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This study concurrently quantified blue crab feeding habits and preference, and examined the inter-relationships between diet, predator preference, and predator and prey abundance and distribution in three subestuaries of lower Chesapeake Bay--the James, York and Rappahannock Rivers, Virginia. Complementary laboratory investigations estimated the combined effect of the functional, aggregative and interference responses upon prey and predator survival and predator foraging rates for blue crabs and a common bivalve prey, Macoma balthica, in this system. Crab abundance, prey abundance and diet were correlated such that blue crabs aggregated in areas of highest preferred (i.e., bivalve) prey abundance, as determined through electivity analyses. Spatial and size-related differences in diet selection occurred. at least two trophic groups were distinguished, based on their relative consumption of bivalves and crabs, including conspecifics (i.e., older juveniles and adults) or polychaetes and small crustaceans (i.e., younger juveniles and new recruits). Spatial differences were reflected by proportional bivalve consumption: crabs always preferred bivalves, but in areas of relatively lower bivalve abundance, opportunistically expanded their diets to include other prey taxa. Cannibalism was common, but the frequency of occurrence varied with crab size, season, river, new juvenile recruit abundance, and the density of alternative preferred prey. Laboratory experiments assessed the joint effects of varying predator and prey densities upon predator foraging rates and prey survival. A full-factorial experimental design involved 2 prey and 3 predator densities with 6 trials per treatment combination. Blue crabs exhibited density-dependent foraging under all conditions: proportionally more clams were consumed at higher clam density. Furthermore, at the higher crab densities, mutual interference was evident in the incidence of wounds and deaths to crabs resulting from cannibalism or intraspecific aggression. The collective results indicate that both predator and prey densities must be examined experimentally for their joint impact upon predator-prey dynamics in marine systems.

No description available.

No description available.

To evaluate the effects of anthropogenic alteration of the Chesapeake Bay (CB) watershed since European settlement, the historical progression of eutrophication and anoxia in the mesohaline region of CB was reconstructed. Lipid biomarker and carbon and nitrogen stable isotopic and elemental composition of CB surficial sediments were examined seasonally in order to identify the present sources of organic matter to CB sediments and the processes controlling their distribution. Temporal variability in surficial sediment composition could be linked to seasonal changes in phytoplankton community composition and biomass while spatial variation was dominated by the delivery of alloclithonous versus autochthonous sources of organic matter. Three cores (3 to 4.5 m in length) collected from the mesohaline region of Chesapeake Bay were dated using a combination of tools including 210Pb and 137Cs radioisotopes, anthropogenic Pb and pollen indices. Enrichments in the carbon and nitrogen isotopic signature of sediments of all three cores deposited between 1790 and 1915 indicated enhanced primary productivity and nitrogen recycling, respectively. at the same time, increases in the flux of total organic carbon (TOC) and episodic enrichments (relative to TOC) of algal and bacterially-derived lipid biomarker compounds signaled a change in the sources of OM to the sediments. More extreme change occurred after the 1915's with further isotopic enrichments, a 1.5 to 2.5-fold increase in TOC deposition and 2 to 5-fold enrichments in algal and bacterially-derived lipid biomarker compounds. No change in the contribution of terrestrially-derived OM was indicated in any of the cores. Changes in sulfur speciation identified the initial occurrence of anoxia/hypoxia in 1790 at the deepest site (26 m) and in 1929 at a 15 m depth site. An examination of both qualitative evidence and quantitative models of degradation indicates that diagenesis cannot account for the observed increases in the total amount and labile quality of OM deposited during the 19th and 20th century in CB. Using diagenetic models, it is estimated that both algal and bacterial production has increased by 100 to 200% relative to pre-Colonial times with a temporal progression similar to the history of anthropogenic alteration of the watershed.

A panel of 48 single nucleotide polymorphism markers (SNPs) was developed for use in a population genetic analysis of the Eastern Oyster Crassostrea virginica sampled from the lower Chesapeake Bay. The SNPs were developed from published and unpublished sequencing data and developed to be used on a Fluidigm Biomark. A selection of 95 SNPs were chosen initially for development and the best 48 were selected for downstream applications. This project was a collaboration with the non-profit Chesapeake Bay Foundation (CBF) to examine their oyster reef restoration project in the Lafayette River, Virginia. The CBF wanted to test a hydrodynamic connectivity model designed to predict where oyster larvae produced in the Lafayette River would settle within the river. To test the model, oysters from Tangier Island, VA and the Haskin NEH hatchery strain were planted in the Lafayette River at locations corresponding to locations within the model with the expectation that the oysters would spawn the following summer. Baseline geographic oyster samples were taken from the nearby rivers; the Lafayette, Elizabeth and James Rivers before deployment of the planted test oysters. Newly recruited oyster spat were sampled from the Lafayette River in the summer following deployment of the planted test oysters. The baseline samples and spat were genotyped and compared to each other with the panel of 48 SNPs. Assignment tests were performed to identify the source population(s) for the spat. There was no population structure defined by FST values among oysters sampled from the lower Chesapeake Bay. The Haskin NEH oysters were genetically different from the other oysters in the study; however, the Tangier Island oysters were not different from the oysters in the lower Chesapeake Bay. The low FST values among the oysters from the lower Chesapeake Bay suggest that the connectivity of the reefs is high. The hydrodynamics of the region mix drifting larvae produced by oysters across the region as seen in the genotypic profile of the spat recovered in the Lafayette River. Heterozygote deficiencies suggestive of a Wahlund effect were observed; however, high rates of migration likely work to counterbalance stable population substructure. Alternatively, the heterozygote deficiencies could represent hidden variation not accessible by the methods used in this thesis. Some population structure exists with increasing geographic distance consistent with a pattern of isolation by distance among the populations sampled for this project. Assignment tests did not identify any spat as a product of the NEH oysters and assignment of spat to Tangier Island origin is inconclusive. The genetic data obtained were not able to provide unequivocal support for the predictions of oyster spat distributions by the connectivity model, although, the data do support the overall circulation patterns in the region predicted by the model.

Establishing the relationships between recruitment indices and estimates of adult abundance using fishery-independent data continues to remain one of the principal challenges faced by fisheries scientists due to the lack of concurrent monitoring programs designed to target different life stages of the same species. In Chesapeake Bay, however, multiple, fishery-independent surveys currently monitor the relative abundance of YOY and adult fishes. Using the available data from these surveys, the relationships between estimates of relative abundance for young-of-year and adults of striped bass (Morone saxatilis), weakfish (Cynoscion regalis), and Atlantic croaker (Micropogonias undulatus) were examined. Year-class strength was reflected in subsequent estimates of age-specific adult abundance; however, the strength of the relationships varied greatly with age. For all three species, the initial lack of significant correlations across all age classes indicated the need for improving the recruitment indices to more appropriately reflect YOY abundance. To ensure that the recruitment indices reflect patterns in abundance of YOY fishes, the following information was examined: assignment of the index period and strata and the distributional assumptions of the YOY catch data. For striped bass, a Bay-wide recruitment index appears to more accurately reflect year-class strength than the individual VA and MD recruitment indices. The recruitment indices for weakfish and Atlantic croaker improved when changes were made to the index period; however, further investigation is necessary to determine how depth influences the distribution and, ultimately, abundance of these two species. Identifying the distribution of the YOY catch data from the VIMS juvenile finfish surveys is also critical for obtaining unbiased recruitment indices. Here, the striped bass and weakfish catch data were gamma distributed; whereas, the Atlantic croaker catch data were lognormally distributed. The application of the delta-lognormal distribution did not improve the recruitment indices for any of the species in this study. An ageing study was conducted to determine if historically-defined length threshold values accurately distinguish YOY fish from older individuals in present day samples of striped bass, weakfish, and Atlantic croaker collected from the juvenile finfish surveys. The length threshold value for striped bass was determined to be approximately 30 mm too high. Although the current recruitment index for striped bass is not likely influenced by the small number of 1-year olds measured as YOY fish, reducing the length threshold value would ensure that only YOY fish are included in the calculation of the recruitment index. Further research is needed to determine if the length threshold values are appropriate for weakfish. For Atlantic croaker, length threshold values for the early portion of the index period (May, June) were appropriate; whereas, values used for the latter half of the index period (July, August) were too high, allowing for older individuals to be considered YOY based on their length. Consequently, the use of an earlier index period for Atlantic croaker would ensure that older fish are not being considered as YOY fish based upon their length.

Polar zooplankton and fish safeguard against the seasonality of food availability by using the summer months to build large reserves of lipids, which in turn are utilized to meet the metabolic demands of apex predators such as penguins, seals, and whales. A warming trend in the northern part of the western Antarctic Peninsula (WAP) has led to a decrease in perennial and summer sea ice, an increase in heat content over the shelf, and lower phytoplankton biomass, which could affect prey quality. We compared prey quality, including elemental (C, N) content and ratios, total, neutral, and polar lipid content, and energy densities, of known top-predator prey items (krill Euphaush1 superba, 17Jysanoessa macrura, and Euphausia crystallorophias; and fish Pleuragramma antarcticum, and Electrona antarctica) along the W AP latitudinal gradient in January of 2009-20 II as part of the Palmer Antarctica Long-Term Ecological Research study. E. antarctica had the highest prey quality in terms of total lipid content and energy density, followed by T. macrura and P. antarcticum, then E. c1ystallorophias and E. superba. For all species, variations in carbon and nitrogen content were best correlated with by the animals' neutral lipid content, in that animals with larger neutral lipid stores had significantly higher carbon and lower nitrogen content. Across all sexes and maturity stages, E. superba in the South had ca. 20% higher total lipid content than E. superba in the North. Total lipid content was also significantly higher in the South for E. crystallorophias, though this was largely due to the presence of larger individuals in the south combined with a significant positive relationship between length vs. weight-specific total lipid content for this species. For all prey species except T. macrura, there was a positive relationship between latitude or 0-120 m integrated Chi a vs. lipid content (neutral, polar, or total lipids), and a negative relationship between 0-120 m mean water temperature vs. lipid content. Trends opposite to those above found for T. macrura, suggest an optimal habitat for this species in the northern W AP which is characterized by warmer temperatures and lower Chi a. Patterns in Chi a were more important than upper water column temperature in explaining the observed latitudinal trends. If regional warming persists, the prey quality trends described for E. superba, combined with their regional abundance decline in the northern, coastal W AP could affect the ability of apex predators that rely on E. superba to meet their energetics demands.

An energetics model was constructed as an alternative method to length frequency analysis for the estimation of growth for juvenile silver perch, Bairdiella chrysoura. The model approach was adopted since estimation of juvenile estuarine fish growth by only collecting length or weight frequency data is difficult due to the large bodies of water and variety of microhabitats that must be regularly sampled over short time intervals. Field and laboratory data was included in analysis of model compartments for growth, metabolism, food intake and energy loss in waste products. Food habit studies indicated that silver perch is a planktivore that feeds on calanoid copepods during the day and mysids at night. Estimated energy utilization for silver perch at maintenance and maximum rations were similar to literature values for yellow perch, Perca flavescens, and brown trout, Salmo trutta. Model simulation of growth for silver perch in the York River, Virginia from July through October 1981, contained less variation in the size prediction than growth rates determined by length frequency analysis. The model did not contain periods of negative growth as did the field data. By the end of the season, model predictions of silver perch length matched the average length of silver perch captured in the lower York River. The construction of energy utilization models through complementary laboratory and field research has been demonstrated to be a viable method for estimating growth for juvenile fishes. Length frequency analysis is limited to only expressing growth over time. The advantages of an energetics model are that it also defines trophic and ecological interactions as well as environmental factors that impact growth.

Previous knowledge of the seed ecology of the clonal seagrass Zostera marina L. (eelgrass) suggests that sexual reproduction is not very important to the population dynamics of eelgrass; however, researchers have hypothesized long-distance dispersal for nearly a century. From a bay-wide sampling effort, viable eelgrass seeds in the seed bank were found throughout most of the lower and middle Chesapeake Bay, but abundance of seeds was highly variable. Lower seed-bank densities were found in middle Chesapeake Bay, the region with slow recovery of eelgrass populations. From natural and artificially created eelgrass populations, regional environmental conditions were found to have a greater impact on reproductive shoot (reproductive effort) and seed (reproductive output) production than small-scale influences of location and patch structure. Detached reproductive shoots of eelgrass (containing viable seeds) held in greenhouse tanks remained buoyant for several weeks before they degraded, sank, and lost all their seeds. In offshore shoal areas, suitable for eelgrass growth and survival, seventy percent of tube caps of the polychaete Diopatra cuprea (found throughout the shallow regions of Chesapeake Bay) had fragmented reproductive shoots built into its walls, suggesting a mechanism for seeding these shallow areas. Viable eelgrass seeds were found throughout the shoreline of south Chesapeake Bay, up to 34 km away from the nearest bed. Additionally, a GIS exercise identified new eelgrass patches up to 108 km from the nearest source population. The use of burlap bags for protecting seeds from predation, burial, or lateral transport maximized germination success over unprotected seeds in the field and provides a new mechanism for restoration efforts. An ecological model of eelgrass reproduction highlighted the potentially significant contribution of seeds to the long-term productivity of eelgrass at different water depths. Exploring theoretical scenarios, the model can be used to predict the total number of seeds produced for one to germinate and successfully establish as a seedling, as well as determine the size of patches, newly created from seeds, based on the number of viable seeds in the seed bank and the vigor of the seedlings that develop.