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. In dividual planktonic components as well as bulk plankton were isotopica lly lightest shortly after overturn in January and heaviest in May, af ter the peak of the annual bloom of the dinoflagellate Peridinium gatu nense. The range of deltaC-13 values observed over an annual cycle and the annual and seasonal means varied considerably between taxa. Withi n the primary producers, P. gatunense (range, - 23.2 to - 17.9 parts p er thousand) was significantly C-13-enriched relative to concurrent na noplankton (- 27.4 to - 19.0 parts per thousand). Zooplankton deltaC-1 3 showed indistinct taxon-specific differences but greater seasonal va riation (- 33.8 to -19.8 parts per thousand) than any phytoplankton co mponent examined. Adult fish exhibited smaller deltaC-13 variability t han the planktonic components. End-member isotope compositions confirm ed the linkages nanoplankton --> zooplankton --> Kinneret sardines, an d Peridinium --> Sarotherodon galilaeus. Likely grazing of zooplankton on isotopically heavy Peridiniopsis spp. in spring and on a yet undet ected 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 wit hin the pelagic food web of a freshwater system.