Reports of unexpectedly 13C-depleted chironomid larvae in lakes have led to an hypothesis that significant transfer of detrital organic matter to chironomid larvae may occur via methane-cycle bacteria. However, to date little is known of how such transfer might vary across species and lakes. We gathered data from 87 lakes to determine how widespread this phenomenon might be and to define boundaries for its likely magnitude. Carbon stable isotope values of chironomid larvae varied greatly between taxa. Very marked 13C-depletion was evident only in certain taxa, especially Chironomus plumosus, C. anthracinus, and C. tenuistylus, all characteristic of eutrophic or dystrophic lakes and known to be tolerant of low oxygen conditions. Furthermore, marked 13C-depletion was only found in larvae from lakes in which late-summer hypolimnetic oxygen depletion near the sediment surface was below an apparent threshold concentration of 2-4 mg O2/L. Similarly, application of a two-source mixing model suggested that methanotrophic bacteria made the greatest contribution to profundal chironomid growth (0-70% of larval carbon) when the late-summer oxygen concentration dropped below approximately 2 mg O2/L. Our study demonstrates that methane-derived carbon is an important, but often neglected, contribution to the flux of carbon through the food webs of many productive or dystrophic lakes.

Chironomus anthracinus generally had a one year life cycle at 8 m depth in Lough Neagh, during the study period. In spring 1975 however only a proportion of the population emerged as usual, the rest remaining a second year in the benthos to emerge in 1976. The mean dry weight of larvae just before emergence was 2.8 mg in 1971 and 2.6 mg in 1974 but only 1.6 mg in 1975, and it seems that larvae must reach a weight of at least 1.0 mg in the early spring to emerge that year. Considering temperature, oxygen saturation and standing crop of phytoplankton as factors influencing the life cycle, mean monthly chlorophyll a (phytoplankton) was weakly correlated with mean monthly increase in weight and % monthly weight increase could be related to temperature.