Experimental δ13C evidence for a contribution of methane to pelagic food webs in lakes

We tested the hypothesis that low stable carbon isotope (delta C-13) values commonly observed for zooplankton in humic lakes are due to their feeding on isotopically light methane-oxidizing microbes, and thus that methane-derived carbon is important in the food webs of these lakes. In replicate laboratory cultures, Daphnia longispina, a common crustacean zooplankter in humic lakes, were fed microbial suspensions with or without enrichment by biogenic methane. The delta C-13 values of Daphnia indicated consumption of C-13-depleted methanotrophic bacteria, while growth rates, survival, and reproduction of Daphnia in cultures enriched with methane were equal to or greater than those in nonenriched cultures. Results from lake enclosures during the autumn overturn period revealed a decrease in delta C-13 of adult Daphnia from -40.5 parts per thousand to -50.3 parts per thousand, reflecting extensive consumption of C-13-depleted methanotrophic bacteria. Methane-derived carbon is a more important contribution to carbon flux through lake pelagic food webs than has previously been suspected.