Differences in feeding behaviour among Chironomus species revealed by measurements of sulphur stable isotopes and cadmium in larvae

1. We set out to determine the feeding behaviours of 15 Chironomus species collected from 16 boreal forest lakes by measuring cadmium (Cd) and stable isotopes of carbon (C), nitrogen (N) and sulphur (S) in larvae. 2. Measurements of S stable isotopes and Cd discriminated between Chironomus species that feed mainly on oxic particles from those that feed mainly on anoxic particles. Our results suggest that C. dilutus, C. entis, C. plumosus and C. staegeri feed mainly on oxic particles (in the water column, in sediment or both), whereas C. anthracinus, C. bifurcatus, C. cucini, C. decorus-group sp. 2, C. harpi, C. nr. atroviridis (sp. 2i), C. ochreatus, C. spp. NAI-NAIII and C. 'tigris' feed mainly on anoxic sediments. 3. In our study lakes, larval C isotopic signatures were relatively high, which suggests that they feed little on methanotrophic bacteria. Although larval C and N signatures differed between some sympatric Chironomus species, these differences were not consistent among lakes. The absence of a trend among lakes suggests that lake-specific factors determine larval C and N signatures. 4. Differences in feeding habits and Cd concentrations among sympatric (cohabiting) Chironomus species suggest that pooling them in ecological, palaeolimnological or ecotoxicological studies could obscure trends in nature, thereby limiting their use as monitors of climate change or pollution.