Comparison of in situ and in vitro rates of methane release in freshwater sediments.

Anaerobic lake sediment incubated in vitro was investigated for its ability to mimic natural in situ sediment activities, using rate of methane production for the comparison. Two lakes with different rates and seasonal patterns of methanogenic activity were compared. There was good agreement (at the 97.5% confidence level) between rates of in situ methane release and initial (lasting an average of 120 h) rates of production measured in vitro in surface (0- to 3-cm) sediment. Evidence from this study, and others, indicated that it is the in situ surface sediment methane production which is primarily responsible for maintaining in situ methane release, and thus the above agreement was what was expected if surface in situ activity was maintained in vitro. When deeper sediment was investigated, however, the sum of in vitro rates from 0 to 20 cm (measured in 1.5- to 3-cm intervals) was much higher than in situ release rates and would have resulted in an impossibly high volume of gas. The extra gas could not have been stored within the sediments. We conclude that the in situ methanogenic activity of the 0- to 3-cm anaerobic surface sediments could be preserved during removal and laboratory incubation. However, similar treatment of deeper sediment appeared to stimulate methanogenic activity.