A model to understand the confounding effects of natural sediments in toxicity tests with Chironomus riparius

Recently, we built a model to link feeding input with growth, emergence, and reproduction of the midge Chironomus riparius exposed to an artificial sandy sediment. This model is based on assumptions about both feeding behavior and use of energy. Here, we show how it can be used for toxicity tests with natural sediments to understand and model the influence of sediment characteristics. We measured growth, emergence, and reproduction of chironomids exposed in beakers to four unpolluted natural sediments and three feeding conditions (no feeding, 0.2 mg Tetramin/larva/d, and 1.4 mg Tetramin/larva/d) and compared the results with data obtained on our artificial sandy sediment. Sediment characteristics had lower influence on growth than feeding level, but their influence could not be neglected. First, we could distinguish between sandy sediments and other sediments. This difference resulted in a significant delay of about 18 h in the growth curves. Second, in case of food limitation, chironomids could use the organic materials in the sediment, provided that the C:N ratio of the sediment was less than 14. Our model proved to be able to incorporate those two phenomena. As for reproduction, we observed a better reproduction (measured in number of eggs per mass) for natural sediments than for artificial sediments. We showed that this difference could be due to the lipid content of the natural sediments.