Effects of low dissolved oxygen events on the macrobenthos of the lower Chesapeake Bay

The effects of low dissolved oxygen or hypoxia (<2 mg l^?1) on macrobenthic infaunal community structure and composition in the lower Chesapeake Bay and its major tributaries, the Rappahannock, York, and James rivers are reported. Macrobenthic communities at hypoxia-affected stations were characterized by lower species diversity, lower biomass, a lower proportion of deep-dwelling biomass (deeper than 5 cm in the sediment), and changes in community composition. Higher dominance in density and biomass of opportunistic species (e.g., euryhaline annelids) and lower dominance of equilibrium species (e.g., long-lived bivalves and maldanid polychaetes) were observed at hypoxia-affected stations. Hypoxia-affected macrobenthic communities were found in the polyhaline deep western channel of the bay mainstem north of the Rappahannock River and in the mesohaline region of the lower Rappahannock River. No hypoxic effects on the infaunal macrobenthos were found in the York River, James River, or other deep-water channels of the lower Chesapeake Bay.     

Relationship between Hypoxia and Macrobenthic Production in Chesapeake Bay

Human development has degraded Chesapeake Bay's health, resulting in an increase in the extent and severity of hypoxia (≤2 mg O₂ l^-1). The Bay's hypoxic zones have an adverse effect on both community structure and secondary production of macrobenthos. From 1996 to 2004, the effect of hypoxia on macrobenthic production was assessed in Chesapeake Bay and its three main tributaries (Potomac, Rappahannock, and York Rivers). Each year, in the summer (late July???early September), 25 random samples of the benthic macrofauna were collected from each system, and macrobenthic production in the polyhaline and mesohaline regions was estimated using Edgar's allometric equation. Fluctuations in macrobenthic production were significantly correlated with dissolved oxygen. Macrobenthic production was 90 % lower during hypoxia relative to normoxia. As a result, there was a biomass loss of ~7,320–13,200 metric tons C over an area of 7,720 km², which is estimated to equate to a 20 % to 35 % displacement of the Bay's macrobenthic productivity during the summer. While higher consumers may benefit from easy access to stressed prey in some areas, the large spatial and temporal extent of seasonal hypoxia limits higher trophic level transfer, via the inhibition of macrobenthic production. Such a massive loss of macrobenthic production would be detrimental to the overall health of the Bay, as it comes at a time when epibenthic and demersal predators have high-energy demands.     

Ecosystem Engineering Effects of <Emphasis Type="Italic">Aster tripolium</Emphasis> and <Emphasis Type="Italic">Salicornia procumbens</Emphasis> Salt Marsh on Macrofaunal Community Structure

This paper examines how perennial Aster tripolium and annual Salicornia procumbens salt marshes alter the biomass, density, taxon diversity, and community structure of benthic macrofauna, and also examines the role of elevation, sediment grain size, plant cover, and marsh age. Core samples were collected on a fixed grid on an intertidal flat in the Westerschelde estuary (51.4° N, 4.1° E) over 5 years (2004–2008) of salt marsh development. In unvegetated areas, macrobenthic biomass, density, and taxon diversity were highest when elevation was highest, benthic diatoms were most abundant, and sediment median grain size was smallest. In contrast, in salt marsh areas, macrobenthic biomass and taxon diversity increased with median grain size, while the effects of elevation and diatom abundance on macrobenthic biomass, density, and diversity were not significant. In fine sediments, macrofaunal community structure in the salt marsh was particularly affected; common polychaetes such as Nereis diversicolor, Heteromastus filiformis, and Pygospio elegans had low abundance and oligochaetes had high abundance. Marsh age had a negative influence on the density of macrofauna, and A. tripolium stands had lower macrofaunal densities than the younger S. procumbens stands. There were no significant effects of marsh age, plant cover, and vegetation type on macrobenthic biomass, taxon diversity, and community structure. The results highlight that ecosystem engineering effects of salt marsh plants on macrofauna are conditional. Organic enrichment of the sediment and mechanical hindering of macrofaunal activity by plant roots are proposed as plausible mechanisms for the influence of the salt marsh plants on macrofauna.     

Patterns and Rates of Recovery of Macrobenthic Communities in a Polyhaline Temperate Estuary Following Sediment Disturbance: Effects of Disturbance Severity and Potential Importance of Non-local Processes

Community-level responses of soft sediment macrobenthos to two relatively large-scale disturbance events associated with dredged material (DM) disposal are examined for subtidal (>10 m) lower Chesapeake Bay. Disturbance severity (DM thickness on initial sampling date following disposal) and date of sampling were important factors explaining the patterns and rates of recovery for species richness, abundance, biomass, and community composition, but sediment disposal had minimal effects when DM thickness was ≤15 cm. It took 1.5 years or less following the cessation of disposal activities for richness, abundance, biomass and community composition at high disposal severity (DM > 15 cm) to attain levels measured at reference stations representing the ambient community of the region. Positive correlations of community structure metrics between the disposal area and reference stations provide evidence that non-local processes influenced patterns of recovery in this estuarine setting. Species interactions and food limitation may also have been important at local scales.     

The Role of Protistan Microzooplankton in the Upper San Francisco Estuary Planktonic Food Web: Source or Sink?

Decline of native pelagic species in estuarine systems is an increasing problem, especially for native fishes in the San Francisco Estuary and Delta (SFE-D). Addressing these losses depends on understanding trophodynamics in the food web that supports threatened species. We quantified the role of microzooplankton (heterotrophic–mixotrophic protists <200 μm) in the food web of the upper SFE-D. We sampled protist plankton abundance and composition at two sites (Suisun Bay and Grizzly Bay) approximately monthly from February 2004 to August 2005 and conducted dilution experiments during spring and summer of both years in Suisun Bay. Heterotrophs dominated the protist community in Suisun Bay and Grizzly Bay, particularly in the <20 μm size range, and peaks in protistan microzooplankton biomass were associated with high phytoplankton biomass. In both years, microzooplankton grazing rates were high (0.5–0.7 day⁻¹) during the spring and lower (~0.2 day⁻¹) during summer. Phytoplankton growth rates peaked in April 2004 (~0.7 day⁻¹) but were much lower (<0.1 day⁻¹) in spring 2005, despite relatively high abundance. Thus, microzooplankton grazing consumed as much as 73% of phytoplankton standing stock during spring and ~15% of standing stock during summer of both years. Combined with earlier results, we conclude that microzooplankton can be important mediators of carbon and energy flow in the upper SFE-D and may be a “source” to the metazoan food web.     

Seasonal Composition of Benthic Macroinfauna Exposed to Hypoxia in the Northern Gulf of Mexico

Bottom-water hypoxia in the northern Gulf of Mexico has increased in severity (duration, frequency, and intensity) since the 1970s and has impacted the less-mobile benthos ever since. From September 2003 to October 2004, the macrobenthic density, species richness, community composition, and vertical distribution were studied at a frequently hypoxic station, C6B (28°52.10′ N and 90°28.00′ W). The polychaete-dominated community was approximately three times less dense and diverse in post-hypoxic months compared to pre-hypoxic months. The lowest oxygen concentrations in July 2004 did not significantly affect the infaunal community as predicted; rather, the response was observed 1 month later after a longer, low-oxygen exposure. The opportunistic, hypoxia-tolerant polychaete, Paraprionospio pinnata, population increased in July 2004 when other common species decreased, thereby maintaining pre-hypoxic densities. Determining the duration and severity of hypoxia prior to sampling rather than at the time of sampling helps to better understand benthic community responses to hypoxia.     

Stable Isotopes Reveal Complex Changes in Trophic Relationships Following Nutrient Addition in a Coastal Marine Ecosystem

Complex links between the top-down and bottom-up forces that structure communities can be disrupted by anthropogenic alterations of natural habitats. We used relative abundance and stable isotopes to examine changes in epifaunal food webs in seagrass (Thalassia testudinum) beds following 6 months of experimental nutrient addition at two sites in Florida Bay (USA) with different ambient fertility. At a eutrophic site, nutrient addition did not strongly affect food web structure, but at a nutrient-poor site, enrichment increased the abundances of crustacean epiphyte grazers, and the diets of these grazers became more varied. Benthic grazers did not change in abundance but shifted their diet away from green macroalgae + associated epiphytes and towards an opportunistic seagrass (Halodule wrightii) that occurred only in nutrient addition treatments. Benthic predators did not change in abundance, but their diets were more varied in enriched plots. Food chain length was short and unaffected by site or nutrient treatment, but increased food web complexity in enriched plots was suggested by increasingly mixed diets. Strong bottom-up modifications of food web structure in the nutrient-limited site and the limited top-down influences of grazers on seagrass epiphyte biomass suggest that, in this system, the bottom-up role of nutrient enrichment can have substantial impacts on community structure, trophic relationships, and, ultimately, the productivity values of the ecosystem.     

Changes in benthos following the clean-up of a severely metalpolluted cove in the Hudson River estuary: Environmental restoration or ecological disturbance?

We studied changes in macrobenthic communities following the environmental clean-up of metal-polluted (cadmium, nickel, and cobalt) sediments in Foundry Cove, a small inlet within the Hudson River estuary of New York. We used a BACI-style experiment to test the hypotheses that high levels of cadmium in sediments change macrobenthic assemblages relative to unpolluted areas, and removal of metals (especially cadmium) by dredging will restore the benthos, such that benthic fauna in Foundry Cove are not different from unpolluted areas. In 1984, prior to the restoration work, there were no significant differneces between macrobenthic assemblages in polluted and unpolluted locations, indicating that cadmium had little effect on community structure. The lack of an observed toxicity effect may have been caused by the compensatory evolution of resistance to cadmium in dominant organisms. Six years after the restoration work and despite a substantial reduction in metal pollution, there were lower abundances of oligochaetes, nematodes, and chironomids and a higher abundance of polychaetes at Foundry Cove relative to reference locations. Correlative analyses identified greater sediment compaction caused by dredging at Foundry, Cove as a possible cause of faunal differences. The results demonstrate that it is difficult to accurately predict, the effects of anthropogenic disturbances and restorations in complex natural systems and that unforeseen side effects are inevitable. Documenting these unpredicted effects and experimentally understanding their causes in past restorations will greatly improve the success and cost-effectiveness of future projects of a similar type.     

Shifts in Zooplankton Community Structure: Implications for Food Web Processes in the Upper San Francisco Estuary

Zooplankton are an important trophic link and a key food source for many larval fish species in estuarine ecosystems. The present study documents temporal and spatial zooplankton dynamics in Suisun Bay and the Sacramento–San Joaquin Delta—the landward portion of the San Francisco Estuary (California, USA)—over a 37-year period (1972–2008). The zooplankton community experienced major changes in species composition, largely associated with direct and indirect effects of introductions of non-native bivalve and zooplankton species. A major clam invasion and many subsequent changes in zooplankton abundance and composition coincided with an extended drought and accompanying low-flow/high-salinity conditions during 1987–1994. In the downstream mesohaline region, the historically abundant calanoid copepods and rotifers have declined significantly, but their biomass has been compensated to some extent by the introduced cyclopoid Limnothoina tetraspina. The more upstream estuary has also experienced long-term declining biomass trends, particularly of cladocerans and rotifers, although calanoid copepods have increased since the early 1990s due to the introduced Pseudodiaptomus spp. In addition, mysid biomass has dropped significantly throughout the estuary. Shifts in zooplankton species composition have also been accompanied by an observed decrease in mean zooplankton size and an inferred decrease in zooplankton food quality. These changes in the biomass, size, and possibly chemical composition of the zooplankton community imply major alterations in pelagic food web processes, including a drop in prey quantity and quality for foraging fish and an increase in the importance of the microbial food web for higher trophic levels.     

A reassessment of the status of the benthic macrofauna of the Raritan estuary

The benthic macrofauna of the Raritan estuary, at the mouth of New York Harbor, has been reported to be severely impacted by pollution, with a significant change occurring between surveys in the late 1950’s and the early 1970’s. We review this assessment using five-fold more macrofaunal data (including biomass), not reported or used before, from a 1973–1974 survey. These data are compared to previous and similar studies in the area. This reassessment suggests benthic community structure is not as degraded as previously reported and is, in many ways, similar to other relatively unpolluted middle Atlanic estuaries.     

Influence of land use on macrobenthic communities in nearshore estuarine habitats

Macrobenthic community indices were examined for their ability to characterize the influence of shoreline alteration and watershed land use in nearshore estuarine environments of the Chesapeake Bay, U.S.A. Twenty-three watersheds were surveyed in 2002 and 2003 for nearshore macrobenthic assemblages, environmental parameters (i.e., dissolved oxygen, pH, total suspended solids, salinity, and sediment composition), shoreline condition, and land use. Two indices of macrobenthic biological integrity, benthic index of biological integrity in the nearshore (B-IBI_N) and abundance biomass comparison (W-value), were evaluated for associations with environmental and shoreline condition, and riparian and watershed land use. Comparisons between nearshore measures of the B-IBI with offshore values (>2 m; Chesapeake Bay benthic index of biological integrity [B-IBI_CB]) were conducted to assess the ability of the index to reflect land use patterns at near and far proximities to shore. Nearshore macrobenthic communities were represented by a total of 94 species (mean number of species =9.2 ± 0.4 sample⁻¹), and were dominated by the phyla Arthropoda, Annelida, and Mollusca. Temporal variability in environmental conditions and macrobenthic abundance and biomass may be attributable to the notable increase in precipitation in 2003 that led to nutrient influxes and algal blooms. For the biotic indices applied in the nearshore, the highest scores were associated with forested watersheds (W-value, B-IBI_N). Ecological thresholds were identified with nonparametric change-point analysis, which indicated a significant reduction in B-IBI_N and W-value scores when the amount of developed shoreline exceeded 10% and developed watershed exceeded 12%, respectively.     

Epifaunal communities thrive in an estuary with hypoxic episodes

We characterized the abundance and species composition of sessile and mobile epifaunal assemblages in the York River, a tributary of the Chesapeake Bay, U.S., during the summer hypoxia seasons in 1996 and 1997. We collected communities on artificial substrates in two areas of the river that have historically experienced different exposure to hypoxia. Despite frequent hypoxic stress, epifauna formed dense communities in both areas. Dominant species comprised a range of phyla and included the polychaetesPolydora cornuta andSabellaria vulgaris, the bryozoansMembranipora tenuis andConopeum tenuissimum, the tunicateMolgula manhattensis, the barnacleBalanus improvisus, the anemoneDiadumene leucolena, and the hydroidsEctopleura dumortieri andObelia bicuspidata. Common mobile species included the nudibranchsCratena kaoruae andDoridella obscura, the amphipodsMelita nitida andParacaprella tenuis, the polychaeteNereis succinea, and the flatwormStylochus ellipticus. We found few differences in species composition between the two areas, even though one area usually experienced lower oxygen concentrations during hypoxic events, suggesting that hypoxia does not exclude any epifaunal species, in the York River. We did find differences between the two study areas in percent cover and abundance of some species. While tunicates, hydroids, and anemones were equally abundant in both areas during both study years, bryozoans and the polychaeteS. vulgaris were more abundant in the area with gereally higher oxygen, suggesting that they may be less tolerant of hypoxic stress. The polychaeteP. cornuta was more abundant in the area that usually had lower oxygen. These results suggest that many epifaunal species have high hypoxia tolerance, and most epifaunal species found in the lowr York River are able to survive in hypoxic areas. Epifaunal species are not necessarily more susceptible to hypoxia than infaunal species in the York River. Epifaunal communities in areas with brief hypoxic episodes and moderate hypoxia (0.5–2 mg O₂ I⁻¹) can persist with little change in species composition, and with few changes in abundance, as oxygen concentrations fall.     

Trophic structure of macrobenthic communities in the Calcasieu estuary,Louisiana

Trophic groups were used to investigate broad patterns and predict species interactions of macrobenthic assemblages in Calcasieu Estuary, Louisiana. Macrobenthic assemblages of the estuary were numerically dominated by deposit-feeding species. Surface-deposit feeders were the most abundant macrobenthos of the upper estuary, subsurface-deposit feeders dominated the lower estuary, and a trophically well-mixed assemblage inhabited the middle estuary. There were periodic shifts in species dominance within each region, but the shifts were without temporal pattern and seldom led to changes in trophic structure of the region. The macrobenthic assemblages of the upper estuary were dominated by several early-colonizing species which switched feeding modes with changes in water flow, suggesting that disturbances in the upper estuary had a direct effect on macrobenthic communities. Disturbances also may have eliminated macrobenthic species before they could establish a well-mixed community. The presence of trophically mixed communities in the middle estuary probably was indicative of fewer disturbances there. The dominance by subsurface-deposit feeders in the lower estuary probably indicates that sedimentary food in Calcasieu Lake was seldom utilized at the sediment-water interface. Rather, most food became buried and was available only to subsurface-deposit feeders.     

California condor associated with spruce-jack pine woodland in the late Pleistocene of New York

A humerus, coracoid, and pedal phalanx of the California Condor, Gymnogyps californianus, were recovered from the Hiscock Site in western New York, in an inorganic stratum containing wood that is 11,000 radiocarbon years old. Associated vertebrates include mastodont, wapiti, and caribou. Pollen and plant macrofossils from the sediments indicate a spruce-jack pine woodland and a local, herb-dominated wetland community. Historic records (all from western North America) and previous late Pleistocene fossils of the California Condor are associated mainly with warm-temperate climates and floras. The New York fossils show that this bird was able to live in a colder climate and in a boreal, coniferous setting at a time when appropriate food (large mammal carrion) was available. The California Condor, which survives only in captivity, has suffered a greater reduction in geographical range than previously suspected. Much of this reduction in range probably occurred ca. 11,000 yr B.P. when the extinction many North American large mammals resulted in severely reduced availability of food for the California Condor and other large scavenging birds.     

Past occurrences of hypoxia in the Baltic Sea and the role of climate variability,environmental change and human impact

The hypoxic zone in the Baltic Sea has increased in area about four times since 1960 and widespread oxygen deficiency has severely reduced macro benthic communities below the halocline in the Baltic Proper and the Gulf of Finland, which in turn has affected food chain dynamics, fish habitats and fisheries in the entire Baltic Sea. The cause of increased hypoxia is believed to be enhanced eutrophication through increased anthropogenic input of nutrients, such as nitrogen and phosphorus. However, the spatial variability of hypoxia on long time-scales is poorly known: and so are the driving mechanisms. We review the occurrence of hypoxia in modern time (last c. 50 years), modern historical time (AD 1950–1800) and during the more distant past (the last c. 10 000 years) and explore the role of climate variability, environmental change and human impact. We present a compilation of proxy records of hypoxia (laminated sediments) based on long sediment cores from the Baltic Sea. The cumulated results show that the deeper depressions of the Baltic Sea have experienced intermittent hypoxia during most of the Holocene and that regular laminations started to form c. 8500–7800 cal. yr BP ago, in association with the formation of a permanent halocline at the transition between the Early Littorina Sea and the Littorina Sea s. str. Laminated sediments were deposited during three main periods (i.e. between c. 8000–4000, 2000–800 cal. yr BP and subsequent to AD 1800) which overlap the Holocene Thermal Maximum (c. 9000–5000 cal. yr BP), the Medieval Warm Period (c. AD 750–1200) and the modern historical period (AD 1800 to present) and coincide with intervals of high surface salinity (at least during the Littorina s. str.) and high total organic carbon content. This study implies that there may be a correlation between climate variability in the past and the state of the marine environment, where milder and dryer periods with less freshwater run-off correspond to increased salinities and higher accumulation of organic carbon resulting in amplified hypoxia and enlarged distribution of laminated sediments. We suggest that hydrology changes in the drainage area on long time-scales have, as well as the inflow of saltier North Sea waters, controlled the deep oxic conditions in the Baltic Sea and that such changes have followed the general Holocene climate development in Northwest Europe. Increased hypoxia during the Medieval Warm Period also correlates with large-scale changes in land use that occurred in much of the Baltic Sea watershed during the early-medieval expansion. We suggest that hypoxia during this period in the Baltic Sea was not only caused by climate, but increased human impact was most likely an additional trigger. Large areas of the Baltic Sea have experienced intermittent hypoxic from at least AD 1900 with laminated sediments present in the Gotland Basin in the Baltic Proper since then and up to present time. This period coincides with the industrial revolution in Northwestern Europe which started around AD 1850, when population grew, cutting of drainage ditches intensified, and agricultural and forest industry expanded extensively.     

Recolonization of Intertidal Infauna in Relation to Organic Deposition at an Oyster Farm in Atlantic Canada—A Field Experiment

A field experiment was carried out to investigate the patterns of macrobenthic recolonization and to determine the effects of biodeposition on benthic communities at an intertidal oyster culture site in New Brunswick, Canada. Total organic deposition in azoic organic-free sediment trays was generally higher within the farm compared to reference sites. Two weeks after deployment of trays, total organic content had reached 1.1%. The abundance, species number, and diversity of the macrobenthic community were positively correlated with the total organic content in the experimental trays, but the correlations between community parameters and organic content were negative in the ambient sediment. The results suggest that organic matter in sediment may have positive effects on macrobenthic infauna at low levels as an additional food source but may be harmful to benthic animals at high levels. This study also indicates that location in the intertidal zone is a major parameter affecting the community structure of macrobenthic colonization.     

The Response of Macrobenthic Communities to Environmental Variability in Tropical Coastal Waters

The relative contributions of spatial and temporal fluctuations are different in shaping natural communities in a tropical coastal/estuarine system. Understanding how coastal communities respond to these fluctuations is still equivocal, and thus, available data are rare. Here, multiple analytical approaches were used to identify key spatial and temporal factors, and to quantify their relative roles in shaping a macrobenthic community through space (contamination degree, physical parameters, and sediment characteristics) and time (climatic factors, season, and year). A dataset of eight sampling times was analyzed over a period of 2 years, in which macrobenthic species abundances were sampled. A total of 33 species were identified, including 18 bivalves, 5 gastropods, and 4 crustaceans. The other taxa were less diverse. The results show that there were no significant temporal changes of macrobenthic community structure, but spatial changes were significant and synchronized with environmental factors (i.e., sediment characteristics, water depth, and the distance from anthropogenic sources). This study demonstrates that spatial factors played a primary role in structuring of macrobenthic assemblages, whereas the influence of temporal factors appeared less across geographically distinct sites. Thus, temporal variation of a coastal macrobenthic community appears to be controlled by partly different processes at different scales.     

Temporal and Spatial Dynamics of Diel-Cycling Hypoxia in Estuarine Tributaries

The distribution and intensity of hypoxia (low dissolved oxygen) in estuaries is increasing worldwide due to cultural eutrophication. This study quantifies the strength of associations between the duration of diel-cycling severe hypoxia (≤2 mg O₂ l⁻¹) in bottom water (∼15 cm above bottom) of a shallow (<2 m) coastal lagoon estuary (Delaware, USA) and abiotic environmental variables (water temperature, insolation, tide, streamflow, and wind) and predicts the duration of severe hypoxia given different combinations of these variables. The intensity and spatial extent and dynamics of diel-cycling severe hypoxia events were defined. Vertical variability in dissolved oxygen (DO) concentration during the daytime was also determined. During the summers of 2001–2005, bottom DO data were collected for periods of weeks to months at multiple sites using automated sondes. Multiple linear regression (MLR) and regression tree analysis (RTA) were used to determine the relative importance of the environmental variables in predicting the number of hours of severe hypoxia per day. Key findings of the study were that severe hypoxia events of minutes to hours in duration occurred frequently in all four tributaries sampled, primarily between 0200 and 1000 hours. Severe hypoxia duration and diel-cycling amplitudes of DO concentration increased in the up-tributary direction. Hierarchically, the duration of severe hypoxia was influenced mostly by the mean daily water temperature, then by preceding days’ total insolation, percentage of morning hours (02:00 to 10:00 a.m.) ebb tide, and daily streamflow. Collectively, the variables examined by the MLR and the RTA approaches accounted for 62% and 65% of the variability in the duration of severe hypoxia, respectively. RTA demonstrated that daily mean water temperature above 26.3°C and previous day’s total insolation below 13.6 kW m⁻² were associated with the longest lasting severe hypoxic events (9.56 h). The environmental variables and combinations of conditions that modulate or augment diel-cycling hypoxia presented in this paper enhance understanding of this widespread and growing phenomenon and provide additional insight regarding the extent to which it can impact food webs in very shallow estuarine waters that often serve as nursery habitat.     

Effects of sediment contaminants and environmental gradients on macrobenthic community trophic structure in Gulf of Mexico estuaries

Macrobenthic communities from estuaries throughout the northern Gulf of Mexico were studied to assess the influence of sediment contaminants and natural environmental factors on macrobenthic community trophic structure. Community trophic data were also used to evaluate whether results from laboratory sediment toxicity tests were effective indicators of site-specific differences in benthic trophic structure. A multiple regression model consisting of five composite factors (principal components) was used to distinguish the effects of sediment contaminants and environmental variables on benthic community trophic structure. This model explained 33.5% of the variation in macrobenthic trophic diversity (p<0.001), a variable derived from the distribution of taxas among nine original trophic categories. A significant negative relatinship was found between principal components reflecting concentrations of sediment contaminants and macrobenthic trophic diversity. Detritivores including surface deposit-feeders (SDF), subsurface deposit-feeders (SSDF), and filter feeders (FF) were numerically dominant at 201 random sites, each group accounting for 25–30% of total macrobenthic abundance. The relative abundance of SDFs was considerably lower (12.1±2.9% to 17.1±4.4%) at sites where sediment contaminant concentrations exceeded minimum biological effects thresholds (ER-L values from Long and Morgan 1990 than at sites sampled at random (29.3±5.7%). SSDFs were proportionally more abundant at contaminated sites (42.0±7.7% to 63.6±10.3%) versus random sites (27.5±5.7%), and the relative abundance of SSDFs was positively correlated with concentrations of particular contaminants. Benthic trophic structure was also found to be a function of salinity, where the proportion of SSDFs was negatively correlated with salinity (p=0.035, r=−0.223, n=326). Silt-clay content loaded fairly strongly on the first principal component, but trophic structure parameters were not significantly correlated with sediment grain size or dissolved oxygen (perhaps due, in part, to covariation). Results from laboratory sediment toxicity tests with mysids were predictive of differences in macrobenthic trophic structure in situ (i.e., mysid survival was negatively correlated with %SSDF; p<0.001, r=−0.292, n=326). Results from laboratory sediment toxicity tests with ampeliscid amphipods were not indicative of site-specific differences in benthic trophic structure.     

Small Spatial Scale Variation in Fish Assemblage Structure in the Vicinity of the Northwestern Gulf of Mexico Hypoxic Zone

Seasonal hypoxia [dissolved oxygen (DO)?≤?2 mg?l^?1] occurs over large regions of the northwestern Gulf of Mexico continental shelf during the summer months (June–August) as a result of nutrient enrichment from the Mississippi–Atchafalaya River system. We characterized the community structure of mobile fishes and invertebrates (i.e., nekton) in and around the hypoxic zone using 3 years of bottom trawl and hydrographic data. Species richness and total abundance were lowest in anoxic waters (DO?≤?1 mg?l^?1) and increased at intermediate DO levels (2–4 mg?l^?1). Species were primarily structured as a benthic assemblage dominated by Atlantic croaker (Micropogonias undulatus) and sand and silver seatrout (Cynoscion spp.), and a pelagic assemblage dominated by Atlantic bumper (Chloroscombrus chrysurus). Of the environmental variables examined, bottom DO and distance to the edge of the hypoxic zone were most strongly correlated with assemblage structure, while temperature and depth were important in some years. Hypoxia altered the spatial distribution of both assemblages, but these effects were more severe for the benthic assemblage than for the pelagic assemblage. Brown shrimp, the primary target of the commercial shrimp trawl fishery during the summer, occurred in both assemblages, but was more abundant within the benthic assemblage. Given the similarity of the demersal nekton community described here to that taken as bycatch in the shrimp fishery, our results suggest that hypoxia-induced changes in spatial dynamics have the potential to influence harvest and bycatch interactions in and around the Gulf hypoxic zone.