Bacterial methanotrophs drive the formation of a seasonal anoxic benthic nepheloid layer in an alpine lake

We investigated the formation and microbial composition of a seasonal benthic nepheloid layer (BNL) in the eutrophic, monomictic southern basin of Lake Lugano. During stratification, a BNL developed at the sediment‐ water interface and progressively expanded 20‐30 m into the water column, following the rising oxic‐anoxic interface. The dominance of the fatty acids C16:1v5 ,C 16:1v6 ,C 16:1v7, and C16:1v8, with d13C values between 262% (v6) and 280% (v7), suggests that the BNL was composed primarily of Type I aerobic methane oxidizing bacteria (MOB). Indeed, MOB contributed . 75% to the fatty acid carbon pool in the fully developed BNL, with cell densities up to 8.5 3 105 cells mL21. In ex situ incubation experiments, CH4 turnover rate coefficients were up to 2.1 d21, which translates into potential CH4 oxidation rates as high as 20 mmol m23 d21 under in situ CH4 concentrations. CH4 oxidation was limited by the diffusive supply of O2, and O2 consumption by aerobic CH4 oxidation (up to 13.1 mmol m22 d21) appears to be the primary driver of the seasonal growth of the BNL and expansion of the hypolimnetic anoxic zone. Methanotrophic activity at the interface between oxic and anoxic water masses can actuate the formation of a BNL, which in turn functions as an effective microbial CH4 filter in the water column, preventing CH4 transport to surface waters and evasion to the atmosphere. In situ biomass production by methanotrophic bacteria may represent, in addition to sediment resuspension and detritus trapping, a novel BNL formation mechanism.