High chemoautotrophic primary production in Lake Kinneret, Israel: A neglected link in the carbon cycle of the lake

Intensive chemosynthetic microbial activity fueled by H2S oxidation with di ssolved O-2 was measured by C-14 fixation in the dark and in presence of 3- (3,-dichlorophenyl)-1,1-dimethylurea in Lake Kinneret waters. This process occur-red in water collected below the photic zone (20 m) at the chemocline in the late autumn (November-January) and close to the sediment water inte rface in May when the chemocline starts to form. Averaged depth-integrated chemoautotrophic primary production at the chemocline was 16% and 24% of th e photosynthetic primary production during May and autumn. respectively. Th e maximal rates were measured in December 1992, reaching values of > 90% of the photosynthetic rate. The delta C-13 of particulate organic matter at t he chemocline ranged between -27 parts per thousand and -39 parts per thous and, the latter being associated with intensive chemosynthesis. These V del ta C-13 values support our earlier hypothesis that chemoautotrophic bacteri a constitute. directly or indirectly (through the microbial loop), a C-13-d epleted food source for the zooplankton in the lake during autumn and early winter. Mass and isotopic balance of carbon and H2S suggest that chemosynt hetic productivity may constitute 20%-25% of the primary production in Lake Kinneret annually.