Deriving effects on Chironomus population carrying capacity from standard toxicity tests.

Translating effects from individual to population level is a crucial issue in ecological risk assessment of bed sediments. We first propose a modeling package to tackle this question for the species Chironomus riparius with biology- and ecology-based models to deduce effects on chironomid production per generation or per year. However, this approach requires the performance of many toxicity tests. Currently, the information available to perform this scaling is limited to a few data from standardized tests, mainly growth and survival after a certain period of time and sometimes emergence after 28 d. We propose here to simplify our model to make it able to deal with only data from standardized tests. The daily death rate is assumed to be constant throughout each instar, the effects on growth to be independent of time, and the effects on reproduction to be negligible. The reliability of this approach is first tested by comparing the population effects on organisms exposed to copper-spiked sediments calculated with the simple model and the more exhaustive one from which it was derived. The simple model just slightly underestimated the effects at the population level. Second, data from tests performed either with unpolluted sediment or with field sediment with different degrees of contamination are used to propose an effect threshold for populations. It appears that control sediments lead to less than 20% effects at population level, which may help define a natural variability range. For polluted sediments, we could find effects up to 42%. Implications of these findings are discussed in a sediment risk assessment perspective.