The effect of a short oxygen exposure period on algal biomass degradation and methane formation in eutrophic and oligotrophic lake sediments

Abstract

Eutrophication is suggested to enhance lacustrine methane emissions, due to enhanced sedimentary decomposition rates of algal biomass, and more frequent occurrence of water column anoxia. We investigated methane emissions from sediments originating from both a eutrophic and oligotrophic lake, and tested the effect of additional algal C inputs. Additionally, we investigated the effect of a pulse supply of oxygen, a mediating measure that is currently being used in the investigated eutrophic lake. Our results show a large legacy effect of eutrophication, but the methane release from new algal biomass additions was the same, although the process proceeded more rapidly in the eutrophic sediments. A short, 3-week pulse of oxygen lowered the emitted methane from both types of sediments by 50%, not only reducing the emissions of algal biomass additions, but also reducing methane emissions from the experiments without fresh organic matter inputs. This effect was relatively long-lasting: its effects were visible for several weeks after anoxic conditions were re-established, making it a potentially interesting measure to lower methane emissions over a longer period. Volatile fatty acid concentrations in the sediments were lowered due to oxygen exposure. Both the methanogenic and methanotrophic community composition showed surprisingly little response to the oxygen or algal biomass pulses. Overall, our results show that providing sediments with a brief pulse of oxygen following an algal bloom event, could strongly mediate the methane emissions following such an event. Such measures could be considered by policy makers to limit greenhouse gas emissions from managed lakes.