Seasonal increase of methane in sediment decreases δ13C of larval chironomids in a eutrophic shallow lake

Recent studies have shown that larval chironomids assimilate 13C-depleted carbon derived from biogenic methane by feeding on methane-oxidizing bacteria (MOB). The dietary contribution of MOB is known to be maximized in the autumn overturn period or winter in eutrophic dimictic lakes due to the increase of MOB biomass following the supply of oxygenated water, but in polymictic lakes, such seasonal variability has not been revealed. We investigated the seasonal patterns of larval δ13C and methane concentrations in the sediment of a eutrophic polymictic lake, Izunuma, Japan. Larval δ13C decreased in late summer and autumn. Methane concentrations above a 6 cm depth peaked in late summer or autumn, while those in the 10–11- and 20–21-cm layers peaked in October. Negative correlations between methane concentrations in the 5–6/10–11-cm layers and larval δ13C were found. This suggests that an increase in the supply of methane stimulated the activity of MOB in a polymictic lake, where water above the lake bottom rarely became anoxic because of frequent overturn, thus increasing the dietary contribution of MOB to larval chironomids.