Abstract
Intensive chemosynthetic microbial activity fueled by H 2 S oxidation with dissolved O 2 was measured by 14 C fixation in the dark and in presence of 3‐(3,4‐dichlorophenyl)‐1,1‐dimethylurea in Lake Kinneret waters. This process occurred in water collected below the photic zone (20 m) at the chemocline in the late autumn (November—January) and close to the sediment water interface in May when the chemocline starts to form. Averaged depth integrated chemoautotrophic primary production at the chemocline was 16% and 24% of the photosynthetic primary production during May and autumn, respectively. The maximal rates were measured in December 1992, reaching values of >90% of the photosynthetic rate. The δ 13 C of particulate organic matter at the chemocline ranged between −27‰ and −39‰, the latter being associated with intensive chemosynthesis. These δ 13 C values support our earlier hypothesis that chemoautotrophic bacteria constitute, directly or indirectly (through the microbial loop), a 13 C depleted food source for the zooplankton in the lake during autumn and early winter. Mass and isotopic balance of carbon and H 2 S suggest that chemosynthetic productivity may constitute 20%–25% of the primary production in Lake Kinneret annually.