Planktonic community production and respiration and the impact of bacteria on carbon cycling in the photic zone of Lake Kinneret

The following parameters were determined in the photic zone of Lake Kinneret from January 2001 through December 2002: primary production (PP), community respiration (CR), bacterial biomass production (BBP), bacterial numbers (BN) and biological oxygen demand (BOD5). Average values over the 2 yr period for these parameters in the photic zone (0 to 15 m) were PP, 1539 mg C m(-2) d(-1); CR, 1653 mg C m(-2) d(-1); BBP, 887 Mg C m(-2) d(-1); BN, 4.6 x 10(6) cells ml(-1); and BOD5, 6366 mg C m(-2). We used these data together with literature-based assumptions about the ratio of net to gross primary production, bacterial and zooplankton respiration to estimate bacterial growth efficiencies (BGE) ranging from 44 to 64 %. Bacterial specific growth rates averaged 0.33 d(-1), ranging from 0.13 to 0.93 d(-1). CR was significantly correlated with both PP and BBP. A significant correlation was found between BBP and BOD5, suggesting that the indigenous bacterial populations were strongly dependent on substrates measured by BOD5. The potentially labile fractions of TOC, as quantified by BOD5, were rapidly cycled by heterotrophic bacteria, within similar to4 d on average. With the exception of 6 to 8 wk in early summer, the photic zone of this warm mesotrophic-eutrophic lake, with low inputs of allochthonous organic carbon, was net autotrophic. BBP and BGE values were high in comparison to those reported from even more eutrophic aquatic systems. Our results indicate that bacteria are by far the major biological agents of organic carbon cycling in Lake Kinneret, and contradict the general perception that the microbial loop plays a less important role in carbon cycling in eutrophic than in oligotrophic aquatic systems.