Virginia Coastal Program: 2003 Coastal Grant Project Description and Final Summary
Project Task:
FY2003 Task 12.05
Grantee:
Virginia Institute of Marine Science
Project Title:
Seaside Heritage Program: Trophic interactions of fish predators in restored seagrass beds of Virginia's seaside bays
Project Description as Proposed:
Seagrass habitats
have long been recognized as important nurseries for numerous estuarine
and marine species. For example, in the Chesapeake Bay region and
elsewhere, seagrass beds are considered important for blue crabs
(and various fish species). Crab densities often exceed that in
adjacent unvegetated habitat by a factor of 10 to 100 (Orth and
van Montfrans, 1987) and since individuals exhibit accelerated growth
in SAV (Perkins-Visser et al., 1996), these habitats are clearly
recognized as critical nurseries for juvenile blue crabs (Heck,
and Thoman, 1984; Chesapeake Blue Crab FMP, 1997; Beck et al. 2001).
Seagrass habitats have such universal intrinsic value that The National
Marine Fisheries Service recently identified grass beds as "Essential
Fish Habitat" (Magnuson-Stevens Act, 1996).
The system of barrier islands, seaside bays, and salt marshes along
the Atlantic coast of the Delmarva Peninsula of Virginia represents
some of the most natural, unspoiled coastal habitat along the U.S.
East Coast. Historically, finfish and shellfish resources in this
region supported large fisheries. However, during the 1930s, this
region underwent a dramatic ecological shift, and seafood harvests
declined dramatically. This shift occurred because seagrasses, primarily
eelgrass, Zostera marina, which was once very abundant in these
seaside bays, underwent a massive decline in the 1930’s attributed
to a wasting disease pathogen, Labyrinthula sp. (Rasmussen, 1977).
The decline was pandemic, affecting not only populations in the
coastal bays but also populations on both sides of the Atlantic.
In August 1933, one of the most destructive hurricanes to influence
the area in the twentieth century also contributed to the decimation
of seagrasses in the bays. Today, the Virginia seaside bays are
primarily salt marsh and macroalgal dominated.
A seagrass restoration program was initiated by the Coastal Program
on the seaside of Virginia’s Eastern Shore with efforts in
Magothy Bay in 1996, and South Bay in 1998, using test plots of
adult transplants. The success of the test plots and the discovery
of several natural patches in South Bay led to the conducting of
seed addition experiments there in 1999 and 2000. The success of
the seed experiments and the sustained growth of previous transplants
in South Bay led the Virginia Marine Resources Commission (VMRC)
to designate a 400 acre area of subtidal habitat in South Bay to
be set aside for seagrass restoration. In the fall of 2001, 3.8
million seeds were broadcast into 24-one acre parcels. Based on
the previous success in this area, the area covered in seagrass
was expected to increase an order of magnitude to 109,983 m2, approximately
11 hectares or 27.1 acres, by 2002.
The re-establishment of seagrass beds in Eastern Shore embayments
provides habitat that has been absent from these systems for decades.
Numerous invertebrates, fish and their predators, many of which
are commercially important, can once again begin to utilize these
structured habitats along Virginia’s coastline. The recent
establishment of these SAV habitats also allows a unique opportunity
to follow trophic interactions and quantify food web linkages as
these habitats develop and spread. This will allow us to document
our habitat restoration effort and better understand the complex
ecology of this system.
Several approaches exist to investigating changes in food-web dynamics
and trophic utilization of newly established seagrass beds. Changes
in ecosystem structure and function due to seagrass restoration
can be documented directly via the analyses of gut contents of fish
predators in newly established grassbeds. More integrative approaches
involve the use of natural isotopic tracers including carbon, nitrogen
and sulfur to identify food web linkages. Stable isotopes have proven
to be useful in studies identifying organic matter sources and food
web linkages (Lajtha and Michener, 1994). Since the pioneering works
of Parker (1964), Haines (1976), and others, an important tool for
identifying the sources and utilization pathways of organic matter,
has involved the use of stable isotopes. The isotopic composition
of light elements (C, N, or S) in biota have proved useful for identify
various sources and pathways of primary production. These approaches
will allow us to identify whether seagrass primary production constitutes
a primary component in the diets of residents of seagrass beds and
adjacent algal/mud dominated habitats.
Thus, the immediate goals of the present study will be to document
the impact of restored seagrasses in South Bay on the higher trophic
organisms through both direct and indirect techniques. VIMS will
sample resident predators throughout their residency in restored
beds, examine their feeding habits directly and quantify their integrated
response to habitat changes by analyzing isotopic tracers in the
tissues of predators, their prey and the primary producers upon
which they ultimately depend.
Our long-term aim is to revisit South Bay after seagrasses are successfully
restored so that we can document changes in energy flow in resident
plant/animal communities using similar approaches. These long-term
objectives will allow us to document changes in ecosystem-level
processes, water quality and food web structure as the South Bay
system progresses from one dominated by salt marsh and algae (both
mico- and macro) primary producers to one where seagrass becomes
a significant component.
The details of this research for examining trophic interactions
using direct and integrative techniques are as follows:
1. DIRECTLY QUANTIFY RESIDENT FISH FEEDING HABITS:
Resident piscine predators in South Bay will be collected seasonally
at regular intervals during spring, summer and fall (i.e., April
– November) using a 150-foot seine deployed within re-established
seagrass and adjacent mud/algal habitats using a standardized approach.
At least three and no more than 5 seine hauls will be made randomly
within each habitat type during the daytime. Subsets of fish from
each sample (up to approximately 10 – 15 specimens per species
or size-class within a species if necessary) will be processed for
length, weight, sex and stomach content analysis. Fish processing
will occur as soon after capture as possible. Stomachs will be labeled,
preserved in normalin and prey identified to the lowest possible
taxon in the laboratory. Prey will be measured, and % number, wet
weight and frequency of occurrence calculated by prey type. Thereby,
the utilization of new and established seagrass habitats by fish
can be compared to the use of adjacent mud/algal-dominated habitats.
2. QUANTIFY INTEGRATED FISH FEEDING HABITS VIA ISOTOPE ANALYSIS
This aspect of the research will be closely coordinated with sampling
efforts described above (see #1 above). We will use stable isotope
analysis of carbon, nitrogen, and sulfur (13C, 15N, and 34S) analysis
to assess the hypotheses: 1) that there is a significant influence
of seagrasses in the diet and trophic structure of the higher trophic
organisms in the restored seagrass areas; and 2) that there is a
significant difference in the isotopic signatures of the diet and
trophic structures of fish and other organisms captured from restored
areas and areas without restored seagrasses. Tissues of plants and
animals as well as seston associated with seagrass beds and adjacent
sites will be collected from each area. Smaller invertebrates will
be placed in seawater overnight to depurate and subsequently frozen.
The samples will be acidified, and rinsed in distilled water and
dried. Animals will be grouped for composite samples, and ground
to powder. Aliquots of the powdered sample (1 to 5 mg) will be weighed
into tin capsules. Tissues from higher trophic animals will be sampled,
extracted with dichlorobenzene to remove lipids, and dried. A portion
of the subsequently powdered sample will be weighed and combusted
in a tin capsule.
Seston will be collected onto pre-combusted GF/F filters using vacuum
filtration. Filters will be fumed in a desiccator with HCl to remove
carbonates. Following removal of carbonates, filters will be placed
in tin capsules for C, N and S isotope compositions. Sediments will
be acidified, and dried 40 °C and ground with a mortar and pestle.
Aliquots of the dried sediment (~10 mg) will be weighed into tin
capsules and analyzed.
Tissue samples will be analyzed for their stable isotopic compositions
(?13C, ?15N, and ?34S) using an elemental analyzer (EA) connected
to a Micromass Optima Isotope Ratio Mass Spectrometer (IRMS). Samples
of approximately 1 to 5 mg of dried tissue will be loaded into tin
capsules, and placed in the carousel on the EA (Micromass Optima
IRMS). Carbon and nitrogen isotopes will be determined with a single
combustion using a dual-furnace system composed of an oxidation
furnace at 1020ºC and a reduction furnace at 650ºC. Samples
for sulfur isotope analyses were separately pyrolyzed at 1050ºC
using a combination oxidation and reduction single-furnace system.
The goal of preparation is the conversion of the organic samples
into suitable gases that can then be analyzed by the mass spectrometer.
Carbon is converted to CO2, nitrogen to N2, and sulfur to SO2. The
resulting gases are purified using gas chromatography, chemically
dried, and directly injected into the source of the mass spectrometer
using continuous flow.
In concert, our research will illuminate the trophic importance
of seagrass beds in Virginia’s seaside lagoons during the initial
stages of seagrass establishment and beyond. The study will serve
as a model example of how habitat restoration and establishment
can influence (and likely enrich) food web dynamics in coastal embayments.
Federal Funding:
$40,000
Project Contact:
Jacques van Montfrans, (804) 684-7391 -vanm@vims.edu
Project Status:
Grant Closed
Final Product Received:
Project Summary Provided by Grantee:
Predatory fish in South Bay were collected monthly (June - September) using 150-foot seine nets deployed within restored seagrass beds and adjacent mud/algal habitats. Each combination of year and density for seagrass plots was sampled randomly three times throughout the four-month period, as were algal habitats. Fish abundance varied seasonally among the 33 species and 5400 individuals collected. Abundance of demersal fish was highest in July, moderate in August and lowest during June and September. Fish abundances were generally higher in SAV than in algal habitats. Feeding habits of fish were evaluated and have been completed for all major demersal species: silver perch (Bairdiella chrysoura), pig fish (Orthopristes chrysoptera), tautog (Tautoga onitus), and Northern pipe fish, (Sygnathus fuscus). These species varied in seasonal abundance with most occurring during all months sampled. Dietary analysis for silver perch revealed differences by habitat for prey consumed. Decapods (mainly shrimp) were important overall; more mysids were consumed in seagrass habitats whereas copepods constituted a greater portion of the diet in algal habitats. Dietary analysis for pigfish revealed differences by habitat for prey consumed, with worms and copepods being important in both habitats, and mysids and decapods (shrimp) occurring mostly in fish that inhabited restored seagrass beds. Tautog diets varied with habitat type. Overall dietary importance of amphipods was greatest for SAV-caught fish and decapods were most important in fish from algal habitats. Northern pipefish diets were dominated numerically by copepods. However, amphipod contributed most to their diet in SAV whereas copepods remained most important in algal habitats.
Integrated trophic relationships were examined by stable isotope analyses. Dominant invertebrates (crabs, shrimp, amphipods and isopods), plant material (SAV and algae) and overlying water were also sampled for stable isotopes of carbon, nitrogen, and sulfur ( 13C, 15N, and 34S) to estimate the influence of primary producers in the diet of resident species (UVA). Approximately 1000 samples of primary producers through higher trophic level fish from South Bay, Virginia (summer of 2004) were analyzed for 13C, 15N, and 34S. Primary producers Zostera marina, epiphytic algae and macroalgae were isotopically distinct. Other samples of lower trophic level animals collected in the summer of 2002 were also analyzed for sulfur. Preliminary interpretation of the bulk isotope analyses indicates that seagrasses from restored beds in South Bay are presently not contributing a significant fraction of fish diet. This may be due to biomass turnover lag in the influence of the seagrass nutrition to the diet of higher trophic level predators. Bulk isotopic values indicate that the isopod Erichsonella sp., amphipods and mud crabs from the seagrass meadows are dietary sources for many of the fish. Overall, direct quantification of fish diets varied by species and by habitat with some evidence of potential seagrass-related influences. Isotopic signatures provide an important basis for tracking changes through time as seagrass habitats expand in Eastern Shore embayments. As these changes occur, it is likely that seagrass primary production will become an important source of nutrition for a variety of species, including those that are commercially or recreationally harvested such as blue crabs and various fish species.
Form C end
Disclaimer: This project summary provides the federal dollars initially awarded to the grantee. Due to underexpenditure or reprogramming of grant funds, this figure may change. For more information on the allocation of coastal grant funds, please contact Laura McKay, Virginia Coastal Program Manager, at 804.698.4323 or email: Laura.McKay@deq.virginia.gov
A more detailed Scope of Work for this project is available. Please direct your request for a copy to Virginia.Witmer@deq.virginia.gov
