Oxidation, efflux, and isotopic fractionation of methane during autumnal turnover in a polyhumic, boreal lake

Abstract

We studied the oxidation and efflux of methane (CH4) in a small, polyhumic lake, Mekkojärvi (southern Finland), during 6 weeks in autumn when the stability of the water mass first weakened, temporarily restabilized, and finally mixed completely. During the summer stratification period, CH4 had accumulated in the anoxic hypolimnion to high concentrations (>150 mmol m−3). Gradual mixing of the water column during the autumn allowed access to both oxygen and CH4 by aerobic methane‐oxidizing bacteria (MOB) deeper in the water column. Thus the bulk (∼83–88%) of the CH4 accumulated in the hypolimnion was subsequently consumed by MOB while only 12–17% was lost from the lake to the atmosphere at a rate varying between 0.05 and 8.8 mmol m−2 d−1. Phospholipid fatty acid (PLFA) analyses revealed that type I methanotrophs (MOB I) were responsible for this CH4 oxidation. The stable carbon isotope ratio of CH4 in the water column (δ13C‐CH4) ranged from −81.2‰ close to the bottom to −45.1‰ at the surface. At CH4 concentrations >1 mmol m−3 the δ13C‐CH4 isotopic value was linearly related to the specific oxidation rate. The carbon isotopic fractionation factor α for aerobic CH4 oxidation calculated from the results (1.037) is within the range of maximum values reported in literature. Our results show that in stratified boreal lakes the greatest MOB activity and effluxes of CH4 both occur during autumnal mixing. Owing to their great abundance and areal cover, small water bodies should not be neglected in global analyses of the CH4 cycle.