United States
Environmental Protection
Agency
Chesapeake Bay
Program
Annapolis MD 21403
Research and Development
EPA-600/S3-83-019 June 1983
Project Summary
Submerged Aquatic Vegetation:
Distribution and Abundance in
the Lower Chesapeake Bay and
the Interactive Effects of Light,
Epiphytes, and Grazers
Robert J. Orth, Kenneth A. Moore, and Jacques van Montfrans
Submerged aquatic vegetation
(SAV), a major ecological resource, has
undergone a major decline in
Chesapeake Bay during the past decade.
Aerial photographs of Lower
Chesapeake Bay taken in 1980 and
1981 were compared with 1978 aerial
photographs of the same Bay area. The
photographs showed that SAV had
declined in every area of the lower Bay
except the lower western shore. A
literature review of the relationship
between epiphytic fouling by
macroalgae and periphyton and the
grazers that feed on these epiphytes
indicates that grazing plays a major role
in preventing over-growth of epiphytes.
Nutrient enrichment can stimulate
the excessive growth of epiphytes,
resulting in death of the plants. The sa-
linity tolerances of one major grazer,
the snail Bittium varium, were studied
to determine whether rapid fresh-water
influx (such as occurred during Tropical
Storm Agnes) would prove fatal to the
snail. Larvae subjected to a rapid
salinity drop (from 22.3 to 11.1 ppt)did
not survive; however, adult snails were
less sensitive. Studies of plant vigor,
under three shading conditions, in the
presence and absence of Bittium
varium, indicated that within each
shading condition plant vigor was
enhanced by the presence of the snail.
The decreased salinities that occurred
with Tropical Storm Agnes were
probably a factor in decreasing
populations of Bittium varium. In turn.
the loss of the snail may permit over-
growth of epiphytes resulting from nu-
trient enrichment and thus contribute
to the decline of SAV in the lower
Chesapeake Bay.
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
Since 1978, submerged aquatic vege-
tation (SAV) has been the subject of an
intensive research program funded by the
U.S. Environmental Protection Agency's
Chesapeake Bay Program (EPA/CBP).
SAV was deemed to be a high priority
area of research in this program because
of its high primary productivity, and
because of its important multiple roles in
the Chesapeake Bay ecosystem -- a food
source for waterfowl, a habitat and
nursery area for many species of
commercially important fish and
invertebrates, a shoreline erosion control
mechanism, and a nutrient buffer. Most
importantly, research was focused on
SAV because of the dramatic, Bay-wide
decline of these species in the late 1960's
and 1970's. The initial emphasis of this
study was an investigation of the changes
in SAV distribution and abundance
between 1978 and 1981.
Numerous aspects of the functional
ecology of these complex systems are still
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poorly understood. One such area
involves the relationship between
epiphytic fouling by both macroalgae and
periphyton (community of diatoms,
microfauna, and paniculate material)
adhering to seagrass blades and the
grazing organisms which rely on these
epiphytes as important food-sources. This
relationship is the subject of a literature
review contained in this report.
The eelgrass fZostera marina L)
epifaunal community in Chesapeake Bay
is a diverse assemblage of species from
numerous taxonomic groups. One
numerically dominant herbivorous
gastropod, Bittium varium Pfeiffer
(Cerithiidae), is investigated in the
present study. B. varium may be an
important consumer. Its grazing action
may have important implications for the
distribution of eelgrass, especially for
those plants living in habitats where light
levels reaching the plant surface may be
only marginally adequate for photosyn-
thetic maintenance.
The productivity of both macro- and
micro-epiphytes of seagrasses is en-
hanced by nutrient enrichment, and the
resulting epiphytic proliferation can be a
factor in the demise of seagrass beds.
Grazers of periphyton can substantially
reduce the biomass of micro-epiphytes,
thereby possibly mediating the effects of
nutrient enrichment on periphyton prolif-
eration. In the absence of grazers, peri-
phyton may have the potential to rapidly
overgrow and shade the host plant, thus
reducing photosynthetic activity.
In Chesapeake Bay, Bittium varium has
been shown to significantly reduce the
biomass of periphyton associated with Z.
marina under laboratory conditions. The
presence of periphyton grazers can
indirectly affect the vigor of the host plant
by preventing periphyton proliferation to
potentially harmful levels. The final
objective of this project was to examine
how the growth of Zostera marina was
affected by the presence or absence of
Bittium varium in laboratory experiments.
Procedure/Methodology
From aerial photographs taken in 1980
and 1981, beds of submerged aquatic
vegetation in the lower Chesapeake Bay
were mapped onto U.S. topographic
quadrangles (1:24,000 scale). To insure
maximum delineation of the SAV beds
and to obtain comparable data, the 1980-
1981 aerial photography was conducted
using similar techniques and film and
under the same constraints observed in
the acquisition of the 1978 photographs
of the same Bay area. Only those topo-
graphic quadrangles in the polyhaline
and mesohaline areas of the lower Bay
were monitored in 1980 and 1981,
resulting in the mapping of 27 quadran-
gles. These maps were compared with
those obtained in the in the 1978 survey.
Field-collected specimens of Bittium
varium were used in laboratory studies of
its salinity tolerances. Tolerances of
laboratory-reared larvae were also
investigated. Effects of rapid salinity
reduction for short periods and gradual
salinity reduction on adult and larval
survival and activity were tested.
To study the effects of grazing on
eelgrass vigor, plants of Zostera marina
were grown in large tanks filled with con-
tinuously-pumped York River water.
Three degrees of shading, in the presence
or absence of of Bittium varium, were
applied and the resultant plant growth
determined. Plant growth was ascer-
tained through leaf area measurements
and dry weight determinations. Growth
of epibiota was also assessed by determi-
nation of dry weight and ash-free dry
weight.
Results/Conclusions
From 1978 to 1980, reductions of SAV
occurred in all sections of the lower Bay
except the lower western shoreline.
Almost no vegetation was found in the
Rappahannock River section in 1980, and
only a slight increase occurred in 1981.
SAV in the James River in 1978, which
existed in a narrow band between Ft.
Eustis and Newport News Point, was
completely absent by 1980.
The predominant SAV beds in 1980
and 1981 were still found in those major
areas identified in 1978: (1) along the
western shore of the lower Bay between
Back River and York River; (2) along the
shoreline of the Mobjack Bay and
immediately adjacent to the Guinea
Marshes at the mouth of the York River;
(3) the shoal area between Tangier and
Smith Island (this represented the largest
and most extensive SAV bed in the entire
Bay); and (4) behind large protective
sandbars near Hungar's Creek and
Cherrystone Creek along the Bay's
eastern shoreline.
Comparison of the 1980 and 1981 data
at the six historical SAV sites mapped in
1978 showed no recovery of any SAV at
the Parrott Island (Rappahannock River)
and Mumfort Island (York River) site.
These two sites remained devoid of any
SAV. SAV at the Fleets Bay (lower
western Chesapeake Bay) site continued
to decline from 1978 to 1980 but showed
a slight rebound in 1981. At Vaucluse
Shores, as well as in the the rest of the
eastern shore, SAV beds remained
relatively stable during this time period.
SAV at the East River (Mobjack Bay) site
declined both in 1980 and 1981. Four
complete surveys showed the decline ol
SAV to have occurred primarily in the
deeper, offshore areas rather than the
inshore, shallower locations. This pattern
was repeated in many other locations in
the lower Bay region. Although total SAV
area showed a slight decline in 1980 and
1981 at the Jenkins Neck (York River]
site, recruitment by eelgrass seedlings
was observed in the vicinity of Aliens
Island in both years, primarily in the more
inshore, shallower areas. These
seedlings grew vigorously and resulted in
numerous patches measuring up to one
m2. This pattern was also observed along
the York River shoreline from Aliens
Island to Sarah's Creek.
In tests of Bittium varium salinit\
tolerance, nearly 100 percent of aduh
snails, subjected to a rapid drop from 22.4
ppt to 10.8 ppt for 72 hours survived,
while less than one percent survived
when subjected to a drop from 22.4 ppt to
6.7 ppt for 72 hours. Greatest mortality
(97.6 percent) at the lower salinity was
seen within 24 hours. Snails subjected to
a gradual drop in salinity (from 21.9 ppt to
3.4 ppt over 456 hours) did not exhibit
significant mortality or change in activity.
Activity did decrease after the first 25
hours spent at 3.4 ppt.
No larvae subjected to rapid salinity
decrease from 22.3 to 11.1 metamor-
phased. However, larvae subjected to a
drop in salinity from 22.3 ppt to 16.3 ppl
after 24 hours, metamorphased, as did
larvae kept at 22.3 ppt. No larvae
subjected to a drop from 16.3 ppt to 11.1
ppt after 24 hours metamorphased
Larvae kept at 16.3 ppt and 22.3 ppl
survived for about eighteen days.
In the study of effects of interacting
factors on plant vigor, the three shadinc
conditions used represented a 43, 58, 69
percent decrease in available light. There
were no significant differences ir
numbers of shoots, leaf weight, or lea
area index among the three shadinc
conditions. However, within a shadinc
level, treatments with Bittium variurr
showed significantly higher values foi
each variable than did tanks containing
no snails.
With respect to periphyton an<
periphyton ash-free dry weights
treatments lacking Bittium varium re
suited in significantly higher weight thai
treatments in the presence of the snail.
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In the presence of high shade with
Bittium variutn, chlorophyll a levels were
significantly lower than with other
treatments, all of which were statistically
similar. Phaeophytin a level increases
with increased shading; the presence of
Bittium varium results in lower
phaeophytin a concentrations at each
shading level.
Recommendations
Since 1978, there have been some
overall decreases in the area vegetated
with SAV in the lower section of the Bay;
this may be simply a function of year-to-
year variations and does not mark a
significant continued decline of the
resource. Continued annual mapping will
permit better definition of this variation.
An annual monitoring program for SAV
using aerial photography is strongly
recommended.
R. J. Orth, K. A. Moore, andJ. van Montfrans are with Virginia Institute of Marine
Science, Gloucester Point, VA 23062.
William A. Cook is the EPA Project Officer (see below).
The complete report, entitled "Submerged Aquatic Vegetation: Distribution and
Abundance in the Lower Chesapeake Bay and the Interactive Effects of Light,
Epiphytes and Grazers," (Order No. PB 83-189 365; Cost: $14.50, subject to
change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 221'61
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Research Laboratory
U.S. Environmental Protection Agency
Gulf Breeze, FL 32561
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Environmental Protection
Agency
Center for Environmental Research
Information
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