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

Abstract. Algal blooms in lakes result in large amounts of labile carbon being transported down the water column towards the sediments, often resulting in temporary water column hypoxia. The algal biomass is deposited at the surface sediments, where it is degraded by the microbial community. Negative effects of algal blooms and biomass depositions are sometimes mitigated by pumping air or oxygen into the bottom waters of lakes. The fate of the algal biomass, in terms of greenhouse gas release, is however often unknown. 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 difference in the control experiments based on the state 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 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.