The long-term (22 yr) record of primary production in Lake Kinneret, Israel, has been examined, together with chlorophyll and microscopically determined algal biomass, in order to discern whether there have been any significant changes in these parameters during this period. During the period 1972 through 1993, annual averages have ranged from 1,223 to 2,3 11 mg C m-2 d-l for primary production, from 127 to 246 mg m-2 for chlorophyll, and from 39.3 to 98.5 g m 2 for algal wet weight. The annual peak of these parameters was in April-May. Over 22 yr, variability in primary productivity was more closely related to changes in chlorophyll than to changes in algal biomass. No evidence was found for consistently increasing long-term trends in primary production, chlorophyll concentrations, or algal wet weight biomass from 1972-1993. Although the annual and semiannual averages of algal biomass were significantly higher in the past 11 yr than those in the previous decade, this pattern could arise from a long-term cyclical but self-compensating trend. The extended record indicates that despite population growth and intense economic development around the lake and in its catchment area, there has been no extreme eutrophication of Lake Kinneret from 1972 to 1993. We suggest that this relative resiliency of the Kinneret ecosystem is due to high ambient levels of alkalinity, calcium, and pH in the lake water acting to limit phosphorus availability, which in turn restricts the outgrowth of phytoplankton.

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A modified assay for alkaline or acid phosphatases associated with microorganisms in aquatic environments has been developed. This is based on collecting microplankton on filters and subsequently determining the enzyme activities spectrophotometrically or fluorometrically, using p-nitrophenyl-phosphate or 4-methylumbelliferyl phosphate, respectively as substrates. The assay is simple and rapid, and has the further advantage of permitting phosphatase activities to be assigned to specific size fractions of the natural microplankton.In samples taken from Lake Kinneret and a nearby reservoir, a consistently high proportion of the total alkaline or acid phosphatase activity was associated with the size fraction x0.8 pm > 0.2 pm indicating the potentially high contribution of bacteria to these activities. This approach can also be used to examine the enzymatic potential of microplankton to release orthophosphate from other organo-phosphate substrates.

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