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Seasonal changes in the intertidal distribution, abundance and population dynamics of the epifaunal amphipod Gammarus palustris were studied in salt marshes bordering 2 estuarine rivers flowing into the Chesapeake Bay. The amphipod populations inhabiting the Patuxent River study site showed abundance peaks during the spring and autumn, and major declines in density during the summer and winter. Populations at 2 study sites in the Rhode River showed only 1 peak of abundance during the late spring and early summer which was followed by a decline in density throughout the remainder of the year. In both rivers, the low amphipod densities observed during the winter corresponded with a subtidal migration. Migrations did not account for the low numbers observed at other times and an examination of the life cycle of this species as well as an egg—ratio analysis of the populations indicated that low densities during the summer (Patuxent) or late summer and fall (Rhode) were not due solely to life—cycle events. The tolerance of G. palustris to 3 environmental parameters was tested in the laboratory. These included low—salinity, heat/desiccation and freezing—stress experiments. Comparisons of the results with observed fluctuations of these variables at the study sites demonstrated that only freezing stress would probably cause significant mortality in intertidal populations of this species. Insufficient tolerance to this stress was postulated as the reason for the observed distributional shift to subtidal areas during the winter. Amphipod distribution within the intertidal zone at other times of the year was highly correlated with Spartina density. Substratum preference experiments indicated that this was due to a strong behavioral preference by this species for Spartina culms. Intraspecific and interspecific competition for food were tested by an analysis of 3 reproductive indices: the estimated birth rate as calculated by the egg—ratio method, the average brood size and the average brood size/ovigerous ♀. The former 2 indices declined during the early summer as a result of natural adult female mortality and a decreased proportion of ovigerous to nonovigerous ♀ ♀. The average brood size/ovigerous ♀ did not decline significantly throughout the reproductive period indicating that food limitation did not induce the observed summer decline in amphipod abundance. Intraspecific competition for space was tested in the laboratory by crowding and competitive—displacement experiments. The results indicated that competition for space was not directly responsible for the summer decrease in amphipod density but did influence amphipod distribution when Spartina culms were a limited resource. In this situation, G. palustris was capable of intraspecific displacement and evidence is presented which indicates that adults are able to displace juveniles from the preferred substratum. Interspecific competition was not examined experimentally because most of the associated fauna inhabiting the marsh beds were infaunal species. Laboratory predati...

Abstract-Matching data on sediment contaminants and macroinfaunal community structure from 231 subtidal stations in southeastern U.S. estuaries were used to develop a framework for evaluating risks of benthic impacts from multiple-contaminant exposure. Sediment contamination was expressed as the mean ratio of individual contaminant concentrations relative to corresponding sediment quality guidelines, that is, to effects range-median (ERM) values, probable effects level (PEL) values, or an aggregate of the two. The probability of a degraded benthos was relatively low in samples with mean ERM quotients Յ0.020, PEL quotients Յ0.035, or combined ERM/PEL quotients Յ0.024. Only 5% of stations within these ranges had degraded benthic assemblages, while 95% had healthy assemblages. A higher probability of benthic impacts was observed in samples with mean ERM quotients Ͼ0.058, PEL quotients Ͼ0.096, or ERM/PEL quotients Ͼ0.077. Seventy-three to 78% of stations with values in these upper ranges had degraded benthic assemblages, while 22 to 27% had healthy assemblages. Only four stations (three with degraded, one with healthy assemblages) had mean ERM or PEL quotients Ͼ1.0, which is the beginning of the range associated with a high probability of mortality in short-term laboratory toxicity tests using amphipods.

SynopsisDemersal fish communities associated with hard bottom habitats in the South Atlantic Bight were investigated in three depth zones (inner, middle and outer shelf) between 30" and 33"N latitudes. Fishes were sampled with trawls and baited fishing gear, and were observed by remotely operated underwater television. Most demersal hard bottom fishes demonstrated seasonal differences in abundance in each depth zone, especially at the inner and outer shelf stations. Diversity values from trawl catches were higher in winter than summer at inner and outer shelf stations, but lower in winter at middle shelf stations. Species richness was higher in summer than in winter at most stations, but H' diversity patterns were more influenced by community evenness. Diversity values were higher than those reported for similar depths in the Middle Atlantic Bight. Mean biomass of demersal teleosts for all stations combined was slightly greater in winter than in summer. There was no significant difference in biomass between stations in summer, however, middle shelf stations had significantly greater biomass than inner or outer shelf stations in winter. Biomass estimates from the hard bottom areas studied were considerably higher than those reported in the literature for sand bottom areas in the South Atlantic Bight, but less than those reported for tropical reefs. Cluster analysis revealed differences in community composition between day and night trawl tows at all stations, and greater seasonal differences in species composition at inner and outer shelf stations than at middle shelf depths. Underwater television provided useful complementary data to trawl catches, documented the presence of large fishes which avoided the trawl, and provided information on the community composition at high relief stations which could not be trawled.

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