In the surface sediments below the oxic-anoxic interface in meromictic Lake Malawi, there is little or no evidence of biological activity. As the water depth increases, even in sediments within the oxic zone microbial activity, as measured by 14C leucine incorporation, decreases; the benthic invertebrates decrease in biomass and diversity; and the amount of detrital material remaining in the sediments increases from <1% at 10 m to >14% at 125 m. This relationship with depth could be owing to several factors. At shallower depths, there is terrestrial material available for bacteria to use, as well as detrital material derived from autochthonous production. This detrital material can be degraded in the sediments by the benthic invertebrates, which release dissolved organic carbon and smaller particles through excretion that bacteria can use. As the water depth increases, the bacteria in the water column are able to degrade much of the planktonic debris, leaving the more refractory carbon particles to settle to the sediment. It appears that when there are enough benthic invertebrates, there is still substantial bacterial activity. As the depth increases, the benthic invertebrates disappear and, although there is still detrital carbon available, the bacteria appear unable to utilize it. We suggest that the benthic invertebrates are essential to the processing of detrital carbon and a shift in the degree of anoxia within the lake could seriously alter the cycling of carbon.

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