Effects of zooplankton grazing and nutrients on the bloom-forming, N2-fixing cyanobacterium Aphanizomenon in Lake Kinneret

A bloom of the filamentous, N-2-fixing cyanobacterium Aphanizomenon ovalisporum Forti occurred for the first time in Lake Kinneret during late summer and fall 1994. During subsequent years (1995-1999), Aphanizomenon also appeared in late summer and fall, but did not bloom. In outdoor microcosm experiments, we examined zooplankton grazing on Lake Kinneret phytoplankton, with and without Aphanizomenon present, and the effects of N, P and N:P ratios on phytoplankton growth. In one-day feeding experiments, clearance and grazing rates of the ambient Lake Kinneret zooplankton assemblage feeding in lake water dominated by Aphanizomenon were 10-fold lower than in water without Aphanizomenon. We suspect that the low grazing rates were due to interference caused by the presence of Aphanizomenon. In 9-day nutrient addition experiments, significant enhancement effects on phytoplankton were detected with additions of either P or N; a high N:P was better for phytoplankton growth than a low N:P. After 7 days, bottles receiving low P and no N additions were dominated by Oscillatoria sp. and Closterium acutum; few Aphanizomenon 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 additions. These results provide evidence that (i) non-grazeability of Aphanizomenon enabled it to gain a competitive advantage over grazeable phytoplankton, and (ii) that nutrient limitation, but not grating, was probably important in the eventual decline of the Aphanizomenon bloom.