The methane cycle in the epilimnion of Lake Constance

Abstract

During summer, methane concentrations measured in the surface water layers of Lake Constance were oversaturated by as much as forty times. Although methane concentrations were less than saturated in the upper water column during the winter of 1994/95, the concentrations increased to more than 2000 nlll (90 nM) in July. The observed spatiotemporal distribution patterns strongly suggest biogenic produc tion of methane in oxic epilimnetic water layers whereas advective transport from the littoral to the pelagic region or from allochthonous river inputs had apparently less im pact on pelagic methane concentrations. Statistically significant correlations of meth ane concentrations with biomass parameters (e.g. phaeophytin, particulate phosphorus) hint at lake-snow as a potential site of epilimnetic in situ production of methane. In contrast, field data did not provide compelling evidence of methanogenesis in the in testinal tract of zooplankton. In diel cycles, high nocturnal methane concentrations al ternate with low concentrations of I3 C-enriched methane during the daytime. This hap pens due to the phenomenon of oscillating oxygen concentrations in littoral water and sediment, which is affected by the antagonism of photosynthetic and respiratory activ ity. Due to the observed oversaturation, epilimnetic methane levels in summer and methane concentrations below saturation in winter, Lake Constance provided a sink for methane from January to April, but for the rest of the year a considerable methane fl ux to the atmosphere was calculated.

Oxic water column methanogenesis as a major component of aquatic CH4 fluxes

“…The enclosures contained very little particulate organic C because of the initial filtration of the surrounding lake water before filling, and there was little or no accumulation of particulate organic C in the bottom of enclosures at the end of the experiment. Moreover, we estimate that zooplankton-derived CH 4 , calculated from the measured zooplankton biomass in the enclosures and published production rates (Supplementary Table 1, Supplementary Note 2), contributed o10% to the estimated gross CH 4 production rates in ambient enclosures, a result that is consistent with previous studies 10,18,19,29 . Membrane permeability and the resulting exchange with the surrounding lake waters were also considered as a potential cause for CH 4 supersaturation.…”

“…3b). Consistent with this conclusion, observations of enriched surface d 13 C-CH 4 in numerous lakes 8,10,18,29,32 have been hypothesized to indicate dominance of acetoclastic methanogenesis in oxic freshwater pelagic zones 18 , and both known acetoclastic genera (Methanosaeta and Methanosarcina) have recently been detected in oxic freshwater 19,26 and terrestrial 27,28 environments. Furthermore, acetoclastic dominance was enhanced with the addition of nutrients and increased algal production (Fig.…”

“…The relationship between CH 4 and phytoplankton observed here (Fig. 2) has been hypothesized before to explain both the presence of metabolically active methanogens, and the recurrent metalimnetic and near-surface CH 4 peaks in oxic lake 9,10,18,19 and marine [13][14][15][16][17] environments. We confirm this link experimentally, and further show that it generates a significant out flux of CH 4 from the mesocosms to the atmosphere (Fig.…”

“…This assumption certainly holds for small, shallow ecosystems where the surface layers are in relatively close contact with sediments. Yet CH 4 supersaturation is also prevalent in large, deep lakes 3,5,[7][8][9][10][11][12] and oceanic surface waters [13][14][15][16][17] , where deeper water columns result in significantly reduced surface water exchange with anoxic sediments. Studies of CH 4 dynamics in surface waters of oceans and large lakes have indeed concluded that pelagic CH 4 supersaturation cannot be sustained either by lateral inputs from the littoral or from benthic inputs alone 9,10,[13][14][15][17][18][19] .…”

“…At the same time, marine studies suggest that the microbial decomposition of methylated compounds such as methanethiol 15 and methylphosphonate 16,[23][24][25] could be an important source of CH 4 in surface waters of the open ocean. Further, metalimnetic peaks of CH 4 are a recurrent feature in many ecosystems, which correlate positively with dissolved O 2 (DO), algal biomass and production [8][9][10]14,15,18,19 . However, despite past efforts, the ecological and biogeochemical significance of oxic water column methanogenesis remain speculative, and we have yet to determine the dominant biochemical pathway, its controls, and its contribution to diffusive CH 4 emissions from aquatic ecosystems.…”