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Abstract:
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 δ¹³C and δ¹⁵N. Intraspecific variability in larval δ¹³C and δ¹⁵N 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 35parts per thousand for δ¹³C and 16% for δ¹⁵N (equivalent to five trophic levels). There was a strong positive relationship between larval δ¹³C and δ¹⁵N, 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
