum, 1963; Nixon and Oviatt 1972;
Nienhuis 1980; Lindeboom and DeBree 1982). All results point to the relative
important roles these communities play in many estuarine and coastal marine
ecosystems.
Comparatively, temperate seagrass communities have received far less
attention than subtropical and tropical communities. In fact studies of
temperate seagrasses along the U.S. Atlantic Coast prior to the mid-1970's
were relatively few and focused attention almost exclusively on a single
species, Zostera marina. It has only been within the past five to ten years
that concerted efforts have been made to investigate the ecology of temperate,
estuarine seagrass communities along the U. S. east coast. Therefore,
historical data bases and comparative studies are limited and relatively few
against which to specifically judge our investigations.
98
-------
o>
E
o
O
cc
CL
UJ
CE
0.
a
800-1
600
Ruppi ' maritime]
> 1
~E 400^
200-1
-200-
-40O-
-600
-800-
/
o /
o o o s
o
oo o / ""SUMMER
SPRING
! I ---r— - — p-
2345
INSOLATION ( E m2- hr'1)
Figure 19. Scatter plots of net apparent rates of 62 productivity in the R^.
maritima dominated coonnunity partitioned into Spring T*C <25) and
Summer ft'C >) seasons. Lines illustrated in the figure are the
simple, linear least squares lines of best fit.
99
-------
Zostera - Ruppia Mix
_
*e
f
»-
E
o
13
o
o
(T
a.
1 3W —
500-
250-
-250 -
-500-^
+ 750-
5 MAY I960
•
*.;
t •".*''
_i • •
•f^ • .
•
14 JUL 1980
500 -j
250-
0 -
-250-
-500-
_ TKn —
1
Jj X* • 8
* ** *•
•*•• * •
•?•*•*• • **
i
E • m hr
2 3
AT CANOPY TOP
Figure 20. Scatter plot of net apparent rates of Oj productivity versus
submarine light (PAR) reaching the plant canopy top in the
co-dominated Z. marina and R. maritima community.
100
-------
Orth et al. (1982) has provided a comprehensive and detailed account of
the relative abundance and distribution of seagrass communities in the Lower
Chesapeake Bay. The characteristic and predictable depth-dependant
distribution and conation patterns of the two dominate macrophyte species, Z_.
•arina and II. maritima, a priori suggests significant differences in the
physiological and production ecology of the two species.
The results of our studies indicate that JS. marina has a lower
temperature optimum, greater photosynthecic efficiency and lower light
requirements than II. maritima. At typical mid-summer water temperatures in
shoal areas of the lower Chesapeake Bay (i.e. 28 to 30*C) £. marina is
temperature-stressed and in terms of total community metabolism is
heterotrophic. Peak periods of productivity, spring and fall, correspond with
optimum in situ light-temperature regimes. The interactive effect of
increasing turbidity and water temperature in the late fall and early summer
is suggested as the principal cause initiating the high summer plant mortality
(die-back). In fact, the two, i.e. light regime and water temperature, may
not operate in phase as controls on production and growth. For example, daily
insolation and turbidity as estimated by mean vertical light (PAR) attenuation
(Figures 3 and 4, respectfully) are the most variable during April and May
when water temperatures are apparently near optimum. In early Summer,
insolation and PAR attenuation are less variable but temperature becomes
suboptimal. The interaction between these and their temporal behavior (or
timing) may result in considerable year to year vatiation in biomass
production and standing crop as indicated by Orth (personal communication,
1982). Assuming a mean daily insolation of 30 E m~* day"* (Figure 3) during
the Spring growing season and a mean PAR attenuation coefficient of -1.50
(Figure A) for the same period, average light intensity over a twelve hour
photoperiod at a depth of 1.5m (approximate mean depth of the Z. marina
grassbed) would equal 155 ^xE m~2 sec~^ which is above the community
compensation level but below the estimated photosynthesis light saturation
intensity. These data suggest that the Z. marina community at the Vaucluse
Shores study area may be light-stressed or at least growing under sub-optimal
light regimes during a significant portion of the growing season. Because of
the detailed similarity in structural characteristics of the Vaucluse area and
other seagrass beds in the Lower Chesapeake Bay (see Chapter 1, this report;
Orth et al, 1982), we assume that these conclusions are applicable to Z.
marina dominated communities for the lower Bay in general. It seems apparent
that "L_. marina communities in terms of growth and survival in the lower
Chesapeake Bay are existing under marginal environmental light and temperature
conditions. This suggests that perhaps relatively small changes in either
magnitude or temporal coincidence could result in significant changes in
structural and/or functional aspects of their community ecology.
Compared to Z_. marina, II. maritima productivity and growth response to
light and temperature are nearly opposite. R. maritima photosynthesis-light
response is characteristic of high-light or sun adapted species. The species
is characterized by a generally higher Pmax, half-saturation light intensity,
compensating light intensity and lower -value or less photoaynthetic
efficiency at low light intensities than Z_. marina. Physiologically, R..
maritima appears well-adapted to the high light-high temperature regimes
characteristic of the shallow, inshore areas. Assuming a mean daily
101
:
J
-------
insolation of 60 E n~2 day'1 during Che summer growing season for R. maritima
(Figure 3) and mean PAR attenuation coefficient of -1.25 (Figure 47 for the
sane period, average light intensity over a twelve hour photoperiod at a depth
of 0.5 m (approximate mean depth of the Fl. maritima grassbed) would equal 750
uE m~2 sec"*. This light intensity is photosynthetically saturating based on
our results and suggests the fl. maritima comnunity is not generally light
United. These data coupled with the apparent high temperature tolerance of
II. maritima suggests these communities in the nearshore areas are existing
near optimum light and temperature conditions. However, this conclusion must
remain tentative due to the apparent afternoon depression in 02 productivity
and our estimates of the various photosynthetic parameters being based only on
morning and noon interval determinations. The characteristic apparent
afternoon depression suggests a physiological stress either directly or
indirectly related to temperature and/or light.
From the standpoint of the entire seagrass bed, production and metabolism
within the dominant macrophyte communities are complimentary which tends to
provide a source of organic matter production throughout the majority of fhe
year even though the specific dynamics are very seasonal and different. In
terms of production and because of these species-specific characteristics, the
co-dominated communities probably represent the most trophically stable
habitat.
Baaed on these results, light and temperature act as primary controls on
seagrass photosynthesis and community metabolism. For Zostera marina, light
or those factors that control submarine light regimes in the shoal benthic
environment is singuarily important in governing growth and survival of the
species in the lower Chesapeake Bay.
102
-------
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107
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Chapter 3
OXYGEN METABOLISM OF A TEMPERATE SEAGRASS (ZOSTERA MARINA L.)
COMMUNITY: PLANT-EPIPHYTE, PLANKTON
AND BENTHIC MICROALGAE PRODUCTIVITY AND RESPIRATION.
L. Murray and R. L Wetzel
Virginia Institute of Marine Science
and School of Marine Science
College of William and Mary
Gloucester Point, VA 23062
108
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ABSTRACT
Monthly studies of gross and net ©2 productivity and respiration by the
principal autotrophic populations of a temperate, Zostera marina dominated
seagrass community are reported. The principal components partitioned by the
studies were the Z_. marina-epiphyte assemblage, phytoplantcton and benthic
microalgae. Seasonally, the dominant autotrophic component varied. The
macrophyte-epiphyte assemblage dominated when water temperatures were at or
below 20*C while the plankton component dominated at higher temperatures.
Benthic microalgae represented a relatively low but constant source of organic
matter input. For both plankton and benthic microalgal communities, 02
production and consumption are nearly mass balanced annually. However,
approximately 40Z of macrophyte-epiphyte production is available for
transport. Integrated gross annual production and percent contribution to
total community production are estimated at 967 gC m~2 y~* (55Z) for the
macrophyte-epiphyte, 488 gC m~2 y"1 (31Z) for phytoplankton, and 225 gC m~2
y~l (14Z) for benthic microalgae. Total gross autotrophic production in this
temperate, Zostera marina aeagrass community is estimated at 1580 gC m~2 y~ .
Of this approximately 600 gC m~2 y~* are directly available for
microheterotrophic and/or macroheterotrophic utilization and secondary
production.
109
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INTRODUCTION
The role and relative importance of seagrasses in estuarine and coastal
marine environments has been the subject of recent extensive research.
Seagrasa meadows are generally considered valuable because they are
characterized by high primary and secondary production and serve as nursery
grounds and refuge areas for many marine and estuarine organisms. Most
studies of autotrophic production in these communities have focused on the
dominant vascular plants, Zostera marina L. in temperate zones and Thalassia
testudinum Konig in the tropics, although other sources of autotrophic
production can provide signiticant input of organic matter (Penhale 1977).
Seagrass communities can have at least four other autotrophic components
that directly support secondary production and are probably the least
understood: benthic macroscopic and microscopic algae, phytoplankton, and
epiphytic autotrophs.
Recent work on seagrass production has provided estimates ranging from
200-3000 gC m~2 yr"1 for Thalassia testudinum (Jones 1968; Bittaker 1975;
McRoy & McMillan 1977) and 200-800 gC nT2 yr'1 for Zostera marina (Nixon &
Oviatt 1972; MeRoy 1974; Neinhuis 1980). Several methods have been employed
to arrive at these estimates, including openwater flow studies (Nixon & Oviatt
1972), chamber incubations of various designs enclosing the intact community
(Neinhuis 1980; Lindeboom & deBree 1982), the uptake of 1AC radiotracers by
excised planes (MeRoy 1974; Penhale 1977; Capone et al. 1979), leaf-marking
techniques (Zieman 1974; Neinhuis 1980) and mathematical regression techniques
using morphometric parameters (Patriquin 1973). Photosynthesis and
respiration of both individual plants as well as the intact plant community
have been investigated using various gas exchange methodologies (02 and CC^)
and *^C radiotracers. Although these methods all measure something slightly
different, the range of estimates among methods are surprisingly similar
(Wetzel et al. 1982b).
Several investigators have demonstrated that autotrophs other than the
macrophyte are responsible for a significant portion of total community
production. Work in freshwater lakes indicated that phytoplankton and
epibenthic algae contribute over 50Z to total lake production 'Wetzel 1964;
Wetzel & Hough 1973). Cattaneo and Kaeff (1980) report that epiphytic algae
on the freshwater angiosperms Myriophyllum spicatum L. and Potamogeton
richardaonnii (Benn.) Rydb. contribute as much as 60Z and 30Z, respectively,
to the total plant-epiphyte complex. Comparable studies in marine ecosystems
have shown similar results. Jones (1968) working in a Florida Thalassia
testidinum grass bed determined that macrophyte production contributed 900 gC
m~^ yr~*, the benthic microflora 200 gC m~2 yr~*, and epiphyte production 200
gC m~2 yr~l or, combining the microalgae components, approximately 30Z of the
total. In North Carolina, Dillon (1971) estimated that Zostera marina and
Halodule beaudettei (den Hartog) den Hartog production contributed
approximetly 85Z to the total organic matter input with phytoplankton making
up the remainder. Bittaker (1975) reported for a Thalassia testudinum grass
110
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bed in Florida that the relative contribution by microphyte* and phytoplankton
were approximately the same as reported by Dillon (1971). In a more detailed
study, Penhale (1977) indicated that macrophyte and epiphyte productivity in a
North Carolina Zostera marina community bed were equal at certain times of the
year and Borum & Wium-Anderson (1980) found similar results in Denmark.
Although these studies report high levels of production for the various
autotrophs in these communities, consumption rates may also be high,
especially in sediments having high faunal densities. Hargrave (1969) reports
a higher benthic carbon consumption rate than can bs supported by vascular
plant production in a freshwater lake. Lindeboom & deBree (1982), found that
although production is less for bare substrates than in nearby Zostera marina
areas, consumption is also less, indicating a higher heterotrophic biomass
within the grass beds. Considering, 1) a significant fraction of the vascular
plant production may not be metabolically unavailable to many heterotrophs, 2)
some plant material is undoubtedly exported and, 3) seagrass beds are
generally characterized by high infaunal and epifaunal biomaas, microalgal
support of heterotrophic production may be greater than is generally
suggested.
Based on these results and observations, it seems appropriate to
hypothesize that significant contributions to community production by
autotrophs other than the vascular plant are characteristic of submerged grass
beds in both temperate and tropical ecosystems. The relative contribution
appears to range between 202 and 50Z of the total community. We report here,
the results of studiee designed to partition the principal autotrophic
components of a temperate, Zostera marina dominated community in terms of
seasonal and annual contributions to total community production and
respiration. Because of a general lack of hard substrates in the Chesapeake
Bay, macroalgae are not a significant portion of the autotrophic communities.
Therefore we limited our studies to the plant-epiphyte, phytoplankton and
benthic microalgae components.
STUDY SITE
A seagrass bed approximately 140 ha in size located on the southeastern
shore of the Chesapeake Bay, Virginia, U.S.A. (37*25' N., 75*59' W.) was used
as the principal study site (see Preface, Figure 1). The entire meadow is
co-dominated by Ruppia maritima in the nearshore areas and by Zostera marina
in the deeper areas with an intermediate area of mixed stands •>£ the two
species. The studies reported here were carried out between transects B and C
and confined to the Zostera marina dominated community type (See Preface,
Figure 1).
MATERIALS AND METHODS
Total Community 02 Exchange
»
Oxygen exchange techniques were used Lo estimate rates of metabolism for
the various autotrophic components. Estimates of total community metabolism
employed large (260 1) plexiglass domes enclosing the entire seagrass
111
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community (see Chapter 2). Oxygen concentration was Monitored continuously
for all incubations. These data were used to calculate areal net apparent
production (NAP), respiration (R), and gross apparent production (GAP) for the
intact community.
A multichannel, Orbiaphere Oxygen Monitoring System (Model #2604) with
#2S insensitive polargraphic probes and self-contained atirrer was used to
measure oxygen concentration. Light as photosynthetically active radiation
(PAR; 400-700 nm) was monitored continuously using a Li-Cor Model 185A Quantum
Meter equipped with surface and submarine quantum sensors. Temperature was
recorded continuously from the Orbisphere which employs thermistors contained
in the probe head.
Area specific rates were calculated as:
iCj+rCj
.
mg 02 m-2 IT1 -AtUi^-ti) • Vd • Aj
where: C^ - (02] (mg I"1), i-0,l...n (hours)
t£ " time (hours) itn interval
V,i - volume of incubation (1)
A • bottom surface area
Daily rates were calculated by integrating the net apparent 02 productivity
hourly rates over the photo-period and respiration was as summed constant over
the 24 hour period. Seasonal estimates were derived by defining "season" as a
function of water temperature with winter <10*C; spring and fall 10 to 20*0
and, summer >20*C. Seasonal estimates were calculated as the algebraic sum
using means between consecutive (monthly) estimates. Because the relative
error is directly proportional to time step (i.e. calendar days between rate
estimates) for these calculations, for the data we report the winter estimates
are the least reliable. Annual estimates are simply the sum of the seasonal
estimates. For comparison to data reported elsewhere, we converted the oxygen
data to carbon units assuming a PQ of 1.25 for the net productivity estimates
and a RQ of 1.0 for the respiration estimates.
Water Column (Plankton) 02 Exchange
Water column plankton community samples were collected by hand and
incubated in triplicate, light and dark standard BOD bottles (300 ml).
Incubations were for four hours over the daytime interval 1000 to 1400 h.
For midday high tide studies, water depth at the study site ranged from 1.0 to
1.7 m and samples from near the surface (approximately 10 cm depth) and from
just above the canopy top were collected and incubated at the depth of
collection. For midday low tide studies, water depth ranged from 0.5 to 0.8 m
and Complete mixing was assumed. For these studies, only mid-depth water
samples were collected and incubated. Water column rates are reported per
square meter surface area and calculated using the average water depth over
the incubation interval. Daily rates were estimated by assuming the mean,
midday hourly rates were characteristic for the photoperiod and respiration
rates determined from the dark BOD bottle incubations were constant over the
112
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diel period. Seasonal and annual estimates were calculated as for the total
community.
Benthic 02 Exchange
For the benthic microalgae triplicate, light and dark, cylindrical
plexiglass chambers (750 ml) were placed on unvegetated sediment within the
bed and incubated as for the BOD bottles. Duplicate clear chambers inoculated
with 10 ml, 10Z (v/v) buffered aeawater formalin (sat. MgCC^) were used to
estimate sediment, chemical oxygen demand (COD). 02 exchange estimates,
corrected for COD, are reported as mg02 m~2 (bottom area) h~*. The amount of
unvegetated area within the Z. marina community was estimated from percent
cover data (Orth and Moore 1982) and the areal rate estimate corrected
accordingly. Daily, seasonal, and annual rate estimates followed the methods
and assumptions for the water column calculations.
Macrophyte-Epiphyte 02 Exchange
Rate estimates for the plant-epiphyte component were calculated as the
difference between total (dome enclosure estimates) and the benthic and
plankton rates estimated over the same time intervals. Respiration for the
plant-epiphyte component was calculated as the difference in nighttime
respiration for the clear dome incubations and rates for the dark chamber and
BOD bottle daytime incubations.
RESULTS
Table 1 summarizes environmental conditions for each date studies were
conducted. The studies covered the nominal water temperature range for the
area (see Chapter 2). For these specific studies, submarine light (PAR)
conditions were generally at or above photosynthetically saturating
intensities for both vascular plant and microalgae except ."or Z_. marina during
the April, early October and January studies. The studies encompass the ma}or
growth and die-back periods for this vascular plant community.
Net apparent productivity estimates for the three, principal components
over the study period are presented in Figure 1. The Z_. marina-epiphyte
component follows the characteristic, bi-modal growth cycle for Zostera marina
in Chesapeake Bay waters (Orth and Moore, 1982; Chapter 2). The arrows on the
figure indicate rate estimates calculated from oxygen exchange studies carried
out under suboptimal light conditions (i.e., less than known saturating
intensities) and are therefore probably underestimates. Over the period of
study, the dominant component is illustrated in the figure by diagonal line
shading for the plant-epiphyte component and stippled shading for the plankton
component. Winter, spring and late fall are clearly dominated by the
plant-epiphyte component and during mid-summer by the phytoplankton with a
possible secondary fall peak. Benthic algae generally had lower net apparent
rates (note scale change in Figure 1) with no clear seasonal pattern
indicated.
113
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PLANT-EPIPHYTE
500-1
ft
o
?
-500 -1
PHYTOPLANKTON
MAR
-100
MAR
MAY
JUL
SEP
NOV
JAN
Figure 1. Mean (*_ S.D.) net apparent productivity for the plant-
epiphyte and plankton components (top panel) and benthic component
(bottom panel). Diagonal line shading and stippled shading in the
top panel illustrate plant-epiphyte or plankton dominance
respectively. The arrows indicate suboptimal light conditions for
Z. marina.
-------
Figure 2 presents the respiration estimates for the three components.
Respiration of the plant-epiphyte component generally tracked net apparent
productivity increasing with increasing water temperature. Plant-epiphyte
respiration paralleled plant growth and dominated total community respiration
during and following the summer die-back period and again in late fall during
the secondary growth period. Plankton respiration showed no clear seasonal
pattern but reached minimum values during the periods of peak plant-epiphyte
respiration and maximum values during minimum plant-epiphyte respiration. As
for net apparent productivity, benthic respiration rates were generally lower
than either the plant-epiphyte or plankton components and paralleled benthic
net apparent productivity estimates.
Gross apparent productivity (calculated as the algebraic sum of net
apparent (Figure 1), and respiration, (Figure 2) rate estimates) is
illustrated in Figure 3. The plant-epiphyte component dominates prior to late
summer (August) followed by an apparent dominance by the phytoplankton in late
summer and fall. Benthic gross productivity never dominated total community
gross productivity but reached maximum rates during the summer.
Table 2 (summarizes the seasonal and annual estimates of gross production
and respiration. Because of our assumptions in the calculations, i.e.
microalgae midday rates are extrapolated over the entire photoperiod and
respiration is constant over the diel period, these estimates probably are
maximized. The pattern of seasonal dominance and rates of production by the
various autotrophic components indicate the important and potential
contribution made by each to total community metabolism. Based on these
calculations gross production by the plant-epiphyte component accounted for
between 37Z and 80Z of total dependent on season. Annually the vaacular plant
component contributed an estimated 967 gC m~2 or 55Z of total community gross
production. Phytoplankton gross production ranged between iOZ and 48Z
seasonally with an estimated gross annual production of 488 gC m~* or 31Z of
total. Similarly, benthic algae contribution ranged between IOZ and 25Z
seasonally with an estimated annual contribution of 225 gC m"2 or 14Z of the
total. Using these estimates of annual gross production by this seagraas
community is 1580 gC m~2. The microautotrophic components account for
approximately 45Z or 713 gC m~2 of this.
Respiration by the various components varied seasonally. The
plant-epiphyte component dominated all seasons except winter accounting for
47Z to 602 of the total. In winter, the plankton component accounted for 73Z
of total community respiration which we consider an overestimate due
principally to the few estimates we have; i.e. only two studies were conducted
at water temperatures below 10*C. Benthic respiration never dominated total
community respiration and accounted for 9Z to 23Z over all seasons. Maximum
rates and Z contribution to total occurred during the summer.
Production to respiration ratios (P/R<~) indicate the generally autotrophic
nature of each component. Both seasonally and annually, total community
metabolism was autotrophic. P/R ratios ranged from a maximum of 6.2 for the
plant-epiphyte component in winter to a minimum of 0.50 for the plankton in
spring. In terms of organic matter input to the seagrass community (i.e.
excess production versus respiration), the plant-epiphyte component clearly
116
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500 -I
o>
E
PLANT-EPIPHYTE
PLANKTON
MAR
MAY
JUL
SEP
NOV
JAN
200 -i
BENTHOS
I I T 11 T I I I I I
MAR MAY JUL SEP NOV JAN
Figure 2. Mean (+_ S.D.) respiration for the plant-epiphyte assemblage
and plankton components (top panel) and benthic component (bottom
panel). Diagonal line shading stippled in the top panel illustrate
plant-epiphyte or plankton dominance respectively.
117
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PLANKTON
PLANT-EPIPHYTE
500-i
'i
0~
MAR
MAY
JUL
SEP
NOV
JAN
200-1
e 100-
o~
f
BENTHOS
MAR
MAY
JUL
SEP
NOV
JAN
Figure 3. Mean (^S.D.) gross apparent productivity for the plant-epiphyte
assemblage and plankton components (top panel) and benthic
component (bottom panel). Diagonal line shading and stippled
shading in the top panel illustrate plant-epiphyte or plankton
dominance respectively.
118
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119
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dominates in the winter and spring, the plant-epiphyte and plankton in summer
and the plankton and benthic components in fall. The actual timing (i.e.
availablity) however differs due to the time lag between vascular plant
production and organic matter input (plant mortality). For the microalgal
dominated components, production and consumption are both spatially and
temporally more closely linked.
The metabolism data were further analyzed by simple, pair-wise linear
regression between selected environmental parameters (temperature and light)
and vascular plant community characteristics (biomass and cover). Table 3
summarizes these results in terms of the correlation coefficients for the
pair-wise regressions. As expected, respiration was significantly correlated
with temperature for all benthic components. Net productivity was positively
correlated with temperature for both plankton and benthos but only
significantly so for the latter. Net apparent productivity of the
plant-epiphyte complex and temperature was negatively and weakly correlated.
This result is consistent with other observations indicating that "L. marina is
stressed at temperatures approaching 30*C (see Chapter 2). As indicated,
earlier, light was optimum for the majority of these experiments and is
supported by the lack of significant correlation between light and net
apparent productivity.
Plant biomass and plant-epiphyte respiration was positively correlated a*
expected. Interestingly benthic respiration and plant biomass were highly
correlated. This result suggests a strong interaction between vascular plant
growth and sediment heterotrophic processes. There was no apparent
correlation between benthic net apparent productivity and percent plant cover
as might be suggested by the observation that vascular plant shading may limit
benthic microalgae photosynthesis. The result may also be an artifact of the
experimental design in that areas within the grass bed were chosen where
obviously the benthic chambers could be placed. Also, the measured light
levels are for 10 cm above the sediment surface due to the underwater sensor
design and thus may not accurately measure light at the actual sediment
surface.
DISCUSSION
Productivity and organic matter production in vascular plant dominated
submerged aquatic communities is divisable into at least four components whose
importance and contribution may vary both spatially and temporally. These are
the plant-epiphyte, benthic microscopic and attached macroscopic algae, and
phytoplankton. Studies designed to investigate organic matter production,
nutrient cycling and various aspects of trophic structure and/or energy-matter
flux in these systems have predominately focused attention on one autotrophic
component: the vascular plant. Obviously, the vascular plants structurally
define the boundaries of the system and govern conditions within which other
processes, both autotrophic and heterotrophic, must operate. However,
functional attributes of the ecosystem such as productivity, nutrient eyeing
and energy-matter flux with regard to the cycles of essential elements and
trophic structure may be partitioned among other autotrophic components that
have escaped the general attention of many studies. The studies reported here
focused attention on the principle autotrophic components of a temperate "L,
marina dominated habitat.
120
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The Plant-Epiphyte Conmunity
Phillips (1974) reported a production range of 10 to 1200 g dry plant
biomasa m~^ y~* for temperate graaa flats. Assuming a mean Z carbon (dry
biomaaa) of 36.4 (see Chapter 2), the range would be approximately 3.6 to 437
gC m~2 y~l. More recent annual estimates for temperate eelgraas communities
comparable to our study area report 350 gC m~2 for "I. marina in a North
Carolina grass bed (Thayer ££ a_K 1975). Using Penhale's estimates (Penhale
1977) for vascular plant and epipyte productivity for the same eelgraas
ecosystem, plant-epiphyte annual production wild equal 402 gC m~2. Penhale's
estimates were based on ^C uptake experiments. If, as Bittaker & Iverson
(1976) report, **C estimates net carbon fixation and as Lindeboom & deBree
(1982) suggest, **C may underestimate gross productivity by 502 for these
seagrasses, Penhale's estimate is very similar to our plant-epiphyte annual
net production estimate of 452 gC m~2 (Table 2). This suggests that
respiratory demand by this autotrophic component is dominated by the vascular
plant and would equal approximately 482 of annual gross production. Orth and
Moore (1982) report data from our study area on aboveground plant standing
crop over 16 consecutive months covering two yearly growing seasons. Using
their aboveground biomass data and the reported mean aboveground to
root/rhizome biomass ratio of 1.0 (Chapter 2), net production in the Z_. marina
dominated community would equal approximately 264 gC m~2. If we assume that
the difference between our estimate and Orth et al. (187 gC m~2) represents
leaf mortality, then approximately 41Z is potentially available for transport
out of the seagrasa system. For the 70 ha Z_. mar int. bed at our study site,
export could provide approximately 130 metric tons rf carbon per year to
contiguous waters.
The plant-epiphyte component dominates tota' community metabolism in
winter, spring and early summer with the highest respiratory demands occurring
after the spring growing season. The significant correlations between
aboveground plant biomass and respiration of the plant-epiphyte, and benthic
components (Table 2) suggests a strong, direct interaction between macrophyte
and sediment metabolism. Based on the work of Wetzel and Penhale (1979) the
two most likely mechanisms appear to be gas transport, primarily 02, to the
rhizosphere supporting aerobic, microheterotrophic processes and/or
extracellar release of dissolved compounds by the root/rhizome system during
the warmer months. This tentative explanation is in part supported by
sediment ATP studies reported elsewhere (see Chapter 2) which indicate that
both the depth distribution ( g ATP cc~l; 0 to 30 cm depth) and total
concentration (pg ATP nf2) parallel the general bi-modal growth pattern of Z,
marina at this study site. However, even if all respiratory demand of the
plant-epiphyte and benthic components are -nported by aerobic oxidation of
vascular plant organic matter, a significant fraction of plant production must
be lost to burial or exported from the system for the component to be mass
balanced. Based on the comparison between harvest data (Orth and Moore, 1982
and our rate estimates, we suggest at this point that a possible major route
is tidally-mediated, advective transport.
The Plankton Community
Comparative investigations of phytoplankton productivity and plankton
community dynamics in shoal and shallow tidal areas of estuaries are,
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surprisingly, relatively few. Moat eatuarine phytoplankton research haa been
concentrated in the deeper, openwater areas. For the Chesapeake Bay, a
reasonable annual production estimate would appear to fall in the range 100 to
200 gC m*2 (Patten e^£l_. 1963; Flemer 1970; Haas 1975; McCarthy jst al . 1975).
Our groas annual estimate of 488 gC m~2 in the seagrass bed (Table iTT
suggests that either these habitats have significantly greater phytoplankton
production than adjacent open-water areas or our estimates are biased by the
winter season values.
At the preseltt we have only limited and indirect evidence to support
either of the above. Thayer et_ jaJL (1975) report data that indicates the
phytoplankton community contributes approximately 30Z to total autotrophic
production where the total was partitioned among eelgrass and epiphytes,
phytoplankton and benthic algae. Our estimate of 31Z using the same principal
components agrees very well with their preliminary assessment. Independent
measures of phytoplankton photosynthesis from our study site (Wetzel et al.
1979) in summer and fall using **C radiotracer techniques indicate a July
average of 170 mgC m~3 h~l and an October average of 93 mgC m~^ h~^. These
agree well with our range of values for net apparent productivity (77 to 225
mg C m~2 h~l) using oxygen. Also, based on the **C studies, the principal
component of the phytoplankton are the nanoplankton (15 m and less) which
account for 67 to 83Z of ^C photosynthesis and A3 to 100Z of the chlorophyll
a_ standing stock (Wetzel e_t fj^. , 1979). Our tentative conclusion is that
phytoplankton photosynthesis and production is generally increased in these
shallow water habitats. This effect, if confirmed by further investigation,
likely results from the phytoplankton residing in more favorable light
conditions throughout the photoperiod, e.g. using the ^C data reported by
Wetzel et_ £K (1979), Ik - 86 yuE m~2 sec~* (SD+12, n-8) which is lower than
typical ambient conditions throughout the shallow water column. The higher
plankton productivity may also result from more favorable nutrient conditions
due to high remineralization rates (e.g. Nixon 1981) and spatial juxtaposition
between photosynthesis and nutrient regeneration in the seagrass bed.
Seasonally, plankton production compliments the plant-epiphyte component.
Plankton metabolism dominates the total from mid-summer through early fall
when the vascular plants are dying back. During the second fall growth period
both components contribute approximately equally to total community
metabolism. However, on an annual basis plankton respiration and gross
production are approximately balanced; 488 versus 434 gC m~2, respectively.
If we assume, as a maximum, autotrophic respiration is 102 of gross
production, then phytoplankton production supports a relatively large
planktonic heterotrophic community. The annual P/R ratio of 1.1 suggests that
production and consumption are tightly coupled over an annual cycle within the
plankton component and are dynamically quite different than the plant-epiphyte
component .
The Benthic Algal Community
Due to the general lack of hard substrates in Chesapeake Bay, benthic
primary production is dominated by sediment microalgae. Annual estimates of
production for a variety of sediment types generally range between 100 and 200
gC m~2 (Pomeroy 1959; Grontved 1960; Pamatmat 1968; Marshall e£ fl_. 1971;
123
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Riznyk and Phinney 1972; Cadee and liegemen 1974, 1977; Gallagher and Daiber
1974; van Raalte and Valiela 1976; Joint 1978; Zedler 1980). Comparative
measures of production and respiration for different substrate types in
Chesapeake Bay shoal areas have only recently been reported (Rizzo and Wetzel,
unpubl. ms.). For five different but geographical close sediment types,
they report an annual gross production range of 107 to 224 gC m~2 with a
subtidal eelgrass site estimated at Ib7 gC m~2. Considering the high degree
of spatial and temporal variability associated with these measures (Rizzo and
Wetzel, unpubl. ms), our annual estimate of 225 gC m~2 is consistent with
these data. We estimate that the benthic microalgae contribute approximately
14Z to total annual production which i» approximately two fold greater than
Thayer et_ al_. (1975) estimate of 8Z.
Seasonally, '.. jnthic productivity and respiration represent a relatively
constant proportion of the total. As for the plankton community, respiration
appears dominated by microheterotrophs. The apparent discrepancy between
benthic annual gross production and respiration probably results from our
respiration measures omitting macroheterotrophic contribution, i.e. amphipods,
isopods and other motile, small mollusc and crustaceans probably escaped
enclosure by the benthic chambers. Orth and van Montfrans (1982) report
densities of individuals in these groups from 1.0 x 10^ to over 1.0 x 10^ m~2.
Therefore, benthic production and consumption are probably more closely
balanced than suggested by our estimates.
SUMMARY
Total community metabolism of a temperate, Zostera marina seagrass
community is both structurally (i.e., principal components involved) and
temporally partitioned among the seagrass-epiphyte assemblage and the
planktonic and benthic microalgae. There is a remarkably similar division in
terms of percent contribution to estimated annual values for both Chesapeake
Bay and North Carolina systems where the principal structural component, Z_.
marina, exists near its southern range limit. The data suggest that for gross
annual production, the sascular plant-epiphyte complex contributes between 50
and 60Z to the total, the plankton, 30Z and the benthic algae 10 to 25Z.
Respiration balances production for the plankton and more than likely for
the benthic community. Assuming that microalgae respiration, both benthic and
planKtonic, is 10Z or less of gross production, the data indicate that
heterotrophs dominate respiratory metabolism and more than likely directly
graze the autotrophic component. Therefore, coupling between production and
consumption in these two subsystems is quite different than the plant-epiphyte
component and probably supports secondary production to a greater extent than
suggested by standing stock estimates alone.
Our results also suggest that failure to include microalgae in estimates
of community productivity and respiratory processes might well result in
significant underestimates of total primary production in seagrass meadows and
omit components that may support heterotrophic processes equal to or perhaps
greater than the direct vascular plant contribution. The generality of this
conclusion awaits further and certainly more detailed study (particularly with
reference to temporal scales). However, qualitatively the conclusion appears
124
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warranted for many aeagraaa ecosystems and where comparative data do exiat,
our findings are consistent.
125
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ACKNOWLEDGEMENTS
We would like to express our appreciation to those who aided directly in
conducting these studies; Frederick Hoffman, Ann Evans, M. E, Bender and R. F.
van Tine. The cooperation and land use provided by Jack Watts, Donald Hammond
and Lloyd Outen is also greatly appreciated. We thank Drs. K. L. Webb, P. A.
Penhale and R. J. Orth, for their reviews and insightful comments made in
manuscript preparation. Especial thanks are due M. Castagna, J. Kraeuter and
J. Moore of VIMS, Eastern Shore Laboratory, Wachapreague, Virginia for their
many and varied contributions in support of these studies. Financial support
for these studies was provided by grants to R. L. Wetzel from the U. S.
Environmental Protection Agency, Chesapeake Bay Program, Grant nos. R805974
and X0003245-1 and the Virginia Institute of Marine Science, Gloucester Point,
Virginia.
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McRoy, C. P. 1974. Seagrass productivity: carbon uptake experiments in
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seagrasses, pp. 53-87. In: C. P. McRoy and C. Helfferich (eds.).
Seagrass Ecosystems: A Scientific Perspective, Marcel Dekker, Inc., New
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Penhale, P. A. 1977. Microphyte-epiphyte biomass and productivity in an
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26:211-224.
Phillips, R. C. 1974. Temperate grata plots pp. 244-299. IN: H. T. Odua
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Poneroy, L. R. 1959. Algal productivity in salt marshes of Georgia. Limnol.
Oceanogr. 4:386-397.
Riznyk, R. Z. and H. K. Phinney. 1972. Manotnetric assessment of interstitial
microalgae production in two estuarine sediments. Oecologia 10:193-203.
Thayer, G. W., S. M. Adams, and M. W. LaCroix. 1975. Structural and
functional aspects of a recently established Zostera marina community,
In, pp. 518-546. L. t. Cronin (ed.), Estuarine Research, Vol. 1,
Academic Press, New York.
van Raalte, C. D. and I. Valiela. 1976. Production of epibenthic salt marsh
algae: light and nutrient limitation. Limnol. Oceanogr. 21:862-872.
Wetzel, R. L., K. L. Webb, P. A. Penhalr, R. J. Orth, D. F. Boesch, G. W.
Boehlert, and J. V. Merriner. 1979. The functional ecology of submerged
aquatic vegetation in the lower Chsapeake Bay. Annual Grant Report Grant
no. R805974, U.S. Environmental Protection Agency, Chesapeake Bay
Program, Annapolis, MD., U.S.A., 152 pp.
Wetzel, R. L., R. F. van Tine, and P. A. Penhale, 1982b. Light and submerged
macrophyte communities in the Chesapeake Bay: A scientific summary.
SRAMSOE 260, Virginia Institute of Marine Science, Gloucester Point, VA.,
U.S.A., 58 pp.
Wetzel, R. G. 1964. A comparative study of the primary productivity of
higher aquatic plants, periphyton, and phytoplankton in a large shallow
lake. Int. Revu ges. Hydrobiol. 49:1-61.
Wetzel, R. G. and R. A. Hough. 1973. Productivity and role of aquatic
macrophytes in lakes: an assessment. Pol. Arch. Hydrobiol. 20:9-19.
Wetzel, R. G. and P. A. Penhale. 1979. Transport of carbon and excretion of
dissolved organic carbon by leaves and root/rhizomes in seagrass and
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Chapter 4
PRELIMINARY OBSERVATIONS ON NUTRIENT ENRICHMENT AND LIGHT
REDUCTION EFFECTS ON ZOSTERA MARINA L. EPIPHYTIC GROWTH
L. Murray and R. L. Wetzel
Virginia Institute of Marine Science
and School of Marine Science
College of William and Mary
Gloucester Point, VA 23062
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ABSTRACT
The short-term effects (two weeks) of nutrient enrichment and ambient
light (PAR) reduction on Zostera marina epiphytic growth was evaluated using
a flow-through aquaria system with transplanted, epiphyte-free (scraped)
plants collected from a nearby seagrass meadow in the lower Chesapeake Bay.
Nutrient effects (ambient, 30 and 70 times ambient) resulted in a twofold
increase in epiphyte biomass. Light reduction (ambient and ca. 50Z
reduction) also resulted in a positive growth response by the epiphytic
community. There was a significant, positive interactive effect between
nutrient enriched and light reduction treatments. Light attenuation due to
epiphyte growth ranged from approximately 45Z to greater than 70Z for a low
light-high nutrient treatment. Attenuation of this degree is sufficient to
reduce light available for macrophyte photosynthesis to a range reported as
compensating intensities for Zostera marina. By the end of the experiment,
epiphytic microalgae accounted for greater than 90Z of both gross and net
apparent productivity estimates in the high-nutrient, low-light treatment.
The combined effects of increased nutrients and reduced light favor epiphytic
growth in the absence of other controls (i.e. grazing and alleopathy). This
results in decreased macrophyte productivity and growth and overtime, would
eventually lead to plant mortality.
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INTRODUCTION
The interactions between submerged aquatic macrophytes and associated
epiphytes range from nutrient transfer relationships (Harlin 1973, 1975;
MeRoy and Goering 1974; Wetzel and Penhale 1979) to epiphytic light
attenuation (Sand-Jensen 1977). Sand-Jensen (1977) suggested that shading
due to epiphytic growth on blades of the eelgrass, Zostera marina, inhibited
photosynthetic carbon fixation by the macrophyte. Microalgal films on leaf
surfaces are potentially competitors for inorganic carbon, gas diffusion
barriers and light attenuating (both quantity and quality) interfaces. Algal
populations generally exhibit rapid and increased growth with nutrient
enrichment (Welch et al. 1972; Ferguson et al. 1976) and, for eelgrass,
epiphytic growth consists primarily of diatoms and filimentous algae
(Sieburth and Thomas 1973; van Montfrans et al. in press). Thus, the
interactive effects of increased dissolved inorganic nutrients and increased
shading concomitant with epiphyte growth on leaf surfaces may potentially act
as a significant control on macrophyte photosynthesis and biomass production.
For example in some freshwater systems, Phillips et al. (1978) found
that diatom growth on the macrophyte Najas marina increased threefold with
the addition of fertilizer (N:P-10) at a rate of 2.0 g P m~2 yr"1. Slides
allowed to colonize in the same waters showed an 842 decrease in light
transmission. Sand-Jensen and Sondergaard (1981) working in lakes of varying
nutrient levels reported that epiphytic growth increased 200 times in lakes
ranging from low to high ambient nutrient concentrations. These authors
concluded that increased epiphytic growth could ultimately lead to mortality
of the macrophyte due to extremely reduced light availabe for macrophyte
photosynthesis. The studies also reported low phytoplankton concentrations
corresponding with nutrient enrichment and suggested that the phytoplankton
are outcompeted by attached, epiphytic algae and played a minor role in water
column light attenuation.
Photosynthetically, light saturation ranges from ca. 200-300 p.E m~^
see"* for £. marina (McRoy 1974; Penhale 1977; Chapter 2). For a Z. marina
bed in the lower Chesapeake Bay, mean daily in situ light intensity during
the early groing season is below this range although the data are quite
variable (Chapter 2). Thus, reduction in ambient light levels by 80Z due to
epiphyte attenuation as suggested by Phillips et al. (1978) and Sand-Jensen
and Sondergaard (1981) would lead to plant mortality. Furthermore, diatoms,
(the principal epiphytic component on Chesapeake Bay seagrasses (van
Montfrans et al., in press; Murray, unpubl. data) exhibit photosynthetic
light saturating intensities (20-50 (JE m~* hr~*) much lower than Z_. marina
(Taylor 1964; Ignatiades and Smayda 1970; Levin and Mackss 1972; and Admiral
1977), indicating a significantly lower liy.ht requirement and a competitive
advantage under reduced light regimes.
Although epiphytic growth may potentially decrease macrophyte
photosynthesis, total community productivity may remain high. The result may
be a shift from macrophyte-d&minated to epiphyte-dominated productivity as
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long as the macrophytes are able to survive. The interactive effects of
increased nutrients and reduced light, although negative relative to
macrophyte photosynthesis, may, overall, result in stimulation of autotrophic
production in these communities due to increase microalgae biomass and
productivity.
Based on these hypothetical and to some extent, demonstrated,
interactions, our studies were undertaken to 1) investigate epiphyte growth
on Zostera marina leafs under controlled nutrient and light levels, 2)
estimate light attenuation due to epiphytic growth on the leaf surface, and
3) estimate the productivity of the macrophyte and epiphyte components under
controlled nutrient and light regimes.
METHODS AND MATERIALS
Experimental Design
Zostera marina plants were collected from a mesohaline area of the York
River eubestuary of the Chesapeake Bay. Plants were returned to the
laboratory and cleaned of epiphytic growth by gently scraping the leaf
surfaces with a flat spatula and then individually planted in pots containing
cleaned sand. Twenty-four individually potted plants were placed in each of
six, 10 gallon flow-through aquaria. The aquaria were incubated on a larger,
flowing sea water table to maintain ambient temperature. The following
experimental treatments were used: two aquaria (numbers 1 and 2) had in situ
(control) nutrient levels; two aquaria (numbers 3 and 4) had nitrogen levels
30 times ambient and two aquaria (numbers 5 and 6) had nitrogen levels 70
times ambient. Nutrient amendments were added according to the Redfield
ratio and tank concentrations were maintained by metering stock solutions of
47 mM ammonia nitrate and 6 mM disodium phosphate into the aquaria with a
multi-speed transmission peristaltic pump. Metering rates were monitored
daily to assure constant treatment throughout the experiment. Three of the
tanks, numbers 1, 3 & 5, were shaded with nc-jtral density screening to in
situ light levels (i.e. light levels normally experienced by the natural
community). The other three tanks, numbers 2, 4 & 6, were shaded to 50! of
the in situ control levels. Water quality parameters measured over the
course of the study included dissolved oxygen (rag I"1), temperature (*C),
salinity (o/00), and light as photosynthetically active radiation (PAR: E
m~^sec~*). Oxygen concentration was determined with an Orbisphere Oxygen
Monitor (Model #2604). Temperature and salinity were monitored daily using a
nax-min reversible thermometer and salinity determined using a refractometer.
Light measurements, taken continuously pver the photoperiod, were made with a
Li-COR Quantum Meter (Model number 135A) at the level of the plant canopy
top. Concentrations of nitrate, nitrite, ammonia and orthophosphate were
determined on replicate water samples from each tank using standard,
Technicon Auto Analyzer techniques at the beginning, mid, and final dates of
the experiment. The experiment was conducted for a total of 2 weeks and kept
relatively short to minimize enclosure effects.
Treatment Effects
To determine epiphytic biomass on _Z. marina leaves, 8 plants were
randomly sampled from each tank at the end of the 2 week period and the leaf
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surfaces scraped into filtered sea water. Plant leaves cleaned in this
fashion were periodically checked for reroval efficiency by examining random
samples using epiflorcsent microscopy. Based on observation, removal
efficiency was greater than 90Z and for the majority of samples no epiphytic
growth was observed on the leaf surface following scraping. There was also
no observable evidence of plant tissue damage by the technique. The
epiphytes were then collected by filtration onto preweighed glass fiber
filters (Whatman GF/C), dried at 60 *C, and weighed to the nearest 0.01 mg.
To estimate light attenuation due to epiphytic growth, two etched
plexiglass slides were placed in each of the tanks and allowed to colonize
for the duration of the experiment. Light measurements before and after
colonization were made by placing the slide over the quantum sensor (in
water) of the Li-Co r Quantum meter. The difference in the two values (Z
change) was used to estimate the light reduction due to epiphytic growth.
Treatment effects on plant and epiphyte productivity was evaluated by
incubating samples of unscraped and scraped plants from each tank in light
and dark 300 ml BOD bottles. All samples were incubated in triplicate. The
bottles were incubated in their respective tanks for a period of four hours
during maximum solar insolation (1000 to 1400 hours EST). Oxygen
concentration was measured at 0, 2 and 4 h with the Orbisphere using a
polarographic probe equipped with a collar designed to fit and seal into the
opening of a standard BOD bottle. Epiphyte productivity was calculated as
the difference between colonized and cleaned leaves thus the estimates are
minimum. Productivity for both plant and epiphyte are reported as the mean
g (dry weight olant)'1 hr~* .
To estimate treatment effects on plant growth, initial and final
measurements of shoot length, fresh weight and leaf number were determined on
eight random plant samples from each tank.
RESULTS
Routine sampling data (Table 1) indicated little variation among tanks
for temperature (26-31 *C), salinity (20-21 °/oo), and midday dissolved oxygen
concentrations (10-18 mg 1 ). Average midday PAR measurements in the
light-control tanks (1, 3 and 5) ranged from 422 to 490 /a E m~2 sec"1 and in
the shaded tanks (2, 4 and 6) ranged from 180 to 220 /jE m~2 sec"1 or about
43Z of the control light levels. Average nutrient concentrations over the
experiment show that for the controls tanks (1 and 2) nitrogen: phosphorus
(N:P) ratios were approximately 7:1 while in the nutrient amended tanks (3,
4, 5 and 6) were 18:1 and maintained according to intended design.
Initial and final total plant weight (g wet weight plant"1) and final
epiphyte biomass (g dry weight plant'1) are given in Figure 1. There waa no
significant differences ( "0.05) in mean whole plant weight changes over the
course of the experiment although all mean final values were lower than
initial conditions. Epiphyte biomass (g dry weight plant'1) at the end of
the experiment was approximately equal in tanks 1 through 5. Tank 6 was
higher and differed significantly from all others.
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TABLE 1. MEAN MIDDAY DISSOLVED OXYGEN, NUTRIENT CONCENTRATIONS AND LIGHT
(PAR) INTENSITY IN THE EXPERIMENTAL TANKS. ENTRIES ARE MEAN OF
ALL OBSERVATIONS MADE OVER THE TWO WEEK STUDY.
Tank
No.
1
2
3
4
5
6
°2 ,
(mg 02 i~l)
11.9
9.52
16.7
10.7
18.5
10.5
(uM)
0.49
0.49
7.74
7.97
13.4
15.1
N02 •»• N03
(uM)
1.19
0.71
72.3
68.4
128.
142.
(uM)
3.57
2.69
59.6
67.1
129.
142.
N:P
(uM) d
7.67
6.94
17.0
17.0
19.2
19.1
PAR
470
180
422
188
490
220
135
-------
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Since epiphyte biomaaa is dependent on plant aurface area, the ratio
(E:P) of epiphyte biomaaa (g dry weight) to plant leaf biomaaa (g dry weight)
was used for data reduction and test for treatment effects. The effect of
ambient light reduction and nutrient treatment on epiphytic growth is
illustrated in Figure 2. In the shaded tanks, epiphytic growth averaged ca.
70Z higher than in corresponding control treatments. In each light
treatment, epiphytic growth per plant increased with increased nutrient
level. In the control light tanks the greatest increase occurrred between
ambient and nutrient level 1, while in the shaded tanks the greatest increase
was between nutrient level 1 and nutrient level 2, suggesting a interactive
effect. Overall, the shaded tanks had a greater increase in epiphytic growth
with increased nutrients.
Table 2 summarizes the results of simple pair-wise comparison of mean
ratios blocked by treatment and gives the probability that the mean ratios
are different. It is obvious that there are significant differnces in mean
ratios due to the light and nutrient treatments. Because the calculated
t-statistics are negative for all comparisons using the blocking design
illustrated, both decreased light and increased nutrient level had positive
effects on the ratio and thus epiphyte growth. Table 3 summarizes the
results of analysis of variance using an ANOVA Model I with fixed effects.
As indicated, there were highly significant main effects and a lower but
significant light-nutrient interactive effect.
The degree of light attentuation due epiphyte growth, i.e. colonization
on the test plates, is presented in Table 4. The data suggest that nutrients
have a greater effect than incident light reduction (i.e. shading
treatments). Figure 3 illustrates the relationship between epiphytic Z light
reduction and the E:P ratio. Percent light reduction increased by an average
of 58Z in the nutrient enriched treatments (dotted line), while in the shaded
treatments remained constant (solid line). The data suggest that percent
light reduction due to epiphytic attenuation remains constant over the
shading treatments but increases logarithmically with increasing nutrient
levels. Overall, there is an increase in percent light reduction with an
increase in epiphyte biomass that appears to asymptotically approach an upper
limit probably governed by leaf surface area.
Figure 4 summarizes the productivity estimates for macrophyte leaves and
epiphytes. The 1st bar in each group represents total apparent net
production, the 2nd bar respiration, and the 3rd bar gross production. The
top area of each bar is epiphyte and the bottom macrophyte contribution
respectively. In tanks I, 2, 3 and 4, epiphyte and macrophyte net and gross
productivity are approximately equal. Tanks with the highest nutrient
concentrations (5 and 6), show a significant increase in total productivity.
While there is only a slight decrease in macrophyte productivity over all
treatments, epiphyte productivity accounted for approximately 90Z of the
total in tanks with the highest nutrient concentration under both control and
shaded light regimes. Respiration remained low and relatively constant for
both epiphytes and macrophytes in treatments 1 through 4 but macrophyte
respiration tended to increase in tanks 5 and 6, suggesting stress.
137
-------
A
4.0-
^ 3.0-
ui
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SM^
o 2-°"
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41
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LIGHT REDUCTION
50%
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rill5! *M
* M
fl li
^
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tank 135 246
nutrient Ox 30x 70x Ox 30x 70x
figure 2. Resulting mean (^ 1 S.D.; n-8) epiphyte (g dry weight to plant leaf
(g dry weight biomass ratio following the atudy. The upper panel
is grouped by light regime and the lower panel by nutrient treat-
ment for convenience.
138
-------
TABLE 2. SIMPLE PAIR-WISE TESTS OF MEAN EPIPHYTE: PLANT LEAF BIOMASS
RATIO DIFFERENCES BLOCKED BY LIGHT (PAR) AND NUTRIENT
TREATMENTS.
Blocks
a. Light
Mean
Comparison
Test
(tanks i-j)
1-2
3-4
5-6
d.f.l
(n)
14
14
14
t-
P2
statistic (x£ J
-3
-1
-2
.14
.18
.70
>
>
>
.995
.800
.990
Xj)
b. Nutrients
1. In situ PAR 1-3
3-5
1-5
2. 50Z In siut PAR 2-4
4-6
2-6
14
13
13
14
13
13
-2.2
-0.714
-6.56
-1.49
-2.20
-3.43
>.975
>.750
>.995
>.900
>.975
>.995
1. d.f. = degrees of freedom for test statistic.
2. P * probability that the two means are significantly different.
139
-------
TABLE 3. MODEL I ANOVA (FIXED EFFECTS) FOR NUTRIENTS, LIGHT AND
INTERACTIVE EFFECTS ON EPIPHYTE: PLANT LEAF BIOMASS RATIOS.
Source
Nutrients
Light
Nutrients X Light
Error
Total
df SS
2 13.1
1 9.97
2 1.55
40 5.33
45 29.9
MS F
6.54 49.1**
9.97 74.8**
.777 5.83*
.133
* Significant @ - 0.05
** Significant @ - 0.01
140
-------
TABLE 4. TREATMENT EFFECTS ON EPIPHYTIC BIOMASS (A), x I? DRY '-'£.
PLANT"1 (••• S. E.) AND LIGHT REDUCTION DUE TO EPIPHYTES
(COLONIZATION OF SLIDES) (B), x PERCENT DECREASE (+ S. E.),
A.
Light 0
Control 47.4
(3.8)
Shaded 91.6
(13.6)
B.
Light 0
Control 43.6
(3.78)
Shaded 45.1
(1.41)
Nutrients*
1
110.
(24.0)
166.
(24.2
Nutrients*
1
70.2
(7.39)
67.0
(0.071)
2
128.
(11.5)
215.
(58.3)
2
76.3
(0.85)
68.3
(3.00)
* 0 - Ambient, 1 = 30X, 2 = 70X
141
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Table 5 summarizes changes in various meriscic characteristics for the
macrophytes. All plants show a net loss in weight and shoot length following
the experiment. Plants in ambient nutrient concentrations show a net gain of
ca. 1 leaf per plant, while there is an average loss of ca. 1 leaf and ca. 2
leaves in nutrient levels 1 and 2 respectively.
DISCUSSION
Submerged aquatic macrophytes are a principal autotrophic component of
many freshwater, estuarine and marine ecosystems. Epiphytic growth on these
living surfaces generally compliments vascular plant production and, as
Penhale (1977) points out, may provide a significant autotrophic input to the
resident heterotrophic community. On the otherhand, the autotrophic
component of the epiphytic community also functions as a compeditor for
light, inorganic carbon and dissolved inorganic nutrients as well as a
physical diffusion barrier. The major controls influencing epiphytic growth
on seagrasses probably result from the interactive effects of light, nutrient
supply, grazing and alleopathy. In a preliminary fashion, we have tested the
short term effects of the first two of these.
Photosynthesis-light relationships for microalgae and the vascular plant
are quite different. Light saturating intensities for microalgae
photosynthesis are much lower than for Che vascular plant (Taylor 1964;
Ignatiades and Smayda 1970; Levin and Mackas 1972; and Admiral 1977).
Therefore, from a competitive standpoint, the microalgae are at the advantage
under many estuarine light conditions. For Z_. marina dominated communities
in the lower Chesapeake Bay, the epiphytic microalgae are probably not
light-limited while the vascular plant may be for a significant part of the
year (see Chapter 2). The results of this study and the experimental
conditions under which it was run indicate a significant, positive response
to reduced ambient light by the epiphytic community. The control light
levels of ca. 460 pE m~* sec"1 (Table I) are well above saturating for both
epiphyte and vascular plant, however, light available for "L. marina
photosynthesis was probably in the range 115-250 jaE m~* sec"' if the
colonized slides accurately estimate 2 light reduction due to epiphy.ic
growth (Figure 3). These PAR levels are suboptimal particularily for the
nutrient enriched treatments. The shaded-treatments light levels of ca. 200 |I
E m~2 sec"' (Table 1) are above saturating for the microalgae and but below
saturating intensities for the vascular plant. Light available under the
shaded treatments for Z_. marina photosynthesis was probably in the range
60-110 p.E m~2 sec"1, well below saturating, and very near the reported range
for compensating light intensities of 50 to 100 pE m~2 sec"* (see Chapter 2).
For the control nutrient treatments, epiphyte growth increased approximately
two-fold with shading. The mean Z light reduction of 45X due to epiphytic
growth under ambient light and nutrient conditions suggests that for the
intact community, in situ light regimes must be near 400 ^E m~^ sec~l
reaching the plant canopy top for maximum rates of vascular plant
photosynthesis to be realized. These light intensities are not typical at
the plant canopy top for Z_, marina dominated seagrass meadows in the
Chesapeake Bay. Thus the plant community may be light-limited during the
growing season due both to water column and epiphytic light attenuation. The
144
-------
TABLE 5. TREATMENT EFFECTS FOLLOWING THE TWO WEEK STUDY ON VARIOUS
MERISTIC PARAMETERS FOR Z. MARINA. ENTRIES ARE THE MEAN
+1 S.D. (n=8). ~
A.
# R wet wt. plant
Nutrients
Light
Control
-.21
(.22)
1
-.44
(.20)
-.04
(.08)
Shade
-.25
(.29)
-.19
(.13)
-.14
(.08)
B.
#shoot length (cm)
Nutrients
Light
Control
Shade
0
-1.2
(0.9)
-1.9
(1.1)
1
-0.3
(2.0)
-1.8
(3.0)
0.42
(0.61)
-1.3
(1.1)
C.
#leaf no.
Nutrient s
Light
Control
•-0.88
(.35)
-0.50
(.31)
-1.50
(.51)
Shade
HO.88
(.35)
-1.14
(.55)
-1.71
(.48)
l'*5
-------
relative in aitu rolea of each of these sources of light attenuation remains
poorly understood.
Nutrient effecta and interactions are even less well underatood.
Nutrient dynamics in these temperate seagraaa communities have been poorly
inveatigated. Zostera marina dominated communitiea in the lower Cheaapeake
Bay generally inhabit sand-silt sediments having organic matter
concentrationa ranging from 2 to 4 Z dry weight and pore-water nutrient
concentrations of 100-125 pM N-Nfy* and £ 1.0 pM N-N<>3 in the top 10 cm of
sediment (Wetzel et al. 1979). These data and the general obaervation that
water column concentrationa are low (K. L. Webb, personal communication,
1982) suggest that the principal route for plant uptake ia via the
root-rhizome system. Orth (1977) demonstrated a rapid *ud positive growth
response by Z. marina to in situ sediment application of a commercial
fertilizer indicating that lower Chesapeake Bay communitiea are nutrient
limited. In general, the kinetic relationships of nutrient utilization by
seagraaaea have only been described to a limited extent (MeRoy et al. 1972;
McRoy and Alexander 1975; Penhale and Thayer 1980). However, the conaenaua
seems to be that pore-water concentrationa represent a source of several
weeks supply while water concentrationa are negligible (Patriquin 1972).
Recycling and new supplies mu«t, it appears, provide the principal nutrient
sources (Capone et al. 1979).
Without knowing the kinetic properties of nitrogen and phosphorus uptake
across the leaf surface, the apparent short-term response to increased
dissolved inorganic nitrogen and phosphorus concentration demonstrated in our
studies was attributable to the epiphytes which resulted in rapid growth.
The increased nutrient supply did not result in either increaaed vaacular
plant growth (Table 5) or apparent photosynthesis (Figure 4). For all
comparisons, increased nutrient supply resulted in increaaed epiphytic growth
that asymtotically approached a maxima probably governed by leaf aurface area
(Figure 3). For the higher nutrient treatments, greater than 90Z of gross
and net apparent productivity was attributable to epiphytic microautotropha
(Figure 4). There ia an obvious interactive effect between decreased light
and increaaed nutrients both of which favor epiphytic growth and would thus
negatively affect vascular plant growth and photosynthesis.
Other controls on epiphytic growth, i.e. grazing and vascular plant
alleopathy which function to surpress the negative effecta of epiphytic
growth, have only recently been investigated for seagraases. In subtropical
Thalassia testudinum graasbeds, Zimmerman et al (1979) report data for
grazing by gammaridean amphipods on various organic matter sources including
epiphytic microalgae. Their reported rates for ingest ion of epiphytic
microalgae (ca. 1 mg (algae) nig"1 (amphipod) day"*) are in a range which at
least theoretically could control epiphyte growth although studies that have
attempted to link epiphyte production and grazing (secondary utilization) are
lacking. For lower Chesapeake Bay i. marina communitiea, van Montfrana et
al. (in press) have shown that grazing by the proaobranch gastropod, Bittium
varium, can effectively remove the epiphytic community. They have suggested
that this biological control may be singularily important with regard to
vaacular plant growth in some bay areas. Because our experimental design
excluded grazing by macroheterotrophs, we can only conclude that in the
146
-------
absence of grazing, the epiphytic community under both ambient and altered
light-nutrient regimes exercises some degree of control over Z. marina
photosynthesis and ultimately production by the community.
Alleopathy in aquatic plants has recently been reviewed by Szcezpanaki
(1977). He concluded that for at least some aquatic species, evidence for
alleopathy is strong. Two studies with Z_. marina indicate that actively
growing, unstressed leaf tissue produces an algal inhibitor (Sand-Jensen
1977; Harrison and Chan 1980). From our experimental design, we are not able
to distinguish an alleopathic response except to note that any treatment
combination which might be assumed to cause plant stress resulted in
increased epiphyte growth. This may in part be explained by reduced
alleopathy rather than directly attributable to light and/or nutrient
treatments. At the present we are not able to distinguish the two.
In summary, these results indicate that epiphytic growth under
conditions of no grazing can limit plant photosynthesis and growth. Ambient
reduction in available light and/or increased dissolved inorganic nutrient
supply interact to favor epiphytic growth and further limit seagrass
production. Under extreme conditions this would ultimately lead to increased
plant mortality and eventually demise of the entire community. For
Chesapeake Bay Z_. marina communities, which appear naturally light-stressed,
any factor or combination of environmental perturbations that might favor
enhanced epiphytic growth (i.e. reduced light, increased nutrients or
relaxation of grazing pressures) would ultimately effect changes in the
distribution and relative abundance of the species.
147
-------
ACKNOWLEDGEMENTS
We would like Co extend our appreciation to all who have contributed to
this study. Our special thanks go to Kr. Damon Delistraty and Frederick
Hoffman for help with maintenance and sampling of the microcosms and to Mr. J
van Montfrans and R. J. Orth for the use of their seawater tables and
flow-through system. Especial thanks are due Drs. R. J. Orth, P. A. Penhale
and K. L. Webb for critically reviewing earlier drafts of the manuscript.
For their editorial and secretarial assistance, we gratefully acknowledge the
efforts of Ms. Carole Knox and Nancy White.
148
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LITERATURE CITED
Admiral, W. 1977. Influence of light and temperature on the growth rate of
estuarine benthic diatoms in culture. Mar. Biol. 39:1-9.
Capone, D. G., P. Penhale, R. S. Oremland and B. F. Taylor. 1974.
Relationship between productivity and N£ (€^^2^ fixation in a Thalassia
tentudinum community. Limnol. Oceanogr. 24:117-125.
Ferguson, R. L., A. Collier and D. E. Meeter. 1976. Growth response of
Thalassiusia paeudonana Hasle and Heimdal Clone 3H to illumination,
temperature and nitrogen source. Ches. Sci. 17(3):148-158.
Harlin, Marilyn M. 1973. Transfer of products between epiphytic marine algae
and host plants. J. Phycol. 9:243-248.
Harlin, M. M. 1975. Epiphyte host relations in seagrass communities.
Aquatic Bot. 1:125-131.
Harrison, P. G. and A. T. Chan. 1980. Inhibition of the growth of microalgae
and bacteria by extracts of eelgrass (Zostera marina) leaves. Mar. Biol.
61:21-26.
Ignatiadea, L. and T. J. Smayda. 1970. Autecological studies on the marine
diatom Rhizosolinia fragilissima Bergon. I. The influence of light,
temperature, and salinity. J. Phycol. 6:332-339.
Lewin, J. and D. Mackas. 1972. Blooms of surf-zone diatoms along the coast
of the Olympic Peninsula, Washington. I. Physiological investigations
of Chaetoceroa armatum and Asterionella socialis in laboratory culture.
Mar. Biol. 16:171-181.
McRoy, C. Peter. 1974. Seagrass productivity, carbon uptake experiments in
eelgrass, Zostera marina. Aquaculture 4:131-137.
McRoy, C. P., R. J. Barsdate and M. Nebert. 1972. Phosphorus cycling in an
eelgrass (Zostera marina L.) ecosystem. Limnol. Oceanogr.
18(6):998-1002.
McRoy, C. Peter and J. J. Goer ing. 1974. Nutrient transfer between the
seagrass Zostera marina and its epiphytes. Nature Vol. 248:173-174.
McRoy, C. P. and V. Alexander. 1975. Nitrogen kinetics in aquatic plants in
Arctic Alaska. Aquatic Bot. 1:3-10.
Orth, R. J. 1977. Effect of nutrient enrichment on growth of the eelgrass
Zostera marina in the Chesapeake Bay, Virginia, U.S.A. Mar. Biol.
44:187-194.
149
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Patriquin, D. G. 1972. The origin of nitrogen and phosphorus for growth of
the marine angioaperm Thalasaia testudinum. Mar. Biol. 15:35-46.
Penhale, Polly A. 1977. Macrophyte-epiphyte biomass and productivity in an
eelgrasa (Zoetera marina L.) community. J. exp. mar. biol. Ecol.
26:211-224.
Penhale, P. A. and G. W. Thayer. 1980. Uptake and transfer of carbon and
phosphorus by eelgraas (Zostera marina L.) and its epiphytes. J. exp.
mar. biol. Ecol. 42:113-123.
Phillips, G. L., D. Eminson and B. Moss. 1978. A mechanism to account for
macrophyte decline in progressively eutrophication freshwaters. Aquatic
Hot., 4:103-126.
Sand-Jensen, K. 1977. Effect of epiphytes on eel grass photosynthesis.
Aquatic Bot. (3):55-63.
Sand-Jensen, K. and M. Sondergaard. 1981. Phytoplankton and epiphyte
development and their shading effect on submerged macrophyte in lakes of
different nutrient status. Int. Revue gea. Hydrobiol. 66(4):529-552.
Sieburth, John McN. and Cynthia D. Thomas. 1973. Fouling on eelgraas
(Zostera marina L.). J. Phycol. 9:46-50.
Szczepanski, Andrzej J. 1977. Allelopathy as a measure of biological control
of water weeds. Aquatic Bot. 3:193-197.
Taylor, W. Rowland. 1964. Light and photosynthesis in intertidal benthic
diatoms. Helgol. Wiss. Meeresuntes. 10:29-37.
Tukey, J. 1977. Exploratory Data Analysis. Addison Wesley Publ. Co.,
Reading, Mass.
van Montfrans, J, R. J. Orth and S. A. Vay. 1982. Preliminary studies of
grazing by Bittium varium on eelgraas periphyton. Aquatic Bot. (in
press).
Welch, E. B., R. M. Emery, R. I. Malsuda and W. A. Dawson. 1972. The
relation of periphytic and planktonic algae growth in an estuary to
hydrographic factors. Limnol. Oceanogr. 17:731-737.
Wetzel, R. G. and P. A. Penhale. 1979. Transport of carbon & id excretion of
dissolved organic carbon by leaves and roots/rhizomes in seagrasses and
their epiphyton. Aquatic Bot., 6:149-158.
Wetzel, R. L., R. F. van Tine and P. A. Penhale. 1981. Light and submerged
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150
A
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Wetzel, R. L., K. L. Webb, P. A. Penhale, R. J. Orth, D. P. Boeach, G. W.
BoehlerC and J. V. Merriner. 1979. The functional ecology of aumberged
aquatic vegetation in the lower Chesapeake Bay. Report to EPA/CBP
(R805974). U. S. Environmental Protection Agency, Philadelphia, Pa. 152
PP.
Zimmerman, R., R. Gibson and J. Harrington. 1979. Herbivory and detritivory
among gammaridean amphipods from a Florida seagrasa community. Mar.
Biol. 54:41-47.
151
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Chapter 5
FUNGI AND BACTERIA IN OR ON LEAVES Of EELGRASS (ZOSTERA MARINA L.)
FROM CHESAPEAKE BAY
Steven Y. Newell
University of Georgia Marine Institute
Sapelo Island, Georgia 31327
152
-------
ABSTRACT
Samples of green and brown leaves of eelgrasa (Zoatera marina L.)
were incubated in aeawater without an additional carbon source.
Parallel leaf samples were used for acridine orange bacterial counting
and water-soluble aniline blue estimation of fungal biovolume. The
incubations produced no evidence that there is an eelgrass counterpart
for the chyCridialean symbiont which is very common in turtlegrass
(Thalasaia testudinum Konig). Sterile mycelium (i.e., living mycelium
without identifiable propagules) was the most prevalent fungal form on
incubated samples from submerged sites, whereas Dendryphiella salina
and Sigmoidea sp. (marina?) were prevalent on brown leaves from the
wrack line. Attempts to assay fungal biovolume in field samples
indicated that the sterile mycelium observed after incubation
represented the outgrowth of formerly dormant propagules or weakly
established microcolonies. It was calculated that fungal biomass
could not account for more than 0.5Z of leaf mass, and it was probably
much smaller than this, for no fungal structures were observed even in
concentrated leaf homogenates. Bacterial densities fell within the
range reported for other paniculate substrates. A speculative
estimate „ • bacterial productivity was 1.4 x the standing stock per
day.
153
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INTRODUCTION
As a part of their study of decomposition of turtlegrass (Thalaasia
testudinum Konig) leaves, Newell and Fell (1980) addressed the question of the
magnitude of the fungal role in this process. Their findings indicate that
(i) the most common active fungus of turtlegrass leaves is a rhizomycelial
chytrid, apparently a symbiont of the living leaves; and (ii) hyphomycetes and
ascomycetes are not major decomposers of leaves which decay in submerged
sites. Newell and Fell speculated that the chytrid (Nowakowskiella or
Cladochytrium sp.) might be capable of causing disease under conditions of
stress to the host plant, and so might be responsible for the wasting disease
of eelgrass (Zostera marina L.) if it or a related form were present in
eelgrass. I report here a test of the hypotheses that (i) there is an
eelgrass counterpart of the rhizomycelial chytrid of turtlegrass; and (ii) the
hyphomycetes and ascomycetes reported in eelgrass (Huges 1975; Johnson and
Sparrow 1961; Jones 1976; Kohlmeyer and Kohlmeyer 1979) are active decomposers
of submerged eelgrass leaves, as the interpretation of Fell and Newell (in
press) of the glucosamine data of Thayer et al. (1977) might suggest. The 1st
hypothesis was tested by seawater incubation of leaf samples (Newell and Fell
1980). The 2nd hypothesis was tested by seawater incubation and by
direct-count estimation of bacterial and fungal biovolumes (S.Y. Newell and
R.E. Hicks, in press).
MATERIALS AND METHODS
Collection and Initial Processing
Collections of eelgrass leaves were made at eight stations on a 200 m
transect perpendicular to the shoreline across the width of a belt of eelgrass
beds on the eastern shore of Chesapeake Bay (37'25'N; 76*51*W) in July 1980.
Water temperature was 29*C, and salinity was 19 to 22°/oo. Station I was at
the outermost (bayward) edge of the bed, station 8 at the innermost (shorward)
edge, where the Zoatera zone graded in the Ruppia maritima L. zone. In
addition, leaves were collected from two sites (9 and 10) within the wrack
line on the shore at the level of the most recent high tide. At each site,
leaves of two types were collected; mature, green, attached (except wrack,
sites) leaves, and brown, detached leaves. Leaves were not scraped or rinsed;
the eelgrass leaves at this location had a much smaller epiphyte or sediment
load than did the turtlegrass and cordgrass leaves, respectively, of the
studies of Newell and Fell (1980) and Newell and Hicks (in press). However,
the eelgrass leaves collected for the present biovolume measurements were
stored in RFFSW, a bacteria-free (0.2p« filtered) 22 solution of formaldehyde
in seawater 20°/oo at 4*C until processed (2 weeks later). This storage
liquid was not included with the sample at the time of homogenization.
Therefore, the storage BFFSW served as a rinse before the preparations, and
the bacteria counted were those most firmly attached.
154
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Frequency of Fungal Occurrence
Park (1972; 1974) and Suberkropp and Klug (1976) recommend ambient water
incubation of natural samples as one means of identifying active aquatic
mycoflorat. I used this method as described by Newell and Fell (1980).
Briefly, Zostera leaves were collected in sterile bags, and duplicate
subsamples (15 mm ) were aseptically removed from each leaf and placed in
dishes of filter-sterilized seawater (50 ml per dish) from sampling sites.
Incubation was for 17 to 21 days in the dark at 25 to 30*C. Penicillin G
(0.05Z), streptomycin sulfate (0.051). cholestrol (0.5 ppn), and FeSO^ (2.0
ppm) were added to the aeawater before filter-sterilization for reasons
discussed by Newell and Fell (1980). Entire surfaces and peripheries of
samples were searched (at a magnification of x50 to x400) for sporangia,
perithecia, and conidia after incubation. Thraustochytrids were not recorded
because of their similarity to protozoan cysts in unbaited natural samples,
but labyrinthulids were recorded, because of their possible connection to the
wasting disease of eelgrass (Johnson and Sparrow 1961; Pokorny 1967; Rasmussen
1977).
Direct-counting and Biomvolume Estimation
Leaves for biovolume estimation were processed as described for Spartina
alterniflora leaves by Newell and Hicks (in press). Briefly, two leaf samples
(one green and one brown) of measured surface area (4,800 mm') from each
station were homogenized (rocor-stator, model BEW-5 Polytron Waring blender
attachment) at low speed (15,000 rpm) for 3 min in 100 ml of BFFSW.
Homogenates were filtered through a 250-m mesh screen. Filtrates were used to
make acridine orange bacterial preparations (Hobbie et al. 1977; Rublee et ai.
1978) or water soluble aniline blue (WSAB) fungal preparation (28; as modified
by Newell and Hick, in press). These are polycarbonate membrane filtration
(Nuclepore) techniques, and direct microscopic counting is performed under
epifluorescence illumination. The modifications of the Paul and Johnson
(1977) technique weie: (i) 0.2-jaun pore size Nuclepore filters were used in
place of 0.4-pm filters ; (ii) the 0.2-pon filters were stained in irgalan black
(0.2Z in 2Z acetic acid) before use; (iii) the sample homogenates were stained
in WSAB for 30 min before filtration; and (iv) the filters plus sample were
mounted in low-fluorescence immersion oil rather than dried. Mycelium from
the seawater incubation dishes was used as a control to verify the capacity of
the WSAB to form fluorescent tnycelial-WSAB complexes. In addition to the WSAB
preparations, cleared membrane filters (Millipore Corp.) (Hannsen et al. 1974)
and concentrated (settling for several hours) homogenatea were prepared with
WSAB and examined for funal structures by epifluorescence and phase-contact
microscopy. Lengths and width of bacterial cells were measured at x2,000, and
individual volumes (Baath and Soderstrom 1979) were calculated as cylinders.
The coefficient of variation for bacterial counts averaged 45t. Blank
filtrations gave counts which were less than 1% of sample counts. The
frequency of dividing cells (FDC) was estimated as described by Hagstrom et
al. (1979); cells with clear invaginations but not a clear separatory space
were counted as dividing cells. Filamentous bacterial cells which were longer
than the width of the microscope field projection of one subsection of the
eyepiece grid used in counting (1 'J pirn) or which consisted of chains of cells
155
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or both were not included in total or FDC counts. They iomed only » small
proportion of cells counted (UZ).
Retentates (250 jam) were dried and weighed for comparison with weights of
unhooogenized leaves, as an estimate or inefficiency of homogti.iiztti - i i'Kewr.1
and Hicks, in press). Retentate weights averaged 56.3 + 6.6Z (standard error,
n " 10) of original sample weights, similar to the value of 65.7Z found for
leaves of S_. alterniflora by Newall and Hicks (in pr ss). A small portion
«0.1Z) of each 250- m retentate was suspended in hrfbW and used to estimate
retention of bacterial cells. Staining (aciidine orange) and filtering
procedures were like those used for the filtered homogenates. Bacterial cells
were counted on microscopically estimated surface areas of particles.
Statistical Analyses
The G-test of Sokal and Rohlf (1969) was used in comparison of fungal
frequencies of occurrence. Mean bacterial counts were subjected to analysis
of variance after transformation ( X + 0.5) (Sokal and Rohlf 1969).
RESULTS
Frequency of Occurrence Fungal Species
There was no evident trend of changing frequency of occurrence of any
fungal species over the transect, except between submerged sites and
wrack-line sites. Therefore, data from each of these two types of site were
pooled for presentation in Table 1. Only the species listed in Table 1 were
recorded at greater than 10* for submerged or wrack sites for either leaf
color. There was no evidence of zoosporic fungi, neither chytridiomycetes nor
oomycetes, in any of the samples with the exception of Labyrinthula sp., which
was recorded only one cc. Sterile mycelium (i.e., living mycelium without
identifiable propagules) and Cladosporium sp. were the prevalent forms on
samples from submerged sites, and they occurred at statistically equivalent
frequencies on the wrack-line material. Occurrences of Sigmoidea sp.,
Dendryphiella salina (Sutherland) Pughet Nicot, and Varicosporina ramulosa
Meyers et Kohlmeyer were largely limited to wrack-line samples, where the two
former species had the highest frequencies recorded on brown leaf samples.
Acremonium sp. and Lulworthia sp. were relatively rare.
Descriptive Note
The species of Sigmoidea Crane (1968) which was observed at high
frequency on emersed intertidal eelgrass leaves was probably Sigmoidea marina
(Haythorn et al. 1980) but in t'."- ee' grass species, the conidia were often
curved in more than one plane, as with the conidia of Angui1lospora Ingold
(1942). This feature was not included in the description of Z. marina. A
further problem here stems from the fact that the original description of the
genus Sigmoidea (Crane 1968) referes to the conidiophorea as phialides or
aleuriophores. There is no evidence of phial id (Kendrick 1971) conidium
production in S_igmo_id_ea_ sp. from «?el grass of j>. marina as described by
Haythorn et al. (1980). Cultures of Sigmoidea sp. from eelgrass are available
upon request from the author (as culture no. SAP 9).
156
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TABLE I. FREQUENCY OF OCCURRENCE ON SAMPLES OF EELGRASS LEAVES AS RECORDED
AFTER SEAWATER INCUBATION.
Species or form
Sterile mycelium
Sigmoidea sp.*
D. salina*
Ciadospor ium sp.
Acretnonium sp.
V. ramulosa*
Lulwortliia sp.
Fungi absent*
Frequency of occurrence on:
Green leaves Brown leaves
Submersed Wrack Submerged Wrack
75a 50a 70a 40a
Oa I0a Oa 60b
Qa oa 33 5Qb
38'i 10a 33a 40a
isa ioa 53 20a
0* 20b Oa 20b
10a Qa j^a Qa
lOab 20n Oa Oa
a. Asterisk after specios n.inc indicates detection of significant
(P ^0.05) differences anone frequencies for types of samples. For each
species, frt-quonr L.'S bc.Trinj; Lht> sant> superscript letter were not
s ipn i f i<_ant 1 v diffc-rent.
b. The number of leaves from t'.ich of which two replicate samples were
taken: both types <• f submerged, 40; both types of wrack, 10.
157
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Bacterial Counts and Volumes
Total numbers of bacterial cells calculated for the filtered (250-po mesh
screen) eelgrass-leaf homogenates and for the 250-m retentates are presented
in Table 2. Calculated values for denisty of bacterial cells per square
millimeter of leaf surface are also given. Densities of bacteria on green
leaves were statistically equivalent (X • 2.0 x 10^ cells per tnm2 of leaf
surface) and two to three times lower than densities on brown leaves, except
on brown leaves from the wrack line. There was a statistically significant
gradient in bacterial density on brown leaves from bayward to shoreward edges
of the bed, with the lowest value being found for wrack-line leaves. Mean
bacterial cell volume did not differ significantly between green leaf and
brown leaf samples; overall mean volume was 0.32 + O.lOum^ (95Z confidence
interval) per cell).
The 250 m retentates contained only a very small percentage of the total
bacterial cells (Table 2); the range was 0.03 to 0.7Z for green leaves and 0.1
to 0.6Z for brown leaves. Homogenization appears to be a satisfactorily
efficient means of releasing adherent bacteria from the type of leaf
substrate.
Frequencies of dividing bacterial cells were 7.2Z for green leaves and
7.62 for brown leaves. Decisions regarding what were and what were not
dividing cells were more difficult than when counts are made of water column
samples (Hagstroin et al 1979; S. Y. Newell and R. R. Christian, submitted for
publication), primarily because many of the bacteria in the eelgrass samples
were long rods (although mean cell length was only 1.5 p.m) with indentations
which may or may not have been constrictions leading to division and because
the background in the eelgrass samples was not as dark as those obtainable
with water column samples.
Fungal Volume
All attempts to estimate fungal biovolume produced the same result; no
structures were observed which could be identified as fungal. This was true
of all types of sample, green and brown, submerged and wrack line. The only
fluorescent structures in WSAB preparations were clearly pieces of eelgrass
(cell walls were green fluorescent^ or algae (orange to red fluorescent).
Under phase-contrast microscopy, no hyphae, conidia, or pieces of ascocarp
were seen, even in the settled concentrates.
DISCUSSION
The complete absence of zoosporic fungi in 100 samples of green eelgrass
leaves indicates that there is no counterpart in eelgrass for symbiotic
rhizomycelial chytrid of turt1fgrass. Since only one point in time and one
geographical location were sampled, this cannot be stated definitively, but it
should be noted that the chytrid symbiont of turtlegrass was recorded at high
frequencies (70 to 98Z, n » 79) in both winter and summer from three widely
separated geographical locales (Newell and Fell 1980). Thus, the suggestion
by Newell and Fell (1980) that a symbiotic rhytrid might be responsible for
the wasting disease of eelgrdss is probably wrong.
158
.* 1
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TABLE 2. TOTAL NUMBER OF BACTERIAL CELLS IN HOMOGENATES OF EELGRASS LEAVES
AND BACTERIAL CELLS PER SQUARE MILLIMETER OF LEAF SURFACE.
1+23
Total no. of bacterial cells in:
Green leaves Brown leaves
3+43 5+63 7+fta 9+ioa \+ia 3+43 5+fta y+ga 9+10*
Filtrateb 2.*>
Retenatec 0.8
Leaf surface^ 2.7
1.9a 2.3a 1.6a 1.43
0.8 0.7 0.8 n. 9
2.0 2.4 1.7 1.4
5.5c 6.JC 4.4b 3.8*> l.&
8.7 7.4 8.4 7.2 9.5
5.9 6.5 4.5 3.9 1.7
a. Surfaces on transect. Stations 1 to 8 were submerged sites; stations 9
and 10 were wrack-line sites.
b. Undiluted hoiiogenate after filtration through 250-^JUn mesh screen; data
is given as the total number of cells x 10^. East superscript letter
designates means which are not s icni f icant ly different from one another
(P N
c. Material collected on a '2iO-fjrn nesh screen; data is given as the total
number of cells x 10^.
d. Data ^iven as the total nunbor .if cells x 10^ per mm^ of leaf surface.
159
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The pattern of occurrence of fungal species other than that for
chytridiomycetes is similar for eelgrass and turtlegrass (Newell and Fell
1980). Sterile mycelium dominated samples of decaying leaves ;rom submerged
sites, and JD. salina was recorded at high frequency from sandy, high
intertidal sites. On eelgrass, D. salina was accompanied at high frequency by
Sigmoidea sp. in the high intertTdal (wrack-line) zone. For turtlegraas
leaves from submerged sites, Newell and Fell (1980) found chat
control-validated, leaf-surface, sterilization greatly reduced recordings of
mycelial fungi. This finding was taken by Newell and Fell (1980) to mean that
fungi other than, perhaps, the rhizomycelial chytrid are not active
decomposers of submerged turtlegrass leaves. In contrast, the high frequency
of conidial fungi which Newell and Fell (25) found in intertidally decaying
turtlegrass leaves, even after surface sterilization, was interpreted as an
indication of a possible active role of higher fungi in the breakdown of
leaves deposited on the shoreline. One could infer from the present fungal
frequency data that the same situation exists for eelgrass.
My findings regarding fungal biovolume in eelgrass leaves support the
inference from species occurrence data, that fungi are not active decomposers
of eelgrass leaves in submerged sites. Since there is little or no fungal
biomass present in these decomposing leaves, fungi cannot be converting
eelgrass matter into fungal matter. Newell and Hicks (in press) estimated the
volume of fungi in dead leaves of S_. alterniflora, using the same sample
volumes and microscopic handling as were used in the eelgrass determinations
of this study. Their estimate of fungal volume in Spartina leaves was about
1/3 of the leaf volume, indicating a dry fungal biomass of 0.2 to 0.5 g/g of
dry leaf. Comparison of average counts of hyphal intersections with an
eyepiece grid for the Spartina samples and for eelgrass samples suggest that
fungal biomass in eelgrass was less than 12 of that estimated for Spartina
leaves (i.e., <0.005 g/g of dry leaf). Since no fungal structures were seen
even in concentrated eelgrass leaf homogenates, the fungal fraction of
eelgrass leaf mass was probably much lower than 0.52.
Since wrack-line eelgrass biovolume samples were just as void of fungal
structures as were samples from submerged sites, the implication is that there
is no more fungal biomass produced in wrack-line litter than in submerged
litter, in spite of the suggestion to the contrary from species occurrence
data. However, both green and brown leaves of the wrack-line samples were
probably from recently deposited leaf litter, since there were many green
leaves within the piles of litter, and eelgrass leaves rapidly turn black
under desiccating conditions (personal observations). Therefore, fungal
invasion of this intertidal material may have been just getting under way.
Since deposition of seagrass leaves in wrack lines may account for a major
share of leaves detached (18, 36) this is a question worth pursuing.
Production of saprotrophic microbial biomass on submerged eelgrass leaves
is apparently limited to bacteria. Standing stocks of adherent bacteria
increased by 2 to 3 times as leaves aged from green mature to detached brown
states, ranging from about I to 7 x 10^ cells per mm^ of leaf surface. This
translates to 2 to 14 x 10^ cells per g of dry leaf, using leaf area-mass data
from this study and adjusting for leaching in BFFSW. Brown leaves in the
recently deposited wrack show 2 to 3 times smaller bacterial standing stocks
160
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Chan submerged brown leaves, perhaps another sign of a change toward an
environment favoring fungal activity. Using the mean bacterial cell volume
for eelgrass (0.32 nr*) and the conversion factor for biovolume to biomass
given by Ferguson and Rublee (1976; 0.28 g (dry weight) per cm^), I calculate
that the standing stock of adherent bacteria is about 2 x 10"^ mg/mm2 of green
leaf surface and 4 to 6 x 10~5 mg/nnn2 of submerged brown leaf surface (• 4 x
10"^ mg of dry bacteria per mg (dry weight) of green leaves and 7 to 11 x
mg for submerged brown leaves). This represents only 0.04 to 0.112 of leaf
mass, and so could account for very little of the elemental chemical content
of the leaves, just as found for other marine litter (Hobbie and Lee 1980; Lee
et al 1980; Rublee 1978; but see Morrison et al 1977). Apparently, the high
glucosamine content of detrital Zostera leaves (about 9 mg of glucosamine per
g of ash-free dry leaves) found by Thayer et al. (1977) is not in the form of
living bacterial material, yet it may be a result of bacterial deposition, as
discussed by Hobbie and Lee (1980).
I counted only tightly adherent bacteria on Zostera leaves; densities of
bacterial cells on Spartina, Thalassia, and other types of litter and
particulate detritus range up to 4 to 14 times the densities which I have
recorded for Zostera (Lee et al 1980; Rublee et al. 1978; Newell and Hicks, in
press; P. A. Rublee and M. R. Roman, submitted for publication). However,
Harrison and Harrison (1980) reported range of bacterial concentrations of 2 x
10^ to 1 x lO-* cells per mar of surfaces of particles of Zostera leaves in
microcosms. Kirchman et al. (1980) reported that bacterial numbers on
surfaces of Zostera leaves from northern temperate waters were approximately
107/cm2, and the values which I report here fall within the range (5 x 10® to
10*0 cells per g of dry substrate) of bacterial densities cited by Fenchel and
Jorgensen (1977) for a wide variety of substrates, including Zostera (Fenchel
1977).
The standing stocks of bacteria on eelgrass are not good indicators of
bacterial productivity, if output of bacteria by predation, sloughing, etc.,
is substantial. Using the FDC estimated for eelgrass bacteria (about 7Z) and
the regression equation describing the relationship between FDC and
instantaneous growth rate ( ) of marine bacteria (Newell and Christian,
submitted for publication; the equation: In - 0.299 FDC - 4.961), I
calculate that instantaneous generation time for bacteria on the eelgrass
sarnies was 8.5 h. If the standing stock of bacteria was in steady state and
had no diel rhythm (a tenuous assumption; see, e.g., Meyer-Reil et al 1979;
Sieburth 1979), this would indicate a 24-h output of about 1.4x the standing
stock. However, there remain several unanswered questions regarding the
validity of the use of FDC in estimating marine bacterial productivity (Newell
and Christian, submitted for publication), and use of the FDC method with
homogenized litter substrates may be invalid due to the effect of
homogenization on dividing cells. Therefore, the above productivity estimate
is highly speculative and needf, testing by alternative -nethods (Fuhrman and
Azam 1980; Hanson 1980; Sieburth 1979).
161
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LITERATURE CITED
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highland including Mew England, and three coastal plain states. Am. J.
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Fenchel, T. 1977. The significance of bactivorous protozoa in the microbial
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Fenchel, T. and B. B. Jorgensen. 1977. Detritus food chains of aquatic
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crop of coastal plankton. Litnnol. Oceanogr. 21:141-145.
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dividing cells, a new approach to the determination of bacterial growth
rates in aquatic environments. Appl. Environ. Microbial. 37:805-812.
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Hanson, R. B. 1980. Measuring microbial activity to assess detritial decay
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Haythorn, J. M., E. B. G. Jones and J. L. Harrison. 1980. Observations on
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Hobbie, J. E., R. J. Daley, and S. Jasper. 1977. Use of nuclepore filters
for counting bacteria by tlurorescence microscopy. Appl. Environ.
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Hobble, J. E. and C. Lee. 1980. Microbial production of extra-
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Ingold, C. T. 1942. Aquatic hyphomycetes of decaying alder leaves. Trans.
Br. Mycol. Soc. 25:339-417.
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Cramer, New York.
Jones, E. B. G. (ed.). 1976. Recent advances in aquatic mycology. Paul Elek
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Kirchman, D. L., L. Mazella, R. Mitchell and R. S. Alberte. 1980. Bacterial
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Spartina alterniflora and the use of ergosterol in estimating the
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Morrison, S. J., J. D. King, R. J. Bobbie, R. E. Benchtold and D. C. White.
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Park, D. 1974. Accumulation of fungi by cellulose exposed in a river.
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164
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Chapter 6
Preliminary Studies on Community Metabolism in a Tropical Seagrass
Ecosystem: Laguna de Terminos, Campeche, Mexico
R. L. Wetzel1
L. Murray1
R. F. van Tine1
J. W. Day, Jr.2
C. J. Madden2
1. College of William and Mary, Virginia Institute of Marine Science,
Gloucester Point, VA 23062.
2. Center for Wetlands Resources, Louisiana State University, Saton
Rouge, LA 70803.
165
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ABSTRACT
Apparent 02 productivity measures of a Thalaaaia testudinum seagraas
community at Estero Fargo in Laguna de Terminos, Campeche, Mexico indicate
Chat maximum areal rates are in the range 5.5 to 7.5 gOj * d~* (1.65 to 2.25
gC m~2 d~M and agree well with published data. The T_. testudinum community
is light-compensated (Pa - R) at approximately 200 ^i£ m~* sec~l and
light-saturates between 700 and 800 E m~2sec~1. Mean above-ground biomass at
the Estero Pargo site was 288 (+86) live and 285 (M47) dead g m~2 (dry
weight) with a mean root to total above-ground plant biomass ratio of 4.11
(+2.28). The relatively low biomass and high root/rhizome to total
aboveground ratio indicates a community that is either in a die-back stage or
environmentally stressed. We hypothesize that light and nutrient interactions
are principal factors governing plant community production at Estero Pargo.
Comparative studies within this important lagoon should help elucidate overall
mechanisms controlling productivity and the role of the seagrasses in the
general energy flow structure of Laguna de Terminos.
166
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INTRODUCTION
Seagraas communities dominated by the turtlegrass, Thalassia testudinum,
comprise an important and productive habitat in subtropical and tropical
ecosystems. Generally, mean standing stocks range from 400 to 3000 g m~* (dry
weight) with daily productivity ranges of approximately 1.0 to 10.0 g m~2 (1.0
to 3.0 mgC g~l(plant)) (McRoy and McMillian 1977). Assuming an average
growing season of 250 days in these areas, the range for annual production
would be 250-2500 gC m~2. These values fall in the upper ranges reported for
a variety of estuarine and coastal vascular plant communities (Westlake 1963;
Keefe 1972) and suggest their potential importance to nutrient cycling, energy
flow and trophic structure.
Most studies estimating productivity and energy flow in T_. testudinum
communities have been carried out in coastal waters of the United States. As
Lot (1977) points out, few, if any, studies on community energetics have been
done in the vast seagrass communities in both Gulf of Mexico and Caribbean
waters of southern Mexico (Caropeche). Nearly all studies in these areas have
been surveys and descriptively oriented. These areas have vast bottom areas
vegetated by T_. testudinum in both lagoon (Laguna de Terminos) and coastal
areas (Yucatan). Estimates of production, environmental factors regulating
productivity and relative importance of seagrasses in these systems are
generally lacking.
This report summarizes our preliminary findings on Thalassia testidinum
community metabolism and the effects on apparent productivity of varying light
regimes and dissolved inorganic nutrient conditions in Laguana de Terminos.
STUDY AREA
The otudy area was located in Laguna de Terminos, Campeche, Mexico.
Laguna de Terminos is a large embayment (ca. 2500 km^) bounded to the south by
the mainland and to the north by Isla del Carmen. Lagoon waters communicate
with the Gulf of Mexico through a net inflow channel, Puerto Real, to the
east, and a new outflow channel, Carmen, to the west. The area is
characterized by extensive mangrove swamps, freshwater marshes and seagrass
beds dominated by Thalassia testudinum. Terminos lagoon is relatively shallow
with a mean depth of 3.5 m. Figure 1 illustrates Laguna de Terminos and the
location of our principal study site, Estero Pargo. The site was chosen
because it is a station routinely sampled by scientist of the Centro de
Ciencias del Mar Licinologia of the Universidad National Autonoma de Mexico
(UNAM) investigating various chemical and biological characteristics of the
lagoon. Also, the site minimized logistical problems for these initial
studies. Depth of our study site averaged 1 m.
167
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METHODS
Metabolism studies were designed and carried out to estimate; 1. net
apparent community 02 productivity (Pa), 2. community respiration (Ra), and
3. 02 exchange by specific community components: water, non-vegetated
substrate, and plant leaves. Also included in our studies were additional
measures of productivity and growth to compare with the 02 data. These
included ' C photosynthesis studies using individual leaves and morphological
measures (leaf width and length) to estimate production according to the
regression equations of Patriquin (1973) and '^C productivity of the water
column. Specific experiments were carried out to evaluate; 1. light
intensity-apparent photosynthesis relations, and, 2. effects of short-term
dissolved inorganic nutrient enrichment.
Community oxygen metabolism
Studies were conducted using 32 1 acrylic hemispheres (domes) that
covered a bottom area of 0.7 m~. The domes were constructed with (I) a 7.5 cm
vertical flange Co eliminate exchange of enclosed water, (2) equipped with
ports for sampling enclosed water and (3) a collar for securing a
polarographic type electrode for oxygen concentration measurements.
Net apparent community 02 productivity and respiration was determined
using clear and opaque domes resoectively. During the incubation period,
incub.it ion continuous water circulation was provided by stirrers integral to
the 02 probe bodies (Orbisphere Laboratories, Inc., Geneva). According to
instrument specifications, the stirrer produces a mean current velocity of 25
cm sec~l at the probe head. Tests with the dome inverted and water filled
indicated complete mixing within 2-3 minutes using Rhodamin WT as a tracer.
02 measurements (mg l~*) were made using temperature-compensated, f^S
insensivive sensors calibrated in water-saturated air. Calibration was
carried out at the beginning of each experiment and checked at the end. For
all studies, pre vs post calibration differed by no more than _* 0.2 mg 1~^.
The domes were placed by hand with care taken to avoid disrupting the sediment
surface and trapping leaves under the vertical flange. After the domes were
placed, a "settling" time of 30 minutes was allowed with all ports open to
ambient water before initial measures were begun. Periodic water samples were
taken by syringe through the sampling ports for dissolved nutrients (NH*4,
N02~, N03~ and PO^"3); 02, temperature (°C) and light (PAR pE m~2 sec"1) at
the top of the plant canopy were monitored continuously using a Dataplex
Signal Scanner (Hampshire Controls) and a dual channel recorder (Soltec,
Inc.). Additionally, written recorders were kept for 02, temperature and PAR
as a check against recorder performance and calibration. PAR measurements
were made using a Ll-COR Model 185 Quantum Meter (LICOR, Inc.) equipped with
submarine and deck quantum sensors and interfaced through a switching panel
with the Dataplex and recording equipment allowing semi-continuous recording
surface light intensity (Io) and light intensity at the plant canopy top (I?).
For all studies, measurement intervals were as close as possible to the
periods of peak isolation (1000 to 1400 hrs. CST).
169
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Water column metabolism
Plankton metabolism was estimated with light-dark BOD bottles (300 ml)
using both standard oxygen and 1*C techniques (Strickland and Parsons 1972).
The oxygen bottles were suspended at mid depth (50 cm) and the 1ZkC bottles
were at the surface. Incubation was four hours for the oxygen measurements
and 4:45 for the 1/1C studies.
Substrate metabolism
Metabolism of non-vegetated substrates (i.e. bare sediments within the
grassbed) and adjacent sand bottoms was determined using transparent and
opaque benthic chambers. The chambers measured approximately 10 cm (diameter)
by 10 cm (height above sediment 31rface) once in place. Incubation volume in
the chambers was ca. 0.8 1. All incubations for the various experiments were
carried out using replicate treatments.
Light/nutrient responses
Net apparent community 02 productivity versus light intensity was
evaluated using four clear, acrylic domes and four levels of light intensity;
100*, 712, 50Z and 302 of ambient conditions. The light levels were
established by using fitted, neutr.1 density screening over the domes.
Because of time constraints, we could not replicate the study but for the
results reported the domes were placed in, at least observationally, a
homogeneous stand of vegetation. Our experience, based on three years of
study in Zostera marina dominated communities in Chesapeake Bay (see Wetzel
1983), indicate that the variance within treatments is much leas than between
study periods.
The short-term effect of increased concentrations in dissolved inorganic
nutrients, NH^*, N03~, and PO^-', was evaluated by "spiking" the domes in a
separate experiment to 50 p»M (NH^ * N03~) and 10 pM ?0^~^. Reported here are
the results for changes in apparent 02 metabolism in these treatments.
Kinetic analysis of the nutrient exchange studies are reported elsewhere
(Boynton et al., unpubl. ms.).
Plant Biomass
For all studies, plant biomass was determined usi..g an acrylic hand co«-er
(0.033 m^). Samples were taken by hand coring to a depth of approximately
25-30 cm to include root/rhizome fractions. The samples were sorted in the
field into above-ground live (AGL), above-ground dead (AGO) (defined as
obvious ch'.orotic and/or brown-black leaf material), and below-ground root and
rhizome (BG) fractions. Wet weight was determined on blotted, fresh materif'
and dry weight determined on the samples following forced-air drying at 60*C
for 48 hours.
RESULTS
Plant Biomass
Table 1 summarizes the results for all core-biomass samples taken during
our the various studies. Total above ground plant biomass averaged 573 g m~^
170
-------
TABLE 1. ESTIMATES OF PLANT BIOMASS FOR VARIOUS FRACTIONS AT THE ESTERO
PARGO STUDY SITE IN FEBRUARY, 1980.
Fract i
g wet weight
(+S.D.)
dry weight
(+S.D)
AGL
12
1980
(797)
288
(86)
AGO
12
2225
(1184)
285
(147)
BL
147H5
(5467)
1979
(370)
171
-------
(dry weight) and was approximately equally divided between live and dead
fractions (288 and 285 g m~2, respectively). Mean root (including rhizome) to
shoot (live + dead fractions) ratio for all samples was 4.1 (S.D. 2.28) but
varied considerable (range 1.89 to 9.71) within what we initially conceived as
a relatively homogenous stand of vegetation. This high and variable
root:shoot ratio probably reflects the generally nutrient poor status of the
area. Biomass estimates also fall in the lower range reported for T.
testadinmn communities. Sediments in this area are coarse, sand-shell mixes
and oxygenated to a depth of at least 15 cm (1.0 mg(>2 I"1, _^ 0.3).
Apparent Thalassia Community 02 Productivity
Figure 2 illustrates the simple linear relationship between net apparent
Oj productivity and light reaching the plant canopy top. The X-intercept
suggests that the community compensation point (P:R • 1) is approximately 200
pE m"^ sec" . The Y-intercept agrees well with measures of community
respiration (200 _+ 63 mg02 nT2 h"'). These data suggest that plant community
photosynthesis is not light saturated in the range 200-650 ^jE m~2 sec"^ (range
of light intensity observed for these experimental conditions).
Figure 3 summarizes the specific rate determinations expressed per unit
of above ground living plant material for all studies. The simple linear
regression line illustrated was calculated for all measurements at light
intensities equal to or less than 2.4 E m~2 hr~^ (667 pE m"2 sec"'). The
rational for this will be discussed in a following section.
Not surprisingly, both the areal and specific rates of net apparent 02
productivity show a strong, positive correlation with in situ light flux. At
the community level, it appears that light intensities above 200 pE m"2 hr~*
are required for net autotrophic production. However, if we assume a 12 hour
photoperiod and a mean areal respiration rate of 200 tngO m~2 h~', in order to
balance the community (P=R) on a daily basis (i.e. net apparent 02 production
of 400 mg02 m"2 hr~'), light intensity at the canopy top must average
approximately 575 j^E m~2 sec"') over the photoperiod. or, based on a mean
vertical attenuance of -1.07 (n*6), 1675 pE m~2 sec~^ insolation.
Community Response to Varying Light Regimes
Figure 4 illustrates the results of the. neutral density shading studies.
The four light levels are identified as: 1.0 (C), 0.71 (L), 0.50 (M) and 0.30
(H) of ambient conditions. Based on th* control (1.0) and light (0.71)
treatments, the autotropnic response apaears to light saturate between 700 and
800 E m sec"' at the canopy top or very near solar maximum isolation
(assuming -kz =• 1.07). The saturated, net apparent community 02 productivity
at these higher light levels is approximately 550 mg02 m hr~* for this time
of year. Using the conversions given in McRoy and McMilla.i (1977), a mean
standing stock of 288 gdw m~-, and a mean community respiration rate of 200
mgOo m~2 hr~ , maximum specific rate of production would be 0.78 mgC g
(plant'1 h"1).
172
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The points circled in Figure 4 identify rates determined in late
afternoon (1400-1700 hr) and suggest an afternoon depression in apparent 02
productivity under optimal light conditions. Figure 5 illustrates the
response in all treatments plotted as the actual 02 concentration versus time
of day. It is apparent that the rate of change approaches zero in all
treatments before there is any significant change in light regime (dashed
line). Plotting the data as illustrated in Figure 6, i.e. rate of apparent 02
productivity versus 02 concentration, suggests that the depression in
afternoon rates is not a function of in situ 02 levels.
However, there is a strong linear relationship between apparent
productivity and in situ light levels during morning and peak insolation
periods. For all regression statistics given in the figures, paired
observations were evaluated resulting in significant and positive
correlations. As a test of light response, a significant improvement in the
correlation coefficient was obtained when a time lag of 10 minutes was used
for light intensity paired with rate estimates: r = .763 versus .913. This
result suggests that for in situ studies of light:photosynthesis relations,
observation intervals must be kept short, particularly in varying
environments, to derive the best estimate of Pa. For the studies reported,
measurement intervals of 10 to 15 minutes were used coupled with the same
integration intervals for light observations.
Nutrient Response
Figure 7 illustrates the response in specific rate of apparent 02
productivity for nutrient enriched and ambient conditions. The X-intercept*s
are not significantly different from previous estimates however the
differences in slope suggest an enrichment effect. The data set used as a
control was run in the same area two days prior to the enrichment experiment.
The nutrient enrichment study was carried out under relatively low light
conditions (maximum intensity approximately 280/jE m~* sec"') and following a
storm. We are therefore hesitant to speculate as to probable cause or even
component involved. It does appear none-the-less, the community responds
rapidly to increased nutrient supply. Within an hour following the spike,
NH^* concentrations in the domes were at ambient (pre-spiked) levels; i.e.
theoretical 25 M-NH^* to approximately 1.0jjM-l»V (Hopkinson, et al.,
unpubl. data).
Water Column Metabolism
Rate of surface net productivity using the '^C method ranged from
0.74-1.75 mgC m~^ hr~^ and averaged 1.16. This is equivalent to 4.7 mg02 m~^
hr~l (Table 2). The results of the oxygen measurements yielded considerably
higher values; 12.5 and 65 mg02 m~3 hr~^. The oxygen respiration values
ranged from -37.5 to -121 mg02 m~3 hr"'.
Sediment Metabolism
Metabolism of the non-vegetated substrate communities are summarized in
Table 2. The net apparent productivity of the bare substrate within the
grassbed averaged 111 mgC>2 m~2 hr~', while that of the sand substrate was 218
176
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Figure 7. Scatter plot of specific rate of apparent 02 productivity vs
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ara linear least squares lines of best fit for all data.
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m~2 hr~*. Both were approximately 54Z of the total plant community
metabolism. The respiration of the bare substrate averaged 41.6 mgC>2 m~2
hr"1, and that of the sand area was -63.5 mg02 m~2 hr~* or 20Z and 30*
respectively of total plant community respiration.
DISCUSSION
Standing stocks of Thalassia testudinum at the Estero Pargo site in
Laguna de Terminos are in the lower range reported by MeRoy and McMillian
(1977): 288 g m~2 (dry weight) (AGL) versus a mean range of 340 to 3100 g m~2
(dry weight) (exclusive of Poraeroy, 1960). The site is characterized by
coarse, sand and shell sediment, high root:shoot ratios, and variable light
conditions (range of kz - .56 to -2.0 for this study period). The study site
is also within several hundred meters of a major inland channel draining an
extensive mangrove swamp. Spectral analysis (HER 100 Spectroradiometer,
Biospherical Instruments, San Diego) of light in the channel outflow and plunie
indicate significant changes in both subsurface light quantity and quality
which may influence light-energy fields in adjacent areas (Wetzel and van Tine
1982).
Highest standing stocks of seagrasses in the lagoon are found in and
around Puerto Real (net inflow channel) and progressively decrease toward
Estero Pargo and Carmen (R. Roman, pers. comm.). We hypothesize, based on
observations in these areas, that light quality and quantity, and sediment
characteristics (implying various nutrient regimes), follow the same gradient.
The conclusion seems that the area studied represents the lower range for
estimates of seagrass metabolism in the lagoon system. This, coupled with the
relatively large fraction of dead above-ground material, suggests that our
rate measurements are probably minimum for this time of year and study site.
Table 3 summarizes the results for measurements of productivity in T_.
testudinum communities in Gulf and Caribbean water. In compiling this data,
carbon (^*C) and biomass data (P and Z) were converted to 02 equivalents using
the ratios given in McRoy and McMillian (1977). There is, surprisingly, a
relatively narrow range of values considering both the diverse nature of the
habitats and methodologies employed. Measurements made during this study fall
well within the range of values reported. Based on our previous discussion,
tttis suggests that production by seagrass communities in Terminos (assuming
this site represents minimum values), is very high. Maximum rates of apparent
productivity at Estero Pargo are in the range 5.5 to 7.5 g02 m~2d~l or 1.6 to
2.2 gCm"2 d~l. If we take McRoy and McMillan's (1977) suggestion of a 250 day
growing season, this would translate to 450 to 650 gC m~2 y ; significantly
higher than production by other autotrophic communities in comparable
ecosystems (Beers et al. 1968).
We have used maximum rates at Estero Pargo for these comparisons.
Without question, these are not realized daily rates but represent potential
input of fixed organic matter to the ecosystem. Our conclusion is, assuming
this area represents minimum estimates, that the seagrass communities in
Laguna de Terminos represent A significant input of organic matter and
nutrients to the lagoon ecosystem.
181
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Controls on production of submerged aquatics have been the subject of *
variety of studies (see Wetzel et al., 1981 and references cited therein).
Generally, light and nutrients are the focus of attention though salinity and
temperature are often included. Our preliminary studies suggest, overall,
that light is singularity important in deriving the "best" estimate of
productivity. The strong dependence (statistically) on predicting net
productivity at the community level based on light regime indicates light, at
least at Eatero Pargo, as a principal factor. We remain at a loss to provide
insight as to the characteristic late afternoon depression in apparent 02
productivity, although several promising avenues of research are apparent
(e.g. see Nafziger and Koller 1976).
Capone et al. (1979) provide data relative to T. testudinum light
response. Their data indicate that Pmax (light saturated rates) occur at
light intensities of 800 to 1000jaE m~^ sec~^. Their measures were derived
from '^C incubations on excised leaf material containing epiphytes.
Considering the assumed problems in measure-neat using various techniques
(Hartman and Brown 1967; Zieman and Wetzel 1980) our estimates are very
comparable. We estimate that the T_. testudinum community light-sacurates at
700 to 800 pE m~2 sec~l. Capone et al. (1979) estimates would fall in the
range 630-1050. We interpret this as support for the oxygen technique in
studies of community metabolism although problems still exist in terms of
recycling and trans location processes. However, the basic arguments against
02 are applicable to l^C02 studies (Kemp et al. 1980).
Nutrient status, i.e. principally nitrogen, is generally presumed to
limit primary production in estuarine and marine ecosystems. Patriquin (1973)
and Capone et al. (1979) have reviewed and summarized contemporary data on T.
testudinum dominated communities. Our data suggests a positive response to
nutrient enriched conditions. We propose that Estero Pargo is a nutrient
limited as well as potentially light-limited environment. Nutrient enrichment
(spikes) obviously result in changes in community behavior. The levels used,
however, are probably never "seen" by the community. Tne community does
respond and available nutrients NH4* and N03~ significantly affect community
behavior as measured by net apparent 02 productivity.
Our preliminary analyses or metabolism in a tropical T_. testudinum
dominated seagrass community suggests that, overall, the community is
responsive to both light and nutrient regimes. In Laguna de Terminos, our
studies indicate that the seagrass community is an important component of
autotrophic production. For example, if we assume a three fold increase in
production at the Puerto Real site (R. Roman, pers. omm.), production would
approximate 1350 to 1950 gC m~^ y~' which are among the higher values reported
(see McRoy and McMillan 1977; Zieman and Wetzel 1980).
The values obtained for water column productivity were very low compared
to open lagoon waters. Day et at. (1982) reported an annual average value of
1.2 gC m~^ day . The low values for the water over the grassbeds is probably
a result of several factors. The shallow w.->ter and high light levels probably
lead to light inhibition as indicated by the extremely low rates measured at
the surface. The plankton are probably also nutrient limited. The generally
low nutrient levels probably result from high uptake by the plant community.
183
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Finally, there is almost certainly heavy preclat ion by consumers living in and
around the grassbeda. These factors combine to limit the importance of
phytoplankton in the grassbed community.
The partitioning of the bare substrate metabolism suggests that the
benthic microalgae contribute significantly to the overall community
productivity. The areal rates of productivity for the bare substrates within
the grassbed repored here are overestimates, because the ratio of
non-vegetated to vegetated areas were not determined and areal estimates
corrected. However, the measurements in both bare substrate areas indicate
that benthic microalgae contribute significantly to the overall plant
community productivity. These findings suggest that a valuable heterotrophic
iood source exists here in the form of benthe microalgae.
Further study is obviously needed in this important ecosystem to
elucidate seasonal and annual characteristics of carbon metabolism both by
autotrophic and het*>rotrophic components of the community and investigate the
apparent major controls on production: light, nutrients and their
interaction.
-------
• - - •
/ .' -' •-' X - ,
Finally, there ia almost certainly heavy predation by consumers living in and
•round the graaabeda. These factors combine to Haul the importance of
phytoplankton in the graaabed community.
The partitioning of the bare aubatrate metabolism auggeata that the
benthic microalgae contribute aignificantly to the overall community
productivity. The areal ratea of productivity for the bare aubatratea within
the graaabed repored here are overestimates, becauae the ratio of
non-vegetated to vegetated areaa were not determined and areal estimates
corrected. However, the measurements in both bare substrate areaa indicate
that benthic microalgae contribute aignificantly to the overall plant
community productivity. These findings auggeat that a valuable heterotrophic
food source exists here in the form of benthc microalgae.
Further atudy ia obviously needed in this important ecosystem to
elucidate seasonal and annual characteristics of carbon metaboliam both by
autotrophic and heterotrophic components of th« community and inveatigate the
apparent major controla on production: light, nutrients and their
interaction.
184
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ACKNOWLEDGEMENTS
A first-time study of this type was necessarily plagued with many
difficulties both procedural and logistical. We want to especially extend
thanks to all our Mexican colleagues who accepted and worked closely with our
contingent of fourteen scientists. Ramiro Roman, Director of the Centre de
Ciencias del Mar Limnologia (UNAM), provided us lodging, space, logistical
support and a friendly atmosphere. Our special thanks go to Dr. Alejandro
Yanes and Drs. Vivianne Solis of the Universidad National foitonoma de Mexico
(UNAM) for their many contributions to the effort.
Without the help of our other American colleagues, Drs. Walter Boynton,
Michael Kemp, J. Court Stevenson, University of Maryland, and Dr. C. S.
Hopkinson, University of Georgia, this effort could not have been done.
We also thank Ms. Carole Knox and Nancy White for their secretarial and
editorial assistance.
185
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