Effects of natural sediment features on survival of the phoxocephalid amphipod, Rhepoxynius abronius

Effects of sediment particle size and water content on the survival of the amphipod, Rhepoxynius abronius, were examined by manipulating these natural sediment features within static laboratory microcosms. Mean amphipod survival in fine, uncontaminated, field sediments (≥ 80% silt-clay) can be 15% lower than survival in native sediment. Storage of sediments at 4°C over 7–14 days did not change sediment toxicity, but handling (i.e. elutriation and recombination) of muddy sediments increased toxicity. Sediment particle size and organic content had greater impact on the survival of R. abronius than did sediment water content in modifying amphipod survival, but we could not independently separate the effects of these two sediment variables. A new set of criteria is proposed to interpret toxicity results from the amphipod bioassay in the light of the mortality associated with fine sediment particle size. The efficacy of these criteria to separate mortality caused by fine particles and chemical contaminants was tested by analyzing field survey data from 78 Puget Sound (WA) Urban sites. Using our new criteria, the toxicity of these sediments was found to closely reflect the degree of chemical contamination. We propose that an approach similar to this be undertaken for toxicity tests whenever natural environmental factors induce mortality above background levels.