United States
Environmental Protection
Agency
Chesapeake Bay
Program
Annapolis MD 21403
Research and Development
EPA-600/S3-83-047 Sept. 1983
Project Summary
Biology of Submerged Aquatic
Macrophyte Communities in the
Lower Chesapeake Bay
(Volume
Robert J. Orth and Jacques van Montfrans
As a part of the Functional Ecology
Program on Chesapeake Bay submerged
aquatic vegetation (SAV), this study
investigated infaunal and epifaunal
trophic dynamics. The work was con-
ducted in conjunction with other major
aspects of the program (see Volumes I,
II, and IV) and represents the culmina-
tion of four years of intensive field and
laboratory investigations.
The main study area established for
investigating the functional ecology of
resident consumers in the lower Chesa-
peake Bay was a large grass bed located
at Vaucluse Shores on the bayside of
Virginia's eastern shore Vaucluse Shores
was chosen as the study site because:
1) the site had been previously studied
and background information was avail-
able; 2) the bed is well established and
historically stable; 3) the area is rela-
tively remote and unperturbed; 4) the
bed contained the two dominant lower
Bay macrophyte species, Zostera marina
and Ruppia maritima; and 5) the bed
was large enough to simultaneously
accommodate varied studies and samp-
ling regimes. This bed was intensively
mapped in 1978 and 1979, and per-
manent transects were established for
sampling reference points.
This Project Summary was developed
by EPA's Chesapeake Bay Program,
Annapolis, MD, to announce key find-
ings of the research project that is fully
documented in a separate report of the
same title (see Project Report ordering
information at back).
Introduction
One of the most notable features about
habitats with SAV is the characteristically
high density of the associated fauna. In-
cluded are epibiota and infauna which are
represented by a diverse and complex
assemblage of micro- and macroalgae,
protozoans, hydrozoans, anthozoans, tur-
bellarians, gastropods, isopods, amphipods,
polychaetes, oligochaetes, bivalves, deca-
pods, and barnacles. Many of these groups
exhibit distinct seasonal pulses of abun-
dance depending on their individual
spawning periods. The epibiotic community
within grass beds is quite distinct from the
communities in adjacent unvegetated areas
Due to the lack of a suitable substrate,
there is usually very little epifauna in bare
sand or mud areas. Epibiota primarily use
the blades as a substratum for attachment
(i.e., barnacles, algae, hydroids, etc.) or, in
the case of microherbivores grazing on the
microalgae that colonize the blades, a
feeding platform. Thus, grass beds provide
substrates, protection, and food resources
which allow for the maintenance of high
densities, which in turn attract and trophi-
cally support numerous migratory utilizers
of SAV habitats such as crabs, fishes, and
waterfowl. These features are fundamental
to the resource value of SAV beds on a
world-wide basis.
Our initial effort in examining the func-
tional ecology of resident consumers was
to determine the structural aspects of the
grass-bed community compared to un-
vegetated areas (Chapter 1). We subse-
quently conducted predator-exclusion ex-
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4
periments to determine the role of preda-
tion in structuring the biotic community in
grass beds (Chapter 2) and examined, in
greater detail, predator-prey interactions
in vegetated habitats (Chapter 3). Having
established which species were numerical-
ly dominant, we calculated the secondary
production of those species which were
trophically or functionally important (Chap-
ter 4). We then focused our attention on
one dominant species, an herbivorous
grazer, and examined its role in controlling
epiphytic fouling on Zostera marina (Chap-
ter 5). Because waterfowl have been the
least studied trophic components of the
grass-bed systems in the lower Bay, we
determined the intensity of utilization by
wintering waterfowl of the Vaucluse grass
system (Chapter 6). We also measured the
impact of feeding by one species (buffle-
heads) on the density of macroinvertebrate
population densities. Finally, we tried to
place into perspective major trophic links
in the Vaucluse Shores grass bed by
examining natural carbon isotope ratios
(13C to 12C) in some of the dominant
species (Chapter 7). Such an approach
enabled us to determine the sources of
primary production utilized by the resident
consumers.
I. Structural Analysis of Benthic Communities Associated with Vegetated and
Unvegetated Habitats
Procedure and Methodology
Three distinct habitats in the lower eastern
Chesapeake Bay (Vaucluse Shores at the
mouth of Hungars Creek) were compared
based on a structural analysis of the as-
sociated fauna. These habitats included a
grass bed (Zostera marina and Ruppia
maritima), large sand patches within the
grass bed, and an offshore sand-bar system.
Within the vegetated habitat, comparisons
were made of the fauna associated with
pure stands of Z. marina, pure stands of/?.
maritima, and mixed stands of both species.
Results and Conclusions
Generally, there was a trend toward a
greatej; species diversity (Shannon-Weiver
Index h) and abundance of infaunal species
and individuals in the vegetated habitats
than in the two sand habitats. Many species
which occurred as one of the top ten in
each habitat persisted throughout the study
(July 1978 to November 1979) and were
characteristic of the habitat examined.
Although infaunal abundances were con-
centrated at the sediment surface in all
three habitats, the grass bed supported a
larger number of individuals deeper in the
sediments than did the other two habitats.
The epifaunal component of the vegetated
habitat comprised a unique and diverse as-
semblage of species which was similar be-
tween each area investigated (i.e., Zostera
marina, Ruppia maritima, and mixed stands).
Few seasonal patterns in epifaunal abun-
dance were evident in the data. Vegetated
areas provided greater habitat heterogeneity
and were therefore capable of supporting
a greater overall diversity of species than
nonvegetated habitats.
II. Predator-Exclusion Experiments in a Chesapeake Bay Grass Community
Procedure and Methodology
The effects of predators on the density
of e.elgrassepifauna and infauna, and sand
infauna were studied using predator ex-
clusion techniques. A large topless pen
(20 m2) and smaller cages (0.25 m2)
within the pen as well as outside the pen,
were set up in a bare sand and an adjacent
grass habitat to test the hypothesis that
predation has a significant effect on the
structure Of associated faunardormnunities.'
Results and Conclusions
The grass habitat consistently had more
infaunal species per core than the sand
habitat for all treatments. There were no
distinct differences among the grass treat-
ments for species infaunal numbers but in
the sand species numbers were higher in
pen and cage treatments compared to the
control. Except for the sand cage treat-
ments, there was no difference between
the pen and cage treatments for number of
species in both habitats.
Density of individuals in the grass habi-
tat treatments was generally higher than
the sand habitat treatments except for the
June cage treatments. In the sand area,
infaunal densities were always higher in
the cage and pen treatments compared to
the control. In the grass habitat only the
September cage and pen treatments were
higher than the control.
Epifaunal densities in the grass habitat
were generally higher in cage and pen treat-
ments than the control. Species responses
to these treatments were variable and con-
trolled by the abundance of grass in the
treatment
The results of this work support recent
evidence for the importance of predation
for the structuring of benthic communities
both in vegetated and non-vegetated habi-
tats.
Predator-Prey Interactions in a Zostera marina (Eelgrass) Ecosystem in the Lower
Chesapeake Bay, Virginia
Procedure and Methodology
Experiments were conducted with arti-
ficial seagrass in small wading pools to
assess the ability of prey to survive preda-
tion at different densities of grass.
Results and Conclusions
Experiments using Mulinia lateralis, a
bivalve, as prey, and adult Callinectes
sapidus, a crab, as predator, showed that
almost no M. lateralis survived at three
different densities of grass. Experiments
with juvenile C sapidus as prey and adult
C sapidus as predator showed greatest
survival at highest densities of grass. The
behavior of the juvenile crabs in relation to
their predators was different in the presence
of the grass than in its absence. It was
believed that the survival of a particular
prey species in a vegetated habitat will
depend upon the life style and life cycle of
both prey and predator and the density
and morphology of the vegetation.
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IV. Secondary Production of Some Dominant Macroinvertebrate Species Inhabiting a
Bed of Submerged Vegetation in the Lower Chesapeake Bay
Procedure and Methodology
Secondary production of dominant macro-
invertebrate species at the Vaucluse Shores
SAV bed was examined from monthly
samples taken in the Zostera-Ruppia area.
Length-weight as well as ash-free, dry
weight calculations were made on all in-
dividuals. Production for each species was
determined using the size-frequency method
and instantaneous-growth method.
Results and Conclusions
The production of the top nine trophically
important species to the higher level con-
sumers at the Vaucluse Shores grass bed
was 40.7 g-m~2-yr1. This is a higher
productivity than reported for most com-
munity production studies. If this rate of
production is projected over the entire
140-hectare grass bed, a total of 53
metric tons of dry tissue was produced
and potentially available for consumption
by other trophic levels. This also represents
6 x 1010 individuals that are born, grow,
and die in a year. The average standing
stock over the year was 4.6 metric tons.
The isopod £ attenuate accounted for
43 percent of the total production for the
nine species. The next two high ranking
producers were C sapidus and G. mu-
cronatus, which, when combined with £
attenuate accounted for 84.8 percent of
biomass produced by the nine species.
Turnover ratios were highest for G. mu-
cronatus (24.5) and lowest for the snail B.
varium (3.2).
V. Preliminary Studies of Grazing by Bittium varium on Eelgrass Periphyton
Procedure and Methodology
The grazing activities of Bittium varium
Pfeiffer on periphyton colonizing live eel-
grass (Zostera marina L) and artificial
eelgrass (polypropylene ribbon) were in-
vestigated Quantitative measurements of
grazing impact on artificial substrates were
determined by periphyton pigment extrac-
tion and dry weight differences between
grazed and ungrazed blades.
Results and Conclusions
Significant differences occurred in
phaeophytm and dry weight calculations
but chlorophyll a measurements were not
significantly different. This suggests that
senescent diatoms constituted the bulk of
the periphyton weight and were selectively
removed over more actively photosynthe-
sizing diatoms.
An examination of scanning electron
micrographs further elucidated the impact
of grazing by Bittium varium. Some micro-
graphs revealed that B. varium removed
primarily the upper layer of the periphyton
crust on both artificial substrates and
living Zostera marina. The diatom Coc-
coneis scutellum Ehrenb. which attaches
firmly to living Z. marina blades was less
commonly removed than Nitzschia or
Amphora. Through its grazing activities, B.
varium may maintain community domi-
nance by tightly adhering diatoms such as
C scutellum. Evidence of the complete
removal of periphyton exposing the Z.
marina epithelium was revealed in other
micrographs.
The grazing activities of Bittium varium,
a species which removes periphyton from
seagrass blades, could have important
implications for the distribution and abun-
dance of Zostera marina in the Chesapeake
Bay.
VI. Waterfowl Utilization of a Submerged Vegetation (Zostera marina and Ruppia
maritime) Bed in the Lower Chesapeake Bay
Procedure and Methodology
A study of waterfowl use of a bed of SAV
was conducted over two winters in the
lower Chesapeake Bay (Virginia). Water-
fowl abundances were estimated from
census counts made during the winters of
1978 to 1 979 and 1979 to 1980. Food
habits of birds were examined using the
stable carbon isotope methods. Water-
fowl exclosure experiments were con-
ducted to assess the effect of waterfowl
feeding on the infauna and epifauna.
Results and Conclusions
In the winter season of 1978 to 1979,
Canada geese (Branta canadensis] were
the dominant waterfowl in the study area.
Goose foraging activity was correlated
with tide stage, and was greatest at low
tide. Consumption by grazing waterfowl
was calculated from bird densities, and
was approximately 25 percent of the
standing crop of vegetation in the shallow
portion of the habitat.
From 1979 to 1980, diving ducks,
primarily buffleheads (Bucephala albeola),
were dominant. Abundance of waterfowl
was influenced by wind parameters; but
tide, temperature, and time of day had little
or no influence on bird numbers. Within-
habitat variation in abundance was ex-
amined, and highest densities were as-
sociated with the deeper Zostera marina
zone.
Gizzard samples and S 13C analysis
revealed that buffleheads fed primarily on
small gastropods and nereid worms char-
acteristic of the grass-bed epifauna. Con-
sumption of important invertebrate prey
items, calculated from exclosure experi-
ments and waterfowl densities, amounted
to nearly 50 percent of the fall standing
crop of these species in Zostera manna.
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VII. Trophic Relationships in a Submerged Macrophyte Bed Based on d 13C Value:
Recommendations
The trophic function of SAV communitie
and the refuge that SAV provides appea
to be the key to understanding the role thesi
habitats play in supporting living resource
of direct importance to man. These two a1
tributes are so functionally interrelated tha'
although it may be necessary to separat
the two for the purpose of modeling th<
system, they must be addressed with a uni
fied research approach.
Procedure and Methodology
Trophic relationships in a lower eastern
shore Chesapeake Bay (Vaucluse Shores
at the mouth of Hungars Creek) seagrass
bed were investigated by examining time
integrated stable carbon isotope ratios
(13C/12C) in primary producer and con-
sumer populations.
Results and Conclusions
The periphyton grazing snail, Bittium
varium, exhibited close ties to the micro-
algae found on Zostera marina leaves.
Dominant isopods (Erichsonella attenuata
and Idotea baltica) were more closely
linked to the seagrasses themselves. In
several other invertebrate and vertebrate
species, trophic relationships were more
obscure. These will be more closely ex-
amined in a forthcoming publication. Over-
all, carbon isotope analysis appears prom-
ising as a method for elucidating general
trophic relationships in seagrass com-
munities.
Robert J. Orth and Jacques van Montfrans are with the Virginia Institute of
Marine Science, Gloucester Point, VA 23062.
David Flemer is the EPA Project Officer (see below).
The complete report, entitled "Biology of Submerged Aquatic Macrophyte
Communities in the Lower Chesapeake Bay: Volume III. Interactions of Resident
Consumers in a Temperate Estuarine Seagrass Community: Vaucluse Shores,
VA, USA," (Order No. PB 83-233 395; Cost: $20.50. subject to change) will be
available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Chesapeake Bay Program
U.S. Environmental Protection Agency
2083 West Street
Annapolis, MD21403
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