Spatial variations in surface water methane super-saturation and emission in Lake Lugano, southern Switzerland

We measured methane concentrations in the surface water of the northern basin of Lake Lugano in spring (May 2012) and autumn (October 2011, 2012), and calculated turbulent diffusive methane fluxes to the atmosphere. Surface water methane concentrations were highly variable in space and time but always exceeded atmospheric equilibrium. Methane concentrations were significantly lower in spring (on average 16 nmol L-1) than during the autumn sampling campaigns (on average 57 nmol L-1 in 2011 and 45 nmol L-1 in 2012). This suggests methane accumulation in the surface mixed layer during the summer productive season. The origin of the methane in the lake's surface waters requires further assessment, but the observed concentration profiles indicate that the excess methane originates from a near-surface source, rather than from the large deep-water methane pool in the anoxic monimolimnion. As a consequence of the higher surface water methane concentrations and increased buoyancy turbulence caused by autumnal cooling of the surface boundary layer, diffusive fluxes were much higher in October (average similar to 97 mu mol m(-2) day(-1), compared to 7 mu mol m(-2) day(-1) in May 2012). The increase in methane concentration in the surface water between spring and autumn suggests links between methane accumulation and the annual biological cycle, yet seasonal changes in wind and temperature forcing of methane emission likely play an important modulating role. While the relative importance of biological versus physical controls on methane emission in Lake Lugano awaits further investigations, our study underscores that lakes can act as an important source of methane to the atmosphere, even when the lake-internal microbial methane filter in the water column seems to work efficiently.