Trophic transfer of methane-derived carbon in lakes: with special reference to benthic food chain pathways mediated by larval chironomids

Recent studies have provided evidence that consumers such as benthic larval chironomids assimilate methane-derived carbon ( MDC ) by foraging on methane-oxidizing bacteria in lake ecosystems. Trophic transfer of MDC can be detected by measuring carbon stable isotope ratios in consumers as biogenic methane is extremely 13 C-depleted. In this paper, I review trophic transfer of MDC mediated by larval chironomids. Trophic transfer of MDC is closely related to methane cycles ( i.e. methane production and oxidation ) in sediment, and is often enhanced by depletion of dissolved oxygen above the lake bottom. The robust tube built in sediment by larval chironomids could function as a microhabitat for methane-oxidizing bacteria, and promote trophic transfer of MDC. In stratified dimictic lakes, autumnal supply of oxygenated water to the lake bottom can stimulate the activity of methane-oxidizing bacteria, resulting in enhanced trophic transfer of MDC. In shallow polymictic lakes, transfer of MDC can be enhanced in late summer or early autumn when methane production increases. As dissolved oxygen could be depleted within aquatic plant vegetation even in shallow waters, seasonality of trophic transfer of MDC within vegetation could be similar to that in dimictic lakes. Although some studies reported that fish assimilated MDC, the extent to which MDC is transported to lake food webs is unclear. Trophic transfer of MDC has been mainly studied in deep stratified lakes. MDC may be transferred to aquatic consumers in shallow waters which are a hot spot of methane production, though further studies are necessary.