SUMMARY: Lake Geneva is a large, holomictic, eutrophic lake with a maximum depth of about 300 m. The sediments in the central basin have a pillow-like appearance. The soft elevations containing the major portion of the recently sedimented detritus are separated by trenches of 5 to 15 cm depth in which the top sediment layers seem to be missing. Bottom-dwelling fishes () prefer the trenches as their habitat and might partly be responsible for the turbation of the trench sediment layers. Thus, within distances of 10 to 30 cm two sediment types can clearly be distinguished. They differ with respect to morphology and chemical stratification. Concentration depth profiles of nitrate, manganese(II), iron(II), sulphate, and methane dissolved in the interstitial water reveal the location within the sediment of microbially catalysed redox processes. The redox transition zone (RTZ) from aerobic to anaerobic is located only a few millimetres below the sediment surface in the pillow sediments, which contain the bulk of the organic detritus. The RTZ is at a depth of approximately 6 cm in the trench sediments, which are poor in oxidizable organic matter. The same thermodynamic sequence of microbially catalysed redox reactions can be observed in both sediment types. As a consequence, microbial activities as well as diffusion fluxes of dissolved substances in and out of the different sediment regions vary greatly. This leads to small-scale horizontal differences in the sediment's abilities to supply nutrients to the bottom water, which is probably a major controlling factor for sediment-borne eutrophication of this lake.


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