The methane cycle in the epilimnion of Lake Constance

During summer, methane concentrations measured in the surface water layers of Lake Constance were oversaturated by as much as forty times. Although me thane concentrations were less than saturated in the upper water column dur ing the winter of 1994/95, the concentrations increased to more than 2000 n l/l (90 nM) in July. The observed spatiotemporal distribution patterns stro ngly suggest biogenic production of methane in oxic epilimnetic water layer s whereas advective transport from the littoral to the pelagic region or fr om allochthonous river inputs had apparently less impact on pelagic methane concentrations. Statistically significant correlations of methane concentr ations with biomass parameters (e.g. phaeophytin, particulate phosphorus) h int at lake-snow as a potential site of epilimnetic in situ production of m ethane. In contrast, field data did not provide compelling evidence of meth anogenesis in the intestinal tract of zooplankton. In diel cycles, high noc turnal methane concentrations alternate with low concentrations of C-13-enr iched methane during the daytime. This happens due to the phenomenon of osc illating oxygen concentrations in littoral water and sediment, which is aff ected by the antagonism of photosynthetic and respiratory activity. Due to the observed oversaturation, epilimnetic methane levels in summer an d methane concentrations below saturation in winter, Lake Constance provide d a sink for methane from January to April, but for the rest of the year a considerable methane flux to the atmosphere was calculated.