Methane production in littoral sediment of Lake Constance

Maximum rates of CH4 production in the littoral sediment were observed in 2-5 cm depth. The CH4 production rates increased during the year from about 5 mmol m-2d-1 in December to a maximum of about 95 mmol m-2d-1 in September. CH4 production rates showed a temperature optimum at 30-degrees-C and an apparent activation energy of 76 kJ mol-1. A large part of the seasonality of CH4 production could be ascribed to the change of the sediment temperature. Most of the produced CH4 was lost by ebullition. Gas bubbles contained about 60-70% CH4 with an average deltaC-13 of -56.2 parts per thousand and deltaD of -354 parts per thousand, and 2% CO2 with an average deltaC-13 of -14.1 parts per thousand indicating that CH4 was produced from methyl carbon, i.e. mainly using acetate as methanogenic substrate. This result was confirmed by inhibition of methanogenesis with chloroform which resulted in an accumulation rate of acetate equivalent to 81% of the rate of CH4 production. Most probable numbers of methanogenic bacteria were in the order of 10(4) bacteria g-1d.w. sediment for acetate-, methanol- or formate-utilizing, and of 10(5) for H-2-utilizing methanogens. The turnover times of acetate were in the order of 2.3-4.8 h which, with in situ acetate concentrations of about 25-50 muM, resulted in rates of acetate turnover which were comparable to the rates of CH4 production. The respiratory index (RI) showed that [2-C-14]acetate was mainly used by methanogenesis rather than by respiratory processes, although the zone of CH4 production in the sediment overlapped with the zone of sulfate reduction

Fate of methane bubbles released by pockmarks in Lake Constance

In the eastern part of Lake Constance, the second largest pre-alpine Lake in Europe, about five hundred pockmarks (morphological depressions on the lake floor) were recently discovered of which ~ 40% release methane bubbles. The carbon isotopic composition of the escaping gas indicated that the methane is of biogenic origin. In our study, we investigated the fate of the released methane bubbles, i.e., the dissolution, oxidation or transport of the bubbles to the surface. At a littoral pockmark site (PM12, 12 m water depth) and a profundal pockmark (PM80, 80 m water depth), we analysed the dissolved methane concentrations and the methane isotopic carbon signature in the water column. At PM80, higher methane concentrations (up to 1523 nM), compared to the control site and the surface waters (225 ± 72 nM), were recorded only on some occasions and only in the bottom water, despite the fact that the released bubbles were dissolving within the hypolimnion based on bubble modeling. The isotope data suggest that most of the dissolved methane is oxidized below 40 m water depth. The isotopic signature of the methane in the surface water at PM80, however, differed from that of the methane in the hypolimnion; therefore, the surface methane at this profundal site is most likely an export product from the littoral zone. Assuming an initial bubble diameter of 5 mm, we calculated that these small bubbles would reach the surface, but approximately 96% of the methane would have dissolved from the bubble into the hypolimnion. At PM12, we observed higher concentrations of dissolved methane (312 ± 52 nM) with no significant differences between seasons or between control sites versus pockmark site. In the shallow water, divers estimated the bubble size to be 10 - 15 mm, which from a release depth of 12 m would barely dissolved in to the water column. The isotopic signature also indicated that there had been almost no methane oxidation in the shallow water column. Thus, the water depth of bubble release as well as the initial bubble size determine whether the methane enters the atmosphere largely unhindered (shallow site) or if the released methane is incorporated into the profundal water column

Lake characteristics influence how methanogens in littoral sediments respond to terrestrial litter inputs.

Shallow lake sediments harbor methanogen communities that are responsible for large amounts of CH4 flux to the atmosphere. These communities play a major role in degrading in-fluxed terrestrial organic matter (t-OM)-much of which settles in shallow near-shore sediments. Little work has examined how sediment methanogens are affected by the quantity and quality of t-OM, and the physicochemical factors that shape their community. Here, we filled mesocosms with artificial lake sediments amended with different ratios and concentrations of coniferous and deciduous tree litter. We installed them in three boreal lakes near Sudbury, Canada that varied in trophic status and water clarity. We found that higher endogenous nutrient concentrations led to greater CH4 production when sediment solar irradiance was similar, but high irradiance of sediments also led to higher CH4 concentrations regardless of nutrient concentrations, possibly due to photooxidation of t-OM. Sediments with t-OM had overall higher CH4 concentrations than controls that had no t-OM, but there were no significant differences in CH4 concentrations with different t-OM compositions or increasing concentrations over 25%. Differences among lakes also explained variation in methanogen community structure, whereas t-OM treatments did not. Therefore, lake characteristics are important modulators of methanogen communities fueled by t-OM.Natural Environment Research Council grant NE/L006561/1 and NBAF Grant NBAF968 to AJ