SEASONALITY OF STABLE CARBON ISOTOPES WITHIN THE PELAGIC FOOD-WEB OF LAKE KINNERET

Seasonal variations in the carbon isotope composition of components of the pelagic food web in Lake Kinneret were recorded and compared with those recorded for bulk plankton from the lake in the early 1970s. Individual planktonic components as well as bulk plankton were isotopically lightest shortly after overturn in January and heaviest in May, after the peak of the annual bloom of the dinoflagellate Peridinium gatunense. The range of deltaC-13 values observed over an annual cycle and the annual and seasonal means varied considerably between taxa. Within the primary producers, P. gatunense (range, - 23.2 to - 17.9 parts per thousand) was significantly C-13-enriched relative to concurrent nanoplankton (- 27.4 to - 19.0 parts per thousand). Zooplankton deltaC-13 showed indistinct taxon-specific differences but greater seasonal variation (- 33.8 to -19.8 parts per thousand) than any phytoplankton component examined. Adult fish exhibited smaller deltaC-13 variability than the planktonic components. End-member isotope compositions confirmed the linkages nanoplankton --> zooplankton --> Kinneret sardines, and Peridinium --> Sarotherodon galilaeus. Likely grazing of zooplankton on isotopically heavy Peridiniopsis spp. in spring and on a yet undetected light component in winter were implicated. The data demonstrate that the deltaC-13 of bulk particulate organic matter samples provides only a simplified view of a complex picture of deltaC-13 dynamics within the pelagic food web of a freshwater system.