The influence of environmental factors on nitrate reductase activity in freshwater phytoplankton

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 NO₃ and/or NH₄ concentrations 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 NO₃ or NH₄ uptake. 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.