Numbers of heterotrophic, methane oxidizing and sulfate reducing bacteria were counted in Lake Vechten. A dynamic distribution pattern was found in the stratified lake. A maximum of heterotrophs (numbers of ~ 109 bact./l) occured in the deepest part of the lake in spring and in the metalimnion during summer-stratification. These bacteria use nearly all available oxygen in the hypolimnion. It was found that the concentration of available organic material and the oxygen tension caused the numbers ofheterotrophs in the metalimnion to be high.
The maximal numbers of methane oxidizers (numbers of > 5.105 bact./1) were found at a depth of maximal methane concentration: the de-oxygenated hypolimnion. Preliminary evidence indicated that these organisms were facultative methane oxidizers and must be regarded as micro-aerophyllics. By oxidizing methane they removed the residual oxygen under the metalimnion.
The sulfate reducing bacteria could be observed in the hypolimnion only. Decreased SO-concentration and increased numbers, of bacteria were found in the bottom water. An association between the methane oxidizers and the sulfate reducers could be deduced. It was assumed that favourable redox requirements for obligate anaerobic sulfate reducers were the results of the activities of the methane oxidizing bacteria.
The dynamic distribution equilibrium of the investigated groups of bacteria was disturbed by the autumn turn-over. The heterotrophic and methane oxidizing bacteria decreased in number at