Experimental d13C evidence for a contribution of methane to pelagic food webs in lakes

We tested the hypothesis that low stable carbon isotope (d13C) values commonly observed for zooplankton in humic lakes are due to their feeding on isotopically light methane-oxidizing microbes, and thus that methanederived 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 d13C values of Daphnia indicated consumption of 13C-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 d13C of adult Daphnia from 240.5% to 250.3%, reflecting extensive consumption of 13Cdepleted methanotrophic bacteria. Methane-derived carbon is a more important contribution to carbon flux through lake pelagic food webs than has previously been suspected. Although lake food webs have been traditionally described as based on algal primary producers, recent investigations have shown that most lakes worldwide are actually net heterotrophic, i.e., community respiration exceeds primary production (Cole et al. 1994; del Giorgio et al. 1999). This imbalance is greatest in lakes with high concentrations of dissolved organic matter (DOM) originating from the catchment (Salonen et al. 1983; Jansson et al. 2000). In small sheltered boreal lakes with a high concentration of allochthonous humic DOM, hypolimnetic anoxia is a typical phenomenon during summer and winter stratification. As a consequence of the anaerobic decomposition of organic matter in the sediment, the concentration of methane (CH4) may be high (.100 mmol m23) in the hypolimnion (Riera et al. 1999; Kortelainen et al. 2000). Most of the CH4 produced (50– 100%) is oxidized to CO2 in the water column in a metalimnetic oxic-anoxic interface zone and partly incorporated into microbial mass (Bastviken et al. 2003; Kankaala et al. 2006). Thus, methanotrophic bacteria could be an important, and hitherto largely ignored, carbon source for zooplankton in some lakes. Stable carbon isotope analyses (d13C) are now widely applied in studies of the sources and fluxes of organic matter in lake pelagic food webs (Grey et al. 2001; Pace et al. 2004). Crustacean zooplankton consume food items (algae, bacteria, heterotrophic protozoa) that can rarely be separated in field samples and hence are usually analyzed as bulk particulate organic matter (POM, .0.5 mm). Lake zooplankton are often 13C-depleted (lower d13C) relative to POM (del Giorgio and France 1996; Grey et al. 2000). This might be accounted for by selective feeding on isotopically light photosynthetic algae, which have utilized dissolved inorganic carbon (DIC) originating from microbial respi1 Email: [email protected]