Stable isotopes and fatty acids reveal that Chironomus riparius feeds selectively on added food in standardized toxicity tests

During long‐term standardized toxicity tests with Chironomus riparius, food additions are a prerequisite for normal development and to avoid false‐positive results. Consequently, larvae may selectively feed on added food rather than on contaminated sediment, which may confound toxicity test results. We designed a feeding study and estimated the degree of feeding on different food resources by using stable isotope and fatty acid (FA) analyses. In one treatment, larvae were offered both artificial sediment (peat, kaolin clay, sand, and calcium carbonate) and added food (TetraPhyllr̀), whereas larvae in the two other treatments had access to either one of these potential food items. The highest biomass and survival were found among larvae with access to both artificial sediment and TetraPhyll. Two‐source mixing models revealed that larval Chironomus that were offered both TetraPhyll and artificial sediment obtained 94 ± 6.9% of their carbon and 90 ± 4.3% of their nitrogen from added TetraPhyll. Larvae with access to only sediment had lower Δ13C and Δ15N (–23.34 ± 0.56%‰ and 0.33 ± 0.52%‰) than those that were offered both sediment and TetraPhyll (–20.95 ± 0.13%‰ and 7.45 ± 0.36%‰) or only TetraPhyll (–20.17 ± 0.20%‰ and 7.82 ± 0.15%‰). In addition, FA composition of larvae that were offered both artificial sediment and TetraPhyll closely resembled that of those fed exclusively TetraPhyll. These results show that larval Chironumus strongly prefer added food, rather than artificial sediment in long‐term toxicity tests. This preferential feeding behavior affects exposure pathways and ultimately toxicity test results.