Benthic communities on the outer continental shelf of the Middle Atlantic Bight are characterized by abundant populations of amphipods in the families Ampeliscidae and Corophiidae. Members of these families occupy tubes at or near the sediment surface and, thus, potentially compete with each other for spatial and trophic resources. This study examines the spatial, temporal and trophic resource use of six numerically dominant species in these families as a means of assessing the relative importance of competition as a structuring force in outer shelf benthic communities. Analysis of abundance data and the use of discriminant analysis to relate abundance to measured environmental parameters indicated that habitat partitioning was of major importance within the ampeliscid group. Spatial partitioning appeared to be of little importance within the corophiid group. Between group patterns suggest that only one species, the corophiid Unciola irrorata, was able to coexist in deep swales with the ampeliscid Ampelisca agassizi. Seasonal abundance patterns were exhibited by all corophiids, but the high degree of temporal overlap in abundance suggests that this was not an important partitioning mechanism. Similarly, persistent abundances over· time (!. agassizi) or variation in a non-seasonal manner suggests that temporal partitioning of resources was not important within the ampeliscid group. Differences in body size which might facilitate differential spatial or trophic resource use among species were observed. These differences were most strongly pronounced in closely related species which frequently co-occured. The use of trophic resources was indirectly assessed by comparing mouthpart morphologies and known feeding behaviors of each species. Quantitative morphological differences among species were evident which relate well to observed habitat preferences. A complex set of physical and biological factors were found to govern the distribution and abundance of these species within the outer shelf zone. In outer shelf swale habitats the ampeliscid, A. agassizi, effectively excludes the other members of this family. This may be the result of the species superior abilities to utilize spatial or trophic resources. Outside . of swale habitats the abundances of Ampelisca vadorum and Byblis serrata may be limited by the availability of trophic resources. The corophiids show little evidence of resource partitioning, although some differences in microhabitat distribution may facilitate coexistence among ·the species in this family, as well as between families. Populations · of these species may be held below the levels at which competitive interactions become important by benthic predators. The corophiids are known to comprise a major portion of the diet of benthic fishes on the outer shelf.

The purpose of this study is to examine the feeding ecology and trophic role/importance of the northern pipefish, Syngnathus fuscus, in a lower Chesapeake Bay seagrass community. The study incorporates; 1) examination of stomach contents in conjunction with prey abundance data, inorder to arrive at conclusions concerning the food preferences of S· fuscus, 2) determination of daily feeding periodicities and stomach evacuation parameters, thus allowing for the determination of a daily ration for S· fuscus, 3) examination of size relationships between S. fuscus and it's major prey species, 4) estimation of pipefish densities at the study site, and 5) examination of the trophic importance of S. fuscus via estimation of the annual quantities of specific prey species consumed at the study site, and comparison of these values with estimated production values for the prey populations. It is suggested, that while S. fuscus consumes only moderate portions of the annual production of it's prey species, it may serve to modulate the production of these prey species by feeding predominantly upon small individuals, thus effectively altering the age-class structure of the prey population, and assumably the production charactoristics as well.

Hypoxia and anoxia have significant deleterious ecological effects on living resources throughout many estuarine and marine ecosystems worldwide. Brief periods of low oxygen facilitate transfer of benthic production to higher trophic levels as many benthic infaunal species have shallower sediment depth distributions during hypoxic events. A baited time-lapse camera equipped with a water quality datalogger was used to document in situ exploitation of oxygen-stressed benthic invertebrate prey organisms by mobile fish and crustacean predators during alternating normoxia-hypoxia cycles in the York River. Based on photographic and diver observations, this hypoxiainduced benthic-pelagic transfer of production is more likely to occur when environmental dissolved oxygen concentrations rise above an apparent threshold between 1 and 2 ml/1. When oxygen concentrations decline below 2 ml/1, the functional response of the predator to increased prey availability is interrupted. There is no energy gain by the predator until oxygen concentrations rise above this critical level when predators return to affected areas and resume feeding activity.

Long chain n-3 polyunsaturated fatty acids (LCn-3 PUFAs) such as EPA and DHA are important biomolecules regulating production in marine ecosystems. This study examined how the interaction at the phytoplankton-zooplankton interface affected the transfer of LCn-3 PUFAs to higher trophic levels. Heterotrophic dinoflagellates contained higher levels of EPA and DHA than their algal prey, suggesting heterotrophic dinoflagellates enhanced the nutritional value of poor quality algae and subsequent transfer to the next trophic level. Formation of EPA and DHA in the dinoflagellates appears to be achieved by the elongation and desaturation of shorter fatty acid chains rather than through de novo synthesis. Fatty acid content in the copepod Acartia tonsa resembled the fatty acid signature of its prey, further supporting the idea that heterotrophs depend on their diet to obtain these nutrients, and their nutritional value is subject to the type of food consumed. Transfer of DHA to A. tonsa, was improved by feeding on a heterotrophic dinoflagellate grown on a poor quality algae, versus feeding on the algae itself. Thus omnivorous copepods may compensate dietary deficiencies by feeding on a variety of prey items. The presence of EPA and DHA can be used as a proxy for the nutritional value of copepods. A. tonsa fed nutritiously poor algae also affected the fatty acid content of its predator. Pseudopleuronectes americanus fed low quality copepods, had lower levels of EPA and DHA than those fed copepods with higher levels of these fatty acids. However, content of these fatty acids did not have a direct effect on the growth rate of the fish. The finding herein does not support consumption of LCn-3 PUFAs as important factors regulating growth in juvenile fish. These results, albeit discouraging, are by no means comprehensive in elucidating the role of n-3 PUFAs for fish health. It is possible that due to food limitation, the effect of food quality was confounded. Field collected data in a nursery ground for juvenile P. americanus showed that the quantity of EPA and DHA in the prey for the fish at the time of sampling was low. The low availability of these fatty acids in the plankton suggests this estuary is at times suboptimal for the growth and development of P. americanus. EPA and DHA are critical for P. americanus growth; however, the low availability of LCn-3 PUFAs does not by itself explain differences in growth rates. It is clear that further field studies should combine physical, biological and chemical factors in order to evaluate the nutritional status of the nursery ground.

The behaviors of oyster larvae are difficult to monitor or experimentally manipulate, especially in field conditions. As a result, little is known of the fate of oysters in the larval portion of their life cycle, prior to recruitment. At the transition from pelagic larvae to benthic adults, larvae are likely to come into contact with many invertebrates resident on oyster reefs. Of these, fouling epifauna are generally believed to reduce the settlement of interspecific larvae through competitive exclusion and predation. Studies of these interactions, however, often utilize artificial settlement panels, which can exhibit different recruitment patterns to those observed on natural substrates. I therefore investigated the interactions between reef-associated fauna and settling oyster larvae on natural shell substrates. Over a series of laboratory microcosm studies, native (Crassostrea virginica) and non-native (Crassostrea ariakensis) larvae were exposed to reef-collected shells, each supporting a single species of reef-associated fauna. The presence of adult bryozoans (Membranipora tenuis) had little effect on either larval settlement rate or mortality. The boring sponge (Cliona sp.) significantly decreased oyster larval settlement, and generally increased oyster mortality. Barnacles (Balanus improvisus) typically facilitated settlement. Barnacle molds and empty barnacle tests, intended to mimic the surface area and rugosity of live barnacles, did not significantly affect settlement. However, in some trials, adult barnacle bathwater enhanced settlement of both oyster species, implicating the role of waterborne cues. Such bathwaters were found to cause oyster larval mortality, as were bathwaters created by adult clamworms or even adult oysters. Predation by clamworms (Neanthes succinea), which were found at very high densities on field-collected oyster shells, caused significant oyster larval mortality in these experiments. The combined roles of both positive and negative interactions between oyster larvae and reef fauna require enumeration under field conditions. The results from this study highlight the need for clarification of these roles in order to optimize shell supplementation restoration efforts, and to more thoroughly understand the settlement behaviors and mortality sources of recruiting oyster larvae

The phytoplankton of the Ross Sea have been intensively studied during the last decade, as opposed to the Amundsen Sea, where virtually nothing is known about phytoplankton taxonomy and distribution. Blooms in the Ross Sea are usually composed of diatoms and the prymnesiophyte Phaeocystis antarctica; diatoms are often dominant in strongly stratified waters during the summer, whereas P. antarctica usually dominates in less stratified waters in the south-central polynya during spring. This study focused on understanding the environmental variables that influence the spatial patterns of phytoplankton assemblages during late summer and early fall, 2007, and late spring and early summer, 2008 in the Amundsen and Ross Seas. Large differences in the distribution of phytoplankton were noted in space and time. In general, P. antarctica dominated the blooms in southwestern part of the Ross Sea. The silicoflagellate Dictyocha speculum was present in relatively high abundance and co-dominated with P. antarctica and diatoms in the eastern part of the Ross Sea during February. In the Amundsen Sea P. antarctica co-dominated with diatoms, and diatom blooms were more common than in the Ross Sea. Shallow mixed layer depths supported the growth of diatoms in the Ross and Amundsen Seas, but it was not the only factor required for diatom bloom development. Blooms dominated or codominated by diatoms were also more frequent in relatively fresher waters of the Amundsen Sea than in the Ross Sea in late summer and early fall. Modified Circumpolar Deep Water (MCDW), a water mass that can potentially be a source of Fe to phytoplankton, intruded the upper waters (from 80-120 m) near the Ross Ice Shelf. I hypothesized that this water mass intrusion may have favored P. antarctica blooms, releasing them from Fe limitation because they occurred in waters where the MCDW was shallow. However, pigment and quantum yield data show that P. antarctica blooms were approaching Fe stress in waters where the MCDW was observed, suggesting that intrusions of MCDW strengthen stratification and restrict Fe inputs into the surface. Because the Ross and the Amundsen Sea have a wide range of environmental and climatic conditions, understanding the factors that influence phytoplankton distribution in these areas will provide information of how phytoplankton respond to a changing climate.

The land-­‐ocean margin is characterized by a shallow water column, which allows light to reach the benthos and supports a diverse community of benthic autotrophs. One group of benthic autotrophs, consisting of benthic diatoms, cyanobacteria and other photosynthetic microorganisms living near the sediment surface (i.e., benthic microalgae) accounts for a substantial amount of this primary production. Benthic microalgae are also tightly coupled to carbon and nutrient cycling processes carried out by the sediment bacterial community. Benthic microalgae exude complex biopolymers, called extracellular polymeric substances (EPS), which consist mainly of carbohydrates, but can contain proteins and nucleic acids. EPS serves multiple functions for the benthic microalgae including: sediment attachment and stabilization and provides a labile substrate that may facilitate the tight coupling between benthic microalgae and sediment bacteria. A novel experimental apparatus, called the perfusionator, was used to examine carbon and nitrogen cycling through the benthic microalgal and sediment bacterial communities during a 48-­‐day field experiment. Dual stable isotopic tracers (H13CO3-­‐ and 15NH4+) were added to the porewater within the perfusionators to trace carbon and nitrogen cycling through inorganic and organic pools in order to assess the role of benthic microalgae in sediment nutrient cycling and sequestration. Physical and biological processes were characterized by: (1) installing covers over half of the perfusionators on day 14 to dampen physical mixing and sediment reworking, (2) using glass beads to track sediment mixing and reworking within the perfusionators, (3) continuous measurements of current speeds using an acoustic Doppler velocimeter and (4) monitoring a suite of environmental variables. The dominant feature of the experimental period was a harmful algal bloom (chl a concentrations peaked on day 14 at 107 μg L-­‐1) dominated by Cochlodinium polykrikoides, which reduced biomass of benthic microalgae. Sediment total organic carbon and nitrogen (TOC and TN), benthic microalgae isolated through Ludox extractions, and EPS were enriched in 13C and 15N during the labeling period. Two fractions of EPS were extracted from the sediments, a hot water (HW-­‐EPS) or intracellular fraction and a hot bicarbonate (HB-­‐EPS) or extracellular fraction. Concentrations of HB-­‐EPS were higher than HW-­‐ EPS, but the HW-­‐EPS showed enrichment in 13C before the HB-­‐EPS, consistent with its association with photosynthetic pathways inside the cell. The carbohydrate content of the EPS was higher later in the experiment when sediment chlorophyll concentrations were lower. The carbohydrate composition of the EPS indicated that a higher proportion of material was probably derived from sources other than benthic microalgae including bacterial sources. Increased concentrations of phaeopigments in surface sediments corroborated increased contributions from decaying or degraded material, likely derived from the harmful algal bloom. Overall, this field experiment traced carbon and nitrogen through the benthic microalgal community in a shallow coastal system, and captured the response of the system to a harmful algal bloom event. This experiment provided new insights about carbon and nitrogen dynamics within shallow systems and benthic microalgae community responses to key environmental events.

The relative strength of bottom-up and top-down processes operating within food webs is a fundamental determinant of community structure and function. In marine systems, inconspicuous but often highly abundant invertebrate herbivores (mesograzers) are implicated as strong consumers of primary production and important prey for higher-order consumers. Because of their small size, however, mesograzer abundance is not easily manipulated in the field, which limits our ability to adequately assess their grazing impacts. Seagrass systems present a pressing need for the study of food web dynamics because anthropogenic nutrient and sediment inputs decrease the amount of light reaching seagrass leaves, which limits the depth distribution of seagrasses via reduced photosynthesis to respiration ratios. Mesograzers benefit seagrass through their consumption of epiphytic algae and thus may mitigate the loss of seagrass beds due to nutrient enrichment. I test the relative impacts of nutrient enrichment and crustacean mesograzer abundance on epiphytes in a natural seagrass bed without using cages. My work presents the first cage-free tests of crustacean mesograzing impacts in natural seagrass communities. I successfully decreased crustacean abundance for extended periods of time in multiple experiments using a degradable chemical deterrent. Crustacean mesograzer reduction led to concomitant increases in epiphytic algal biomass, while nutrients increased epiphytes only in the absence of mesograzers. My results validate early work from mesocosm and field cage studies designed to test grazing impacts of mesograzers and support the hypothesis that mesograzers indirectly benefit seagrass through a positive indirect interaction.

The Mid-Atlantic Ridge (MAR) is the largest topographic feature in the Atlantic Ocean, yet little is known about the food web structure and trophic ecology of fishes inhabiting mesopelagic waters along the MAR. To better understand the food web structure and to compare the feeding ecology of abundant MAR fishes to those in offridge areas, MAR-ECO, a Mid-Atlantic Ridge ecosystem field project of the Census of Marine Life, sampled the pelagic fauna of the northern MAR (Iceland to the Azores) during June-July, 2004. Samples were collected with two midwater trawls outfitted with multiple opening and closing cod ends to sample discrete depths from the surface to >2300 m at predefined stations within four ridge sections. Fishes of the family Myctophidae were the most abundant fishes collected, with the myctophid Benthosema glaciale the numerically dominant species. The feeding ecology of three myctophid species, B. glaciale, Protomyctophum arcticum, and Hygophum hygomii, were quantified through dietary analysis. Each species was predominantly zooplanktivorous, consuming copepods, euphausiids, ostracods, and other crustacean zooplankton, with each myctophid species exhibiting unique dietary patterns. Different abiotic parameters influenced variability in the diet of B. glaciale and P. arcticum as determined by canonical correspondence analysis. Of the measured parameters, ridge section and time of day were significant explanatory variables in the diet of B. glaciale, while only depth was significant in the diet of P. arcticum. Daily consumption by B. glaciale and P. arcticum was highest at the Reykjanes Ridge and Charlie-Gibbs Fracture Zone, respectively; consumption by H. hygomii was only measured at the Azorean Zone. Daily consumption of all three species was less than 1% of dry body weight. Active transport of carbon out of the euphotic zone by diel vertically migrating MAR myctophids through respiration of CO2, excretion of dissolved organic carbon, and egestion of particulate organic carbon (POC) was also estimated, and compared to passive sinking of POC in the North Atlantic. Active carbon flux by the 0-200 m integrated migrant myctophid biomass, uncorrected for trawl efficiency, ranged from 0.01-0.4% of sinking POC flux at 150 m and 0.02-0.95% at 300 m. If myctophid biomass was corrected for low gear efficiency, flux increased to 0.1-1% and 0.3-3.5%, respectively, of sinking POC. Lower MAR myctophid biomass resulted in lower active carbon transport compared to myctophids in the Pacific and zooplankton in the North Atlantic estimated in previous studies, but myctophid active transport should still be considered in models of MAR carbon cycling. Due to the role of myctophids as both predators of numerous zooplankton taxa and as prey of higher taxa, information on the feeding ecology and daily consumption of these fishes is necessary to accurately assess their role in the MAR food web and the overall trophic structure of this hydrodynamically and topographically unique ecosystem.

The northern Mid-Atlantic Ridge, from Iceland to the Azores (MAR), ranges in depth from 800 – 4500 m and extends over an area of 3.7 million km2. Recent evidence from MAR-ECO, a Census of Marine Life field project, reported increased abundance and biomass of deep-pelagic fishes below 1000 m on the MAR. Among the fishes sampled, Bathylagus euryops was found to be the biomass-dominant species and ranked third in total abundance. In this thesis, we characterize the distribution and feeding ecology of B. euryops as a function of physical, biological, and life history parameters along a mid-ocean ridge system. Multiple biologically plausible general linear models were fitted to B. euryops catch-per-unit-effort (CPUE) data to investigate the role of various combinations of explanatory variables on the distribution of this species. Results indicated that a model containing categorical depth and geographic location variables provided the most parsimonious description of B. euryops CPUE data. Vertical migration analyses were also conducted to investigate the vertical distribution of B. euryops along the MAR and results indicated that time of day had little influence, whereas ontogeny likely influenced the vertical distribution of B. euryops. To describe the feeding ecology of B. euryops, a general diet composition was determined. Multivariate analyses, including a cluster analysis and a canonical correspondence analysis, were utilized to investigate factors that cause variability within the diet of B. euryops. Results revealed that fish size and geographic location significantly influenced the diet of B. euryops. Furthermore, daily rations were estimated to better understand the role of B. euryops in the food webs of the North Atlantic and results were consistently estimated to be less than 1% of the average wet weight along the MAR. The general trend observed was a southward increase in daily ration estimates along the MAR.