Size-specific responses of the amphipod Eohaustorius estuarius to clay in sediment toxicity testing

A number of lines of evidence suggest that the amphipod Eohaustorius estuarius has variable tolerance to clay in sediments. In the current study, two laboratory dose-response experiments were conducted with kaolin clay to evaluate whether clay effects varied with amphipod size. The results indicated that smaller amphipods (< 0.9 mg dry wt.) were significantly more tolerant of clay than larger individuals up to 2.9 mg dry wt. Average survival in clay/sand mixtures with > 70% clay was 88%, 71%, and 52% for small (0.6–1.2 mg), medium (1.4–1.9 mg), and large (1.8–2.9 mg) amphipods, respectively. Standard 96-h reference toxicant tests with cadmium chloride (CdCl₂) were conducted to determine whether there were size-specific differences in response to this metal reference toxicant. The CdCl₂ median lethal concentrations (LC50s) for small, medium, and large amphipods were 6.78, 5.13, and 4.63 mg/L, respectively. Responses of all three size classes to cadmium were within historic confidence intervals for this reference metal, and were not significantly different from one another based on overlapping confidence intervals. Additional experiments with high clay reference site sediments from San Francisco Bay were conducted to confirm the size-related response with field sediments, but were only partially conclusive. Overall results suggest that the use of smaller amphipods in routine monitoring of high clay sediments will reduce the influence of this factor on test results.