Stable isotopes reveal that chironomids occupy several trophic levels within West Greenland lakes: Implications for food web studies

Stable isotope analyses of nitrogen (delta N-15) and carbon (delta C-13) were conducted on individual chironomid taxa from low arctic lakes to investigate the range in trophic levels covered by this diverse insect group. Five lakes were sampled, including two freshwater, two oligosaline, and one glacier influenced lake, representing the major lake types in southwest Greenland. There was a large difference in the offset of isotopic values among the different lake types. In the oligosaline lakes, we ascribe low delta C-13 values to the use of recycled CO2, while the high delta N-15 signal is possibly controlled by microbial processes. In order to compare among lakes, the delta N-15 signal of the chironomid taxa was normalized to common primary consumers (Psectrocladius sordidellus group and Psectrocladius limbatellus groups) that show consistently low delta N-15 values and belong to the subfamily Orthocladiinae. Normalized delta N-15 values spanned more than 5%, indicating that the chironomid community covered at least two trophic levels in these low arctic lakes. The delta N-15 values of different chironomid taxa within a single habitat differed by 1% to 5 parts per thousand. The results suggest that chironomids should not be considered as one group in food web studies. Even interpretation of trophic position on subfamily level can be problematic as a result of the large differences observed in their delta N-15 values. The level of detailed trophic information on individual chironomid taxa presented here has not previously been reported. Chironomids are an abundant and important group of organisms in arctic lakes, and the observed variation in their trophic level indicates a high complexity of the food web structure of arctic lakes.