Methane‐Derived Carbon Contributes to Fish Biomass in a Deep, Eutrophic Reservoir

ABSTRACT Methanotrophic bacteria oxidise large amounts of biogenic methane produced in freshwater and bind the original methane carbon in their biomass. When these bacteria are consumed by other organisms, methane‐derived carbon enters food webs. Methane‐derived carbon can support a substantial part of invertebrate biomass in some freshwater ecosystems. However, the transfer of methanogenic carbon further up the food web to higher‐level consumers has rarely been studied. Here, we used stable isotopes and mixing models to investigate the quantitative contribution of methane‐derived carbon to fish consumers in a deep and eutrophic reservoir, the Římov Reservoir, Czechia. We measured δ 13 C and δ 15 N in the five most abundant fish species, bleak ( Alburnus alburnus ), bream ( Abramis brama ), roach ( Rutilus rutilus ), perch ( Perca fluviatilis ) and ruffe ( Gymnocephalus cernua ), and their potential food sources, such as littoral macroinvertebrates, pelagic zooplankton, and profundal chironomid larvae. We expected that profundal chironomids would be the main agents for transferring methane carbon to higher‐level consumers, and thus we predicted that fish species better adapted to foraging in the profundal zone, such as bream and ruffe, would assimilate more methane‐derived carbon. The isotope values of littoral macroinvertebrates and pelagic zooplankton were too high to suggest any significant incorporation of methane‐derived carbon. In contrast, both the δ 13 C and δ 15 N values of profundal chironomids were very low. The results from an isotope mixing model considering methanotrophic bacteria and sedimentary organic carbon as sources indicated that profundal chironomids obtained 35%–71% of their carbon biomass from methane. Using these outcomes and the contributions of profundal prey to the five fish species, we estimated that the population carbon biomass derived from methane ranged from 0.2%–0.5% for perch to 4.2%–8.6% for ruffe. In line with our predictions, we found that bream and especially ruffe contained more methane‐derived carbon than the other fish species. We concluded that 2.2% of the total fish carbon in the Římov Reservoir could have come from biogenic methane. Our results suggest that partial support of fish communities in eutrophic lentic waters by methane‐derived carbon may be a common but hitherto overlooked phenomenon.