The fate of organic carbon

In Lake Kinneret, the majority of photosyntetically produced organic carbon (OC) is cycled through the microbial loop. Taken together, bacterial production (BP) and bacterial respiration (BR), i.e., bacterial carbon demand (BCD), accounted for about 65 % of gross primary production (GPP), measured biweekly and averaging 2.3 g C m^-2day^-1during the last decade (2001-2011). Community respiration (CR) was 2.1 g C m^-2day^-1.The major contributors to total CR were bacterial and phytoplankton respiration (~80%) while zooplankton respiration accounted for the reminder. Most (~ 83 %) of the OC input were eventually respired, ~3 % lost to outflows, while ~15 % of the total OC input were transferred annually to the sediments. Here oxic mineralization is gradually replaced by anoxic processes as a function of the availability of suitable electron acceptors. After the depletion of oxygen in the hypolimnion, sulfate (500 μM) becomes the dominant oxidant. Depending on the settling flux of OC sedimentary sulfate reduction (SR) rates were measured from 0.01 to 1.67 μmol cm^-3day^-1in December and July, respectively. SR is the dominant anaerobic terminal decomposition process in Lake Kinneret and is responsible for the accumulation of sulfide in the hypolimnion to concentrations up to 400 μM. Methanogenesis is restricted to those sediment layers that are depleted of sulfate (below 3-5 cm). Seasonal profiles and 13C signatures of dissolved methane in the sediment pore water of Lake Kinneret have indicated anaerobic methane oxidation in the deeper sediments (below 20 cm), with Fe(III) as electron acceptor. Lake Kinneret resembles the first aquatic ecosystem where the existence of this process could be verified. Changes in the watershed and lake environment are suggested as possible causes for the apparently significant declines in bacterial numbers, BP, and BCD that have taken place over the last decade in Lake Kinneret.