Tom M. Dillon

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 Daphnia magna. Sediments differed in their effects on survival, age at first reproduction, the number of broods produced and the total number of young 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 matrix population model was constructed and parameterized with survival and reproduction data from each sediment. By including estimates of variability in vital rates, the model output provided quantitative estimates of uncertainty in projected population size. Sediment effects on survival and reproduction translated into large differences in projected population growth; mean estimates of projected population size at day 28 of the simulations ranged over two orders of magnitude among the 17 sediments. Reproductive timing (e.g. age at first reproduction), followed by fecundity and survivorship, had the largest effect on population growth. Results of this study also indicate that the presence of suspended sediment in elutriates may confound toxicity evaluations using cladocerans. The concentration of total suspended solids was negatively correlated with age at first reproduction and positively correlated with measures of fecundity and population growth. In order to realize the potential benefits of chronic testing we must develop ecologically relevant ways of interpreting sediment bioassay results and expressing the uncertainty associated with our estimates of ecological risk.

Intraspecific density effects in Nereis (Neanthes) arenaceodentata Moore (Polychaeta: Nereidae)

Effects of intraspecific density on Neanthes arenaceodentata Moore were examined under laboratory conditions by rearing animals for 28 days, in the presence of sediment, at densities of 230, 575, 1150, and 1730 worms/m2. The lowest density examined in this study was similar to the mean reported field density for N. arenaceodentata. Food was allocated to each beaker on a per worm basis to focus the experiment on competition for space. Individual worm growth increased with increasing density. This result contrasts with other studies of intraspecific density effects in this species conducted in the absence of sediment where negative effects on growth were observed. Estimated individual dry weight and ash free dry weight of worms in the highest density treatment were significantly greater (13 and 18%, respectively) than worms in the lowest density treatment at the end of 28 days of growth. This increase in growth was associated with a decrease in the number of ovigerous females, suggesting a delay in reproductive development, and an increase in the number of worms regenerating posterior segments, suggesting an increase in the number of aggressive encounters. At the end of the 28 day growth experiment, mated pairs were established in separate beakers. Intraspecific density did not have a lasting effect on time to egg deposition and juvenile emergence or fecundity. The benefits of larger body size at higher densities may include the fact that larger worms are more successful at defending territories and finding mates. The results of this study emphasize the importance of sediment as a structuring agent in soft substratum habitats and the potential for complex density dependent effects.