Effects of zooplankton grazing and nutrients on the bloom-forming, N-2-fixing cyanobacterium Aphanizomenon in Lake Kinneret

A bloom of the filamentous, N-2-fixing cyanobacterium Aphanizomenon ovalisp orum Forti occurred for the first time in Lake Kinneret during late summer and fall 1994. During subsequent years (1995-1999), Aphanizomenon also appe ared in late summer and fall, but did not bloom. In outdoor microcosm exper iments, we examined zooplankton grazing on Lake Kinneret phytoplankton, wit h and without Aphanizomenon present, and the effects of N, P and N:P ratios on phytoplankton growth. In one-day feeding experiments, clearance and gra zing rates of the ambient Lake Kinneret zooplankton assemblage feeding in l ake water dominated by Aphanizomenon were 10-fold lower than in water witho ut Aphanizomenon. We suspect that the low grazing rates were due to interfe rence caused by the presence of Aphanizomenon. In 9-day nutrient addition e xperiments, significant enhancement effects on phytoplankton were detected with additions of either P or N; a high N:P was better for phytoplankton gr owth than a low N:P. After 7 days, bottles receiving low P and no N additio ns were dominated by Oscillatoria sp. and Closterium acutum; few Aphanizome non were present. In contrast, bottles receiving high P and N additions had large increases of Aphanizomenon, as well as Oscillatoria and Closterium. There was a tendency for more green algae and diatoms with increasing N add itions. These results provide evidence that (i) non-grazeability of Aphaniz omenon enabled it to gain a competitive advantage over grazeable phytoplank ton, and (ii) that nutrient limitation, but not grating, was probably impor tant in the eventual decline of the Aphanizomenon bloom.