Methane cycling in the sediment and water column of mid-boreal hyper-eutrophic Lake Kevätön, Finland

Methane (CH 4 ) dynamics were studied in the sediments and the water column of mid-boreal, hyper-eutrophic Lake Kevaton, Finland. We determined the CH 4 production and oxidation potentials in deep and shallow profundal sediments, water column CH4 oxidation potentials, and seasonal variation in CH 4 oxidation in water column in situ. The sediment CH 4 production and oxidation potentials were greater in summer than in winter. In the shallow profundal, the CH 4 oxidation potential of the surface sediment was equal to or higher than the sediment CH4 production potential, whereas in the deep profundal sediment, CH 4 production potential exceeded the CH 4 oxidation potential. The entire water column had the potential to oxidize CH 4 . The CH 4 oxidation rates (shown as negative values) in the water column in situ, ranged from 0 to -27 mmol m -3 d -1 , and were lower than the potential CH4 oxidation rates, which ranged from -9 to -160 mmol m -3 d -1 . The supply of CH 4 regulated its oxidation rate in water in situ, resulting in a clear seasonal variation in the level of CH 4 oxidation. The highest oxidation rates occurred in the hypolimnion during summer and winter stratifications when there were the highest CH 4 concentrations. During these stratifications, the CH 4 oxidation contributed significantly to the hypolimnetic O 2 consumption. In winter, there was CH 4 oxidation both in the aerobic and anaerobic water column. The depth integrated CH 4 oxidation in the water column was highest during stratification, in winter -31 and in summer -15 mmol m -2 d -1 . During the autumn overturn, the water column oxidized some CH 4 , but in spring, after the ice melt, the water column had a very low CH 4 production. Oxidation of CH 4 both in the sediments and water column reduced the emissions of CH 4 to the atmosphere.