Stable isotope analysis of chironomid larvae from some Finnish forest lakes indicates dietary contribution from biogenic methane

Larval chironomids (mostly Chironomus tenuistylus) were collected from the sediments of five small forest lakes in southern Finland during summer 2002. Carbon and nitrogen isotope ratios of larvae were determined by isotope ratio mass spectrometry. Isotope ratio variability among 23 individuals from one lake was appreciable, with [ta13C showing a range spanning 1.6% and [ta15N values a range spanning 5.6%. Between the lakes, larvae showed considerable variation in [ta13C with values as low as –55%. [ta15N values were strongly correlated with [ta13C values. The 13C-depletion found in the benthic chironomid larvae correlated with that found previously in zooplankton from the same lakes. It is argued that the low [ta13C values are most likely attributable to incorporation of carbon from biogenic methane, and that methane production can therefore make an important contribution to benthic as well as to pelagic food webs in these small forest lakes.