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 behaviour 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 TetraminÒ /larvae/day and 1.4 mg TetraminÒ /larvae/day) 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 hours 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 inferior to 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. / Nous avons récemment proposé un modèle pour relier nutrition, croissance et reproduction. Ici, nous utilisons ce modèle pour rendre compte des effets de confusion due aux caractéristiques des sédiments naturels au cours des tests de toxicité sur Chironomus riparius. Nous avons observé des effets limités : un léger retard de croissance est noté si le sédiment n'est pas sableux, et une amélioration de la croissance apparaît pour des sédiments dont le ratio C:N est inférieur à 14. L'extension proposée à notre modèle peut prendre en compte ces phénomènes. D'autre part, la reproduction est meilleure sur sédiment naturel que sur sédiment artificiel. La présence de lipides pourrait expliquer l'écart observé.