SEASONAL VARIABILITY OF N P RATIOS IN EUTROPHIC LAKES

The ratios of different forms of nitrogen to phosphorus (particulate, total, total dissolved, and dissolved inorganic N : P) from four eutrophic lakes are presented to assess their variability during two consecutive growing seasons. Particulate and total N : P ratios showed the lowest amplitude of fluctuations, while the total inorganic N : P ratios showed the highest. All N : P forms demonstrated substantial variations and seasonal pattern over the growing period April-October. It was the short-lasting spring minima of the N : P ratios, ranging from less than 1: 1 to 6 : 1, which triggered the onset of nitrogen-fixing cyanophyte blooms. Seasonal mean values were as high as 20: 1 to 30: 1 and misleading in assessing nitrogen limitation. The nitrogen fixation rapidly restored the N : P values to normal levels (TN: TP of 15: 1 and over). © 1990 Kluwer Academic Publishers.