Water samples were taken from three regions in the Kinneret ecosystem (the Watershed (WS), the National Water Carrier (NWC) and the lake) over a 14 month period and nitrate reductase (NR) activity measured by a simplified assay previously developed for field studies. A complex relationship between NR activity and ambient NO3and/or NH4concentrations was observed. NR activity was not generally predictable from ambient nutrient concentrations. No correlation between NR activity and the ambient orthophosphate (SRP) concentrations or the dissolved, inorganic N : P atomic ratio was observed, although NR activity appeared to correlate with both NO3or NH4uptake. Our results suggest that NR activity is related to the N nutritional status of the cells and their nutrient prehistory rather than ambient nutrient concentrations.
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