Benthic communities and bioturbation history of estuarine and coastal systems: effects of hypoxia and anoxia

Abstract

The stratigraphic record preserved in sedimentary deposits of coastal and estuarine systems is increasingly recognized as a source of information on historical trends in pollution, eutrophication and related biotic changes. For this study, spatial and historical patterns in organism–sediment interactions and bioturbation were evaluated for three coastal and estuarine systems: Chesapeake Bay, Long Island Sound and parts of the Baltic Sea. Observed patterns are used to evaluate the hypothesis that degradation of benthic community functioning can be attributed to increasing eutrophication in these systems. For all areas studied, gradients in bioturbation in near-surface sediments (0-20 cm) were coincident with apparent gradients in oxygen stress. In areas characterized by severe oxygen stress (generally deeper channel or basin areas characterized by restricted water flow), benthic communities were characterized by the absence of large infaunal organisms, bioturbation was reduced and sediments were physically laminated. In areas not influenced by severe oxygen stress, large infaunal organisms and bioturbated sediments predominated. The observed patterns of bioturbation in each system contrast with patterns expected on the basis of physical sediment reworking due to wave and tidal action (i.e. increasing dominance of bioturbation with increasing water depth). Long sediment cores from each area suggest that laminated sediments are a natural phenomenon in some areas, especially deeper channels or basins, but that conditions favoring bioturbation have fluctuated through time in some shallower regions.