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

Abstract

We studied methanotrophic activity in the water column in relation to heterotrophic bacterial production and efflux of methane (CH 4) 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 23 . 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 22 d 21 ) 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 22 during 6.5 months) was of the same order of magnitude as that during the ice-free period (0.22 mol CH4 m 22 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 CH 4 m 22 yr 21 ) was released to the atmosphere. During the ice-free period, bacterial production measured as [ 14 C]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 23 . 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.