CHRONIC TOXICITY OF GREAT-LAKES SEDIMENTS TO DAPHNIA-MAGNA - ELUTRIATE EFFECTS ON SURVIVAL, REPRODUCTION AND POPULATION-GROWTH

Seventeen Great Lakes sediments ranging in degree of expected toxicity were evaluated using a 21 day sediment elutriate bioassay with Daphni a magna. Sediments differed in their effects on survival, age at first reproduction, the number of broods produced and the total number of y oung produced per adult. Sediments producing low survivorship (50-60%) also had negative effects on reproduction. However, both positive and negative effects on reproduction were found among sediments producing high survivorship. To integrate all test end-points, a stochastic mat rix population model was constructed and parameterized with survival a nd reproduction data from each sediment. By including estimates of var iability in vital rates, the model output provided quantitative estima tes of uncertainty in projected population size. Sediment effects on s urvival and reproduction translated into large differences in projecte d population growth; mean estimates of projected population size at da y 28 of the simulations ranged over two orders of magnitude among the 17 sediments. Reproductive timing (e.g. age at first reproduction), fo llowed by fecundity and survivorship, had the largest effect on popula tion growth. Results of this study also indicate that the presence of suspended sediment in elutriates may confound toxicity evaluations usi ng cladocerans. The concentration of total suspended solids was negati vely correlated with age at first reproduction and positively correlat ed with measures of fecundity and population growth. In order to reali ze the potential benefits of chronic testing we must develop ecologica lly relevant ways of interpreting sediment bioassay results and expres sing the uncertainty associated with our estimates of ecological risk.