Hypoxic response of two marine benthic communities

Benthic macrofauna communities were sampled with their 'undisturbed' sediments from 2 soft-bottom sites on the Swedish west coast. At one of the sites the top sediment had a positive redox potential (Expt 1) and at the other the sediment was comparatively more reduced close to the surface (Expt 2). The benthic communities at these 2 sites were each exposed to 3 treatments of oxygen concentrations: (1) normoxia (>8.0 mg I-', >90% oxygen saturation), (2) moderate hypoxia (-1.0 mg I-', 12% oxygen saturation) and (3) severe hypoxia (-0.5 mg I-', 61G oxygen saturation). Redox potential profiles in the sediments showed a significant increase with time under normoxic conditions, whereas they showed an overall decl.ine with t ~m e In moderate and severe hypoxia. The number of individuals and species at the 2 field sites did not d~ffer significantly from corresponding numbers In the experimental normoxia (laboratory control) at the end of those experiments In Expt 1 survival of the number of individuals was reduced in severe hypoxia compared to in moderate hypoxia and normoxia. The number of species was significantly reduced between all treatments. In Expt 2 community abundance and the number of species were significantly reduced in moderate and severe hypoxia compared to normoxia. Behaviour of dominant specles is described: in general they left their position in the sediment (for Pectinaria korerli with t h e ~r tubes) and occurred at the sediment surface in the moderate and severe hypoxic treatments. Comparison of the results of Expt 1 with those of Expt 2 suggests that the comparatively more reduced sediment in Expt 2 resulted in a greater negative response, implying the significance of sediment conditions for benthic macrofauna response to hypoxia.