Methanotrophic activity in relation to methane efflux and total heterotrophic bacterial production in a stratified, humic, boreal lake

We studied methanotrophic activity in the water column in relation to heterotrophic bacterial production and efflux of methane (CH4) from the lake surface in a small, stratified, humic, boreal lake (Valkea-Kotinen, southern Finland). During summer and winter stratification, the highest methanotrophic activities were in the metalimnion, where oxygen concentration was <6 mmol m−3. During an incomplete spring turnover and summer stratification period, 3-5 times more CH4 was consumed by methanotrophs in the water column than was released to the atmosphere. The highest CH4 effluxes (1.2–5.1 mmol m−2 d−1) to the atmosphere occurred during the autumnal turnover despite observed methanotrophic activity in the whole water column. In winter, the amount of CH4 consumed by methanotrophs (0.20 mol CH4 m−2 during 6.5 months) was of the same order of magnitude as that during the ice-free period (0.22 mol CH4 m−2 during 5.5 months). Annually ~80% of CH4 diffused from the sediment was consumed by methanotrophs in the water column, and only 20% (0.11 mol CH4 m−2 yr−1) was released to the atmosphere. During the ice-free period, bacterial production measured as [14C]leucine uptake showed a bell-shaped relation to CH4 concentration. The highest production was found in the metalimnion at CH4 concentrations ranging from 5 to 10 mmol m−3. During summer stratification, net production of methanotrophs corresponded to 23–47% of total bacterial production, but during the autumn turnover, this proportion was higher (27–81%), indicating that methanotrophs offer a potentially significant source of carbon to zooplankton in stratified humic lakes.