Phoxocephalid Amphipod Bioassay for Marine Sediment Toxicity

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Abstract
The relative toxicity of marine sediment can be accurately determined through acute, static bioassays with the phoxocephalid amphipod Rhepoxynius abronius. Mortality and sublethal effects on emergence from sediment and reburial behavior are determined after ten day exposure in 1-L beakers containing 175 mL of test sediment, 775 mL of seawater (25 ppt, 15°C), and 20 amphipods. Response of amphipods to test sediment is compared with response in control sediment collected from the species' natural habitat. Mean survival under control conditions is 95%. With five replicates the bioassay is 75% certain of detecting statistical significance when mean survival is reduced by 15%. The method can be applied to a great variety of sediment types because of the tolerance of R. abronius to a broad range of sediment grain sizes and levels of organic enrichment. The bioassay can be applied to determine (1) the toxicity of sediment subject to regulatory decision (e.g., dredging or disposal), (2) the spatial distribution of sediment toxicity along pollution gradients or near point sources, and (3) the LC50 of contaminants added to unpolluted sediment. The species' sensitivity to low salinity limits the method to sediment from the coastal zone and lower portion of estuaries. A detailed description of the bioassay procedures is appended to this report.