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The rate and quantity of methane produced by sediments of Wintergreen Lake were estimated by separately measuring methane lost by ebullition using bubble traps and by estimating methane lost to the water column by diffusion. The maximum rates of methane loss by ebullition occurred in late summer and were 35 mmol. rnw2. d-i in 1972 and 37 in 1973. The average dissolved methane flux for 1972 was estimated to be in the range of lo-46 mmol. rnw2. d-l. The minimum value was estimated by summing dissolved methane concentrations in the water column for each sampling date. The maximum value was estimated using the eddy diffusion coefficient of 4.9 x 10e3 cm2-s-l, calculated from the temperature method of Hutchinson.This methane diffusion rate was about two times the average rate of methane lost by ebullition (21 mmol*m-2*d-1) over the same time period. Dissolved methane was detected only in anoxic waters during summer stratification and within a meter of both the sediment and ice during winter. Ebullition also occurred during winter.

The kinetic parameters Ki, V,,,m, Tt (turnover time), and v (natural velocity) were determined for H2 and acetate conversion to methane by Wintergreen Lake sediment, using short-tern (a few hours) methods and incubation temperatures of 10 to 140C. Estimates of the Michaelis-Menten constant, Km, for both the consumption of hydrogen and the conversion of hydrogen to methane by sediment microflora averaged about 0.024 pmol g-' of dry sediment. The maximnal velocity, Vmax, averaged 4.8 ,umol of H2 g h-1 for hydrogen consumption and 0.64 ,umol of CH4 g-1 h-1 for the conversion of hydrogen to methane during the winter. Estimated natural rates of hydrogen consumption and hydrogen conversion to methane could be calculated from the Michaelis-Menten equation and estimates of Km, Vmax, and the in situ dissolved-hydrogen concentration. These results indicate that methane may not be the only fate of hydrogen in the sediment. Among several potential hydrogen donors tested, only formate stimulated the rate of sediment methanogenesis. Formate conversion to methane was so rapid that an accurate estimate of kinetic parameters was not possible. Kinetic experiments using [2-14C]acetate and sediments collected in the summer indicated that acetate was being converted to methane at or near the maximal rate. A minimum natural rate of acetate conversion to methane was estimated to be about 110 mnol of CH4 g-' h-', which was 66% of the V.,= (163 nmol of CH4 g-1 h-1). A 15-min preincubation of sediment with 5.0 x 10-3 atm of hydrogen had a pronounced effect on the kinetic parameters for the conversion of acetate to methane. The acetate pool size, expressed as the term Km + Sn (S. is in situ substrate concentration), decreased by 37% and Tt decreased by 43%. The Vmax remained relatively constant. A preincubation with hydrogen also caused a 37% decrease in the amount of labeled carbon dioxide produced from the metabolism of [U-14C]valine by sediment heterotrophs. Anaerobic decomposition is an important but little studied route of carbon flow in aquatic ecosystems. Methane production becomes the terminal step when alternative electron acceptors are absent, which is probably the case in most eutrophic lakes. For Wintergreen Lake, the subject of this study, we have estimated that as much as one-third of the primary production could be recovered as methane over a summer season (Strayer and Tiedje, Limnol. Oceanogr., in press), and Molongoski and Klug (unpublished data), in a 2-year study, have evidence that at least 39% of the sedimnenting carbon could be accounted for as methane. Thermodynamic considerations and reasoning from intert Journal Article no. 8256 of the Michigan Agricultural Experiment Station; contribution no. 344 of the W. K. Kellogg Biological Station.

A method of transferring dissolved H2 to a CO2 headspace and then absorbing out the CO2 to concentrate the H2 before gas chromatographic analysis was developed to measure low concentrations of dissolved H2. A detection limit of 10 pmol of H2 ml-l of water was achieved. When used to monitor H2 changes in a bovine rumen, a 10-fold increase in H2 was noted 1 h after feeding and then declined rapidly to the normal steady-state concentration of 1 ,uM.

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