Water quality management in Lake Kinneret (Israel): hydrological and food web perspectives

Long term (1969-2001) data record of nutrient and plankton temporal distribution, and hydrological parameters in Lake Kinneret, combined with metabolic parameters of zooplankton, which were experimentally measured, were statistically (ANOVA) analyzed. Trophic relations between food web compartments were quantitatively considered to evaluate directional combination of ecological forces. Monthly data of inflow discharges, and lake volume were used to calculate residence time values and the data were incorporated into the ecological analysis. The seasonal fluctuations of the hydrological parameters, nutrients, and plankton inventories represent typical subtropical climate conditions: high level in winter and low in summer months. It was found that nitrogen inventories in the lake declined and the biomass of grazable phytoplankton was enhanced since early 1980’s. Dissolved phosphorus was decreased mostly in summer months when the lake is nutrient limited, as a result of phytoplankton uptake. Zooplankton was declined until 1993 and increased later. Zooplankton preferably feed on chlorophytes and diatoms with supplemental resources of detritus, bacteria and protozoa. The most abundant zooplanktivorous fish, Lavnun (Bleak, Acanthobrama spp.) populated the lake very densely during 1993-95 and biomanipulation management of subsidized fishery caused lowering of predation pressure resulted in zooplankton enhancement and suppression of additional primary produced matter. It is concluded that zooplankton in Lake Kinneret is not food limited and fishery management (Lavnun removal) might be efficient to enhance zooplankton grazing capacity and algal suppression if phosphorus flux is reduced. Long term changes of nano-phytoplankton are affected by both phosphorus availability and zooplankton grazing and fish predation has a significant impact on zooplankton density. Fishery management aimed at algal suppression might be efficient if phosphorus supply is reduced. The similarity between seasonal fluctuations of food web components and residence time is a result of the natural subtropical conditions. The major effect on nutrient loads is due to inputs from the catchment and to a lesser extend to internal processes. Consequently, management implications aimed at lake water quality protection should be mostly directed towards nutrient removal in the drainage basin and loads reduction by pumping water for supply. When such a management is successfully implemented biomanipulation (fish removal) might be efficient.