Primary production

The synthesis of organic compounds from aqueous carbon dioxide, the aquatic primary production, principally occurs through the process of photosynthesis, which uses light as its source of energy, but it also occurs through chemosynthesis, which uses the oxidation or reduction of chemical compounds as its source of energy. Here, we present both pathways and their contribution to the primary production in Lake Kinneret. Phytoplankton density, measured as chlorophyll a (Chl a), and photosynthetic primary productivity (PP), measured as radiocarbon isotope uptake, have been monitored in the lake for more than four decades. The average Chl a areal concentration in the lake is 208 mg m^-2 and the PP is 1.66 g C m^-2 day^-1. Both parameters displayed marked seasonal patterns, with highest values occurring in April and May. From July to December, Chl a and PP values were on average only 24 and 64%, respectively, of the spring maxima. However, since the mid-1990s, a definite weakening of the seasonal periodicity of Chl a and PP was observed. Intensive chemosynthetic microbial activity, fueled by H₂S oxidation, was measured at the chemoclines during its deepening below the photic zone in late autumn, 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 in May and during autumn, respectively. The δ¹³C of particulate organic matter at the chemocline ranged between - 27 and - 39%, suggesting the involvement of intensive chemosynthesis. Mass and isotopic balance of carbon and H₂S suggest that chemosynthetic production contributes between 20 and 30% of the total primary production in Lake Kinneret annually. This component should be considered when the lake’s carbon budgets and food webs are assessed.