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 Mex ico were studied to assess the influence of sediment contaminants and natur al environmental factors on macrobenthic community trophic structure. Commu nity trophic data were also used to evaluate whether results from laborator y sediment toxicity tests were effective indicators of site-specific differ ences in benthic trophic structure. A multiple regression model consisting of five composite factors (principal components) was used to distinguish th e effects of sediment contaminants and environmental variables on benthic c ommunity trophic structure. This model explained 33.5% of the variation in macrobenthic trophic diversity (p < 0.001), a variable derived from the dis tribution of taxa among nine original trophic categories. A significant neg ative relationship was found between principal components reflecting concen trations of sediment contaminants and macrobenthic trophic diversity. Detritivores including surface deposit-feeders (SDF), subsurface deposit-fe eders (SSDF), and filter feeders (FF) were numerically dominant at 201 rand om 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 mi nimum 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%) vers us random sites (27.5 +/- 5.7%), and the relative abundance of SSDFs was po sitively correlated with concentrations of particular contaminants. Benthic trophic structure was also found to be a function of salinity, where the p roportion of SSDFs was negatively correlated with salinity (p = 0.035, r = -0.223, n = 326). Silt-clay content loaded fairly strongly on the first pri ncipal component, but trophic structure parameters were not significantly c orrelated with sediment grain size or dissolved oxygen (perhaps due, in par t, to covariation). Results from laboratory sediment toxicity tests with mysids were predictive of differences in macrobenthic trophic structure in situ (i.e., mysid surv ival 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 structu re. Results from this study demonstrated that sediment contaminants can be quite important in structuring macrobenthic communities in soft-bottom estu arine habitats. The fact that macrobenthic trophic diversity decreased sign ificantly with increasing sediment contamination indicates that important g eneral differences in benthic community function may exist between contamin ated and random sites. These data suggest that benthic trophic structure an alysis may be an effective tool for assessing integrated community response s to chronic sublethal exposure and may be useful for assessing toxicologic al responses at ecologically relevant levels of organization.