A total of 12,323 summer flounder were tagged and released in Chesapeake Bay, the coastal waters off Virginia Beach, and the Eastern Shore during 1987-89. Excluding immediate recaptures, a total of 675 were recaptured for an overall return rate of 5.5%. Returns from the commercial fishery (56.2%) outnumbered those from the recreational fishery (35.7%). The majority of returns (47.1%) came from either Virginia waters or areas to the south, while only 12.8% were from areas to the north of Virginia. offshore returns accounted for 8.5% of the total. Returns with insufficient location data made up the remaining 31.5%. of the returns with sufficient recapture location data, 69% were from Virginia waters or areas to the south. Differences in length at tagging were noted between these groups with smaller fish accounting for a larger proportion of the returns from northern waters. The returns from Virginia waters and areas to the south were more representative of the entire size range of fish tagged. No behavioral differences were noted between tagged and untagged summer flounder held in the laboratory. No tag losses were noted in fish held over the course of one year. Scales were used to age summer flounder and were found to be adequate for ages 0-3, older fish were more difficult to age with scales. Percent agreement between scale and otolith determined ages (n = 170) was 100% for ages 0-5. Length frequencies of summer flounder were used to validate scale determined ages during 1987-89. Prominent modes in length frequencies represented ages 0-2, afterwards, differential growth between the sexes obscured the modes. Marginal increment analysis proved that the marks formed on scales were annular for ages 1-4 and that annulus formation occurred in May-June, at sizes ranging from 179-367 mm TL. Overall mean backcalculated length and the mean observed length in May for age-1 fish were 262 vs 265.4 mm. Mean backcalculated lengths for males were 249, 337, 393, and 455 mm TL for ages 1-4. Mean backcalculated lengths for females were 273, 379, 470, and 550 mm TL for ages 1-4. Backcalculated lengths at ages 1-7 for the pooled sexes were 262, 377, 473, 546, 600, 655, and 696 mm TL. Length-weight relations were calculated by sex for 2,172 fish. Overall sex ratio was 1:1.32, males to females. The sex ratio was approximately 1:1 for length groups up to 360 mm TL. Females outnumbered males at sizes greater than 360 mm TL. Male summer flounder reached 50% maturity at 261-270 mm TL, while females attained 50% maturity at 361-370 mm TL. Overall catch per unit effort decreased from 1.65 fish caught per minute to 0.4 from 1987 to 1989. Instantaneous rates of mortality for summer flounder caught in Chesapeake Bay and coastal waters ranged from 0.964 in 1988, to 1.655 in 1987. Instantaneous rates of mortality for summer flounder caught at Wachapreague ranged from 0.844 in 1989, to 3.608 in 1987.

Natural ecosystems are strongly affected by changes in resource supply (bottom-up forces) and by changes in upper trophic levels (top-down forces). The extent to which these processes impact a system depends largely on the responses of organisms at middle trophic levels. In seagrass beds, a group of mid-level consumers known as mesograzers form a critical link in the chain of impact, connecting seagrass and epiphytic algae with predatory fishes and crustaceans. I observed dramatic seasonal and interannual changes in mesograzer abundance and species composition in eelgrass (Zostera marina) beds of lower Chesapeake Bay, Virginia, and endeavored to explain the top-down and bottom-up causes and consequences of those changes with field studies and controlled experiments. A field cage experiment showed that grazing, predation and nutrient enrichment all had strong effects on the eelgrass community, but that the effects of each factor varied for different community components (Chapter 1). A second experiment delved deeper into the predation dynamic by manipulating the diversity of both predators and mesograzers in macroalgal mesocosms. Increasing predator diversity increased the strength of predation, but increasing mesograzer diversity conferred resistance to some types of predation (Chapter 2). to assess the influence of top-down and bottom-up forces in a more natural context, I analyzed the long-term changes in biotic and abiotic components of an eelgrass bed at the Goodwin Islands National Estuarine Research Reserve. I found that abiotic processes had strong effects on both consumer and resource abundance, and could therefore initiate either top-down or bottom-up control of eelgrass community structure (Chapter 3). to examine this top-down and bottom-up control in more detail I explicitly compared the ecological relationships seen in the field to those observed in mesocosm experiments. Mesocosm experiments tended to find a greater influence of top-down effects and a lesser influence of bottom-up effects, relative to field observations (Chapter 4). Finally, I took a snapshot of the eelgrass food web itself by examining the gut contents and stable carbon and nitrogen isotopic ratios of predators, mesograzers, and plants. I found that direct grazing on eelgrass does occur, but that microalgae and detritus provide the main trophic support for the epifaunal community (Chapter 5). Overall, my results suggest that both top-down and bottom-up forces control eelgrass community structure via mesograzers, but that top-down control in the field is more subtle and more intimately tied with bottom-up control than has been indicated by some manipulative experiments.

Estuaries are dynamic physical environments. The stability of the sediment-water interface is influenced by sources and rates of sediment delivery and physical reworking of sediments by currents, tides, waves and biology, but effects of disruption of this interface on benthic biology are poorly resolved. For this study, I investigated effects of prevalent gradients in seabed disturbance processes and associated seabed characteristics on estuarine benthic community structure and function in the mesohaline York River, a tributary of Chesapeake Bay, USA. I used a variety of approaches to characterize the seabed, including sediment grain size, sediment water content, maximum depth of 7Be, depth of the oxidized sediment layer, profiles of sediment Eh, physical structure of the sediment, sediment chlorophyll a, and sediment organic content. Differences in magnitude of deposition and subsequent reworking of sediments by physical processes were documented among the five benthic subenvironments sampled (south shoal, secondary channel, main channel flank, main channel, and north shoal). Temporal and spatial variations in spring recruitment were observed among subenvironments sampled weekly for recruits: the south shoal, secondary channel and main channel flank. Total recruitment was greatest in the main channel flank, which experienced the highest sediment deposition, and was limited in the secondary channel, which had the strongest tidal currents. The five benthic subenvironments sampled for patterns of community structure and estimates of secondary production were dominated by estuarine opportunist species. Total abundance was greatest in the north shoal, which experienced minimal deposition and physical reworking of sediment. Biomass and secondary production estimates were driven by presence of deep-dwelling bivalves, and were greatest in subenvironments that experienced deposition. These results suggest that variations in seabed characteristics across relatively small spatial scales can influence estuarine benthic community structure and function. Laboratory experiments were conducted to further elucidate the effect of sediment deposition on estuarine organism survival. Species representing both infaunal and epifaunal taxa ranged from highly susceptible to highly tolerant of burial by sediment. Survival was a function of organism motility, residence depth and perhaps physiological adaptations. Small, shallow-dwelling juveniles of some common estuarine species were highly tolerant of burial.

Section 1. Ecotoxicological effects of creosote contamination on benthic bacterial communities in the Elizabeth River, Virginia were investigated using both structural and functional microbial parameters. Results indicated that cell specific and total heterotrophic bacterial production parameters were depressed in a dose dependent manner with increasing sediment PAH concentrations. Toxicity effects upon production were modified by temporal trends associated with temperature as well as spatial sediment characteristics. of the parameters employed, the tritiated thymidine production assay was found to be the most sensitive for detection of ecotoxicological effects. Section 2. Bacterial abundance and production were examined during a destratification event in the lower James River, Virginia. Bacterial abundance, although significantly different between stations, did not change over the study. Bacterial production (&\sp3&H-Tdr incorporation) in surface waters was significantly less during the mixed period (187 &\mu&g C&\cdot&1&\sp{lcub}-1{rcub}\cdot& d&\sp{lcub}-1{rcub}&) compared to the most stratified state (324 &\mu&g C&\cdot&1&\sp{lcub}-1{rcub}\cdot& d&\sp{lcub}-1{rcub}&). Correlations between bacteria and chlorophyll were diminished during the mixed period. Total and flagellate specific grazing rates upon bacteria were reduced during the onset of destratification. Relationships between bacterial and nutrient parameters also indicated a strong influence of destratification. These results indicate that destratification changes trophic interactions within the microbial loop, which are not necessarily reflected in temporal patterns of bacterial abundance. Section 3. Bacterioplankton production, and ammonium assimilation and remineralization were examined between April and August 1988 in the lower York River, Va. Size fractionation enabled estimates of bacterial contribution (&&15 &\mu&m) towards ammonium cycling processes. Bacterial ammonium assimilation accounted for 19-95% of total dark ammonium assimilation, with station means of 46-48%. Station means of ammonium remineralization in the &<&1.6 &\mu&m treatment was 92, 48, and 38% of unfractionated values from lower to the upper river stations respectively. Regression statistics indicated that assimilation was best predicted by bacterial production. Remineralization was less well predicted by all variables. These results indicate the importance of bacteria in ammonium cycling can be greatly disproportionate to their biomass and production.

The structure of surf-zone fish communities at Cassino (western South Atlantic, WSA) and the Chesapeake Bight (western North Atlantic, WNA) is described using historical beach seine data, to examine similarities between geographically isolated fish communities. Numerical classification, ANCOVA, Shanon-Wiener diversity and its components, and species rarefaction are used to describe fish community structure: (a) within habitat (Cassino); (b) within region (Chesapeake Bight); (c) within the western Atlantic (comparison between Cassino and the Chesapeake Bight). The surf-zone fish community at Cassino had a low diversity and was dominated by a few species. Seasonal periods identified by cluster analysis correlated well with seasonal environmental changes. The seasonal occurrence, abundance and diversity of fish species reflected recruitment patterns of juveniles, and seasonal variation in the marine and estuarine circulation patterns. Fish species associations usually fitted into three broad categories: year-round surf-zone residents; (b) spring to fall, estuarine related, pelagic planktivores or shallow water omnivores; (c) summer to fall coastal marine occasionals. Faunal similarity between Cassino and other selected locations of the WSA, decreased towards lower latitudes due to faunal replacements. In the Chesapeake Bight community structure was determined by differences between the two sampled localities. The community at Sandbridge was characterized by estuarine related species associated with the Chesapeake Bay. Sub-tropical coastal marine species at Cape Hatteras indicated influence of Gulf Stream waters. The seasonal species occurrence, abundance and diversity was determined by an enhanced seasonal migration along the coast, juvenile recruitment, and the thermal regime of the Chesapeake Bight. Ubiquitous surf-zone residents were replaced by a wintering group in the colder months. The diversity of estuarine related species at Sandbridge indicated that locally the surf-zone acted as a peripheral habitat for those species. Faunal similarities between the Chesapeake Bight and other WNA localities decreased both northward and southward due to faunal replacements. Similarities between surf-zone fish communities within the western Atlantic (Cassino x Chesapeake Bight) were correlated with the temperature range and habitat structure.

Dissolved nutrient concentrations and invertebrate grazing activity regulate epiphytic biomass. Because epiphyton may limit light and carbon at leaf surfaces and the consequent productivity of submerged macrophytes, factors which influence epiphytic biomass may indirectly affect macrophyte abundance. I measured the simultaneous effects of water column nutrients (ambient or 3x ambient concentrations of nitrogen and phosphorus) and grazing (presence or absence of epifaunal community) on epiphyton and macrophytes seasonally in eelgrass (Zostera marina L.) microcosms on lower Chesapeake Bay. Grazing was more important than nutrients in controlling accrual of total epiphytic biomass, although effects on epiphytic components varied; numbers of diatoms responded to grazing, whereas numbers of cyanobacteria responded to nutrients. Numbers of heterotrophic microflagellates mimicked those of bacteria. The indirect effects of nutrients and grazing on macrophytes depended upon the relative magnitude of each factor and the physiological demands of the macrophyte. Under low grazer densities of early summer, macrophyte production (g m&\sp{lcub}-2{rcub}& d&\sp{lcub}-1{rcub}&) was reduced with grazer removal and nutrient enrichment independently. In contrast, under high densities of late summer, production was reduced by enrichment with grazers absent only. There were no macrophyte responses to treatment during the spring and fall, regardless of differences in epiphytic biomass; this may have been related to comparatively low light requirements of eelgrass at low temperatures. I used a simulation model to extrapolate microcosm results to predictions for community persistence. The model included ranges of environmental variables specific to lower Chesapeake Bay, where declines in eelgrass abundance in recent decades were correlated with nutrient enrichment, reduced grazer populations, and increased turbidity. Simulations indicated that neither nutrient enrichment nor loss of grazers alone would limit eelgrass survival, but together would cause community instability. Simulations indicated further that with grazers present, nutrient enrichment with a slight decrease in submarine irradiance would cause macrophyte loss. Measured rates of epiphytic accrual on artificial substrata in situ suggested that with grazers present, light reduction actually reduced the absolute rates of biomass accumulation despite nutrient enrichment. Predictions for macrophyte community stability must thus consider the relative effects of both direct (acting on macrophytes) and indirect (acting via epiphyton) environmental controls.

A study of channel utilization and behavior of subadult loggerhead turtles was conducted in the St. Mary's River entrance channel area, Georgia. A total of 38 loggerheads were captured by trawling and instrumented with radio and depth sensitive sonic tags from April to November 1993. Turtles were monitored for 6 to 36 hours continuously for approximately 30 days in the spring (13), summer (13), and fall (14). On a daily basis turtles generally confined their diving activity to one location for one to twelve hours in 10 to 20 m water depths then moved 1 to 5 kilometers to a new location. Overall the turtles spent only a small percent of time in/near the channel. Duration of diving variables varied widely among and between individuals. The loggerhead turtles spent a small percent of time and short durations at the sea surface and large percent of time and long durations associated with the sea floor. A turtle would typically surface 1-4 times per hour. In general, bottom durations, submergence durations, and dive cycle durations were longer at night than during the day, with twilight durations intermediate between the two. Turtles surfaced more frequently during the day than at night. No day/night pattern was observed for surface duration, descent duration, ascent duration, ascent rate, and descent rate. Ascent duration was longer and ascent rate slower than descent duration and descent rate, respectively. Mean surface durations were greater in the spring than summer and fall. Surface/submergence frequency was less in the fall than in spring and summer. Bottom durations, submergence durations, and dive cycle durations were longer in the fall than in the spring or summer. Surface durations, descent durations, ascent durations, ascent rates, and descent rates did not differ seasonally. In the spring, summer, and fall loggerhead turtles spent most of their time on the sea floor outside the channel, dove more actively during the day than at night, and remained in one location for up to 12 hours at 10-20 m water depths.

Snapping shrimp in the family Alpheidae are remarkably abundant in coral reef ecosystems worldwide. The second most speciose genus, Synalpheus , includes species usually small and reclusive, and their general morphology is notably uniform, yet the number of species is staggeringly high, as is the intraspecific variability. These features have contributed to a poorly resolved taxonomic status, as evidenced both by recurring lumping and splitting of species and by lack of proper definition of several common species. I examined western Atlantic species of Synalpheus in the large and regionally dominant "Gambarelloides group of species", an informal but widely used guild within the genus. Analysis of external anatomical features of preserved and live specimens enabled a reassessment of morphological characters traditionally used in taxonomy, and incorporation of novel characters. Identity and validity of every species described from the western Atlantic has been reevaluated. Detailed ecological records and observations on live specimens under laboratory conditions yielded ancillary data useful in distinguishing the species. Seven new species are preliminarily described. Recognition of the Gambarelloides group as a distinct subgenus within Synalpheus is recommended. A systematic account of 17 species with selected illustrations and color photographs is included, together with a dichotomous identification key. The evolution of morphological characters was traced onto the current best reconstruction of the phylogenetic relationships among the species of the Gambarelloides group. The phylogenetic tree was obtained by analyzing molecular data from segments of two mitochondrial genes (COI and 16S) from 39 species of Synalpheus, with Alpheus cylindricus as the outgroup. Most of the morphological characters studied bore weak phylogenetic signal and homoplasies were frequent. Nevertheless, two characters provided consistent synapomorphies defining the Gambarelloides group: the dorsal brush of setae (used in feeding) on the dactyl of the minor first chela, and the coxal lamella on the third pereiopods. Evolution studies of Synalpheus are particularly interesting because of the high number of closely related species, similar general ecological preferences with microhabitat specificity, and variation in social organization. This reassessment of western Atlantic species provides a solid taxonomy and an important frame of reference for future studies of Synalpheus.

Laboratory experiments determined the effects of two levels of habitat complexity upon pipefish (Syngnathus fuscus) foraging for amphipods. Habitats were composed of equal densities of either narrow (low complexity) or wide (high complexity) leafed artificial seagrass. The response to habitat, as measured by rate of encounter with amphipods, probability of attack after encounter, probability of success after attack, and overall rate of amphipod consumption, was determined for combinations of two fish size classes and three amphipod size classes. Small fish did not experience visually inhibitive effects in either habitat, while large fish had their visual fields impinged upon in the wide leaf habitat and encountered fewer amphipods. There was a general trend for encounter rate to increase with amphipod size. Large fish attack probability was positively related to amphipod size in the narrow leaf habitat, but negatively related to amphipod size in the wide leaf habitat. Small fish attack probability was negatively related to amphipod size in both habitats. Success was negatively related to a ratio of prey size to fish size, and showed no overall effect of habitat. Pipefish have flexible behaviors, allowing them to minimize unsuccessful attacks. Due to their position in the structure of vegetation, amphipods have a distribution of vulnerabilities; a criterion by which pipefish select prey. Size-selective predation on gammarid amphipods by pipefish was examined utilizing simulation modeling and laboratory experimentation. Three computer simulation models were developed: (1) a mechanistic model based on empirically derived size-dependent mechanisms of pipefish-amphipod interaction, (2) an optimal diet breadth model in which the rate of energy intake is maximized, and (3) an optimal diet breadth model where switching from energy maximization to time minimization occurs as consumption becomes limited by gastric processing (i.e. satiation). None of these models successfully accounted for the observed pattern of prey size selection. Pipefish concentrated their feeding upon smaller, energetically more profitable amphipods, in excess of what was predicted by either the mechanistic or optimal diet breadth models. This pattern of selection was evident through out 4 hour feeding bouts, indicating that diet breadth compression did not occur.

Analyses of EPA long-term datasets (1985--1994) combined with field studies and ecosystem model development were used to investigate phytoplankton and nutrient dynamics in the York River estuary. Analysis of the EPA dataset showed that algal blooms occurred during winter-spring followed by smaller summer blooms. Peak phytoplankton biomass during the winter-spring blooms occurred in the mid reach of the mesohaline zone whereas during the summer bloom it occurred in the tidal fresh-mesolialine transition zone. River discharge appears to be the major factor controlling the location and timing of the winter-spring blooms and the relative degree of potential nitrogen (N) and phosphorus (P) limitation. Phytoplankton biomass in tidal fresh water regions was limited by high flushing rates. Water residence time was less than cell doubling rate during seasons of high river flow. Positive correlations between PAR at 1m depth and chlorophyll a suggested light limitation of phytoplankton in the tidal fresh-mesohaline transition zone. A significant relationship between the delta of salinity between surface and bottom water and chlorophyll a distribution suggested the importance of tidal mixing for phytoplankton dynamics in the mesohaline zone. Accumulation of phytoplankton biomass in the mesohaline zone was generally controlled by N with the nutrient supply provided by benthic or bottom water remineralization. In general, phytoplankton dynamics appear controlled to a large extent by resource limitation (bottom-up control) rather than zooplankton grazing (top-down control). The dynamics of phytoplankton size structure were investigated in the freshwater, transitional and estuarine reaches of the York River over an annual cycle. The contribution of large cells (micro-plankton, >20 mum) to total biomass increased downstream during winter whereas that of small cells (nano-, 3--20 mum) pico-plankton, <3 mum) increased downstream during summer. I conclude from these studies that spatial and seasonal variations in size structure of phytoplankton observed on the estuarine scale are determined both by the different preferences of micro-, nano-, and picoplankton for nutrients and by their different light requirements. Analyses of phytoplankton size structure are, thus, necessary to better understand phytoplankton dynamics and to better manage water quality in estuarine systems. An ecosystem model was developed to integrate these data and to investigate mechanisms controlling the size-structured phytoplankton dynamics in the mesohaline zone of the York River estuary. The model developed in Fortran90 included 12 state variables describing the distribution of carbon and nutrients (nitrogen, phosphorus) in the surface mixed layer. Forcing functions included incident radiation, temperature, wind stress, mean flow and tide including advective transport and turbulent mixing. Model results supported the general view that phytoplankton dynamics are controlled by abiotic mechanisms (i.e. bottom-up control) rather than biotic, trophic interactions in the York River estuary. Model sensitivity tests showed that small cells (pico-, nano-sized) are more likely regulated by temperature and light whereas large cells (micro-sized) are regulated by physical processes such as advection, and tidal mixing. Microphytoplankton blooms during winter- pring resulted from a combination of longitudinal advection and vertical diffusion of phytoplankton cells rather than in-situ production.