BIOTIC FACTORS MODIFYING ACUTE TOXICITY OF AQUEOUS CADMIUM TO ESTUARINE AMPHIPOD LEPTOCHEIRUS-PLUMULOSUS

A 96-h exposure to aqueous cadmium (Cd) is the recommended reference t oxicity test for IO-day sediment bioassays with the estuarine amphipod , Leptocheirus plumulosus (US EPA 1994), This cater-only test was used to assess the influence of organism size, sex, and nutritional status on the sensitivity of laboratory-cultured L. plumulosus to Cd. In add ition, the response of field-collected amphipods was compared to simil arly sized laboratory animals to assess potential seasonal changes in Cd sensitivity. Lipid content of test organisms was measured in these seasonal experiments and those evaluating effects of nutritional statu s because of its potential as an indicator of physiological condition, LC50 values of laboratory animals size-sorted on nested 500-, 710-, a nd 1000-mu m mesh sieves, increased with size class: 0.36, 0.65, and 0 .88 mg Cd/L, respectively, Gravid females were less sensitive than mal es or mature females to aqueous Cd. Studies on the influence of the mo lt cycle on Cd toxicity indicated enhanced sensitivity of immediate po stmolt animals that may explain some of the observed differences in Cd tolerance. Nutritional effects were investigated by comparing the sen sitivity of fed and starved laboratory-reared amphipods. Starved juven iles and adults were significantly smaller than their fed counterparts and exhibited a 28-43% reduction in Lipid content, respectively, Howe ver, comparison of LC50 values indicated no significant differences in sensitivity to Cd between starved and fed juveniles (0.23 vs 0.30 mg Cd/L) or adults (0.37 vs 0.52 mg Cd/L). Field-collected amphipods were typically more sensitive to Cd than laboratory animals, regardless of the season, although their Lipid content varied, ranging from 6.6% in August to 13.7% in November. Results are discussed with respect to th e use and interpretation of toxicity tests with this species.