High intraspecific variability in carbon and nitrogen stable isotope ratios of individual lake chironomid larvae

Stable isotope analyses of chironomid trophic interactions have recently indicated the potential importance of isotopically light biogenic methane as a carbon source. Mass balance of isotope ratios suggests that small proportional differences in ingestion of such an isotopically distinct basal resource by individual consumers can result in considerable intraspecific variability. To test this, we collected individual larvae of two closely related chironomid species (Chironomus anthracinus and Chironomus plumosus) from six lakes and analyzed their 13C and 15N. Intraspecific variability in larval 13C and 15N values was greater in lakes where chironomids were more 13C depleted. C. plumosus exhibited higher intraspecific variability relative to C. anthracinus. In two lakes, individual C. plumosus exhibited a range of 35‰ for 13C and 16‰ for 15N (equivalent to five trophic levels). There was a strong positive relationship between larval 13C and 15N, both between individuals from the same lake and also between lakes, suggesting that the underlying causative mechanisms are similar. Furthermore, larvae from deeper sites, which are more susceptible to prolonged anoxia, exhibited greater intraspecific variability, and larger larvae were significantly 13C depleted. Such high intraspecific variability can confound the interpretation of benthic food web stable isotope values. We advocate the reporting of more intraspecific isotopic variability as a means to further examine niche breadth and feeding behavior