The filamentous cyanobacterium Aphanizomenon ovalisporum was observed for the first time in Lake Kinneret in August 1994 and formed a prominent bloom from September through October. Aphanizomenon ovalisporum reappeared in diminished amounts in the summer and fall of 1995. These events are the first record of significant quantities of a potentially toxic nitrogen-fixing cyanobacterium in this lake. No definite provenance of inoculum has been identified, although A.ovalisporum was also observed in a newly reflooded area (Lake Agmon) in the catchment. Unusually high water temperatures and low wind inputs were observed prior to and during the A.ovalisporum bloom period. These, together with possibly enhanced availability of phosphorus or other growth factors, may have contributed to the cyanobacterium growth in 1994. Phosphorus limi tation, as indicated by high cellular alkaline phosphatase activity, the onset of stormy conditions and a fall in water temperatures led to the demise of the 1994 bloom. Although the A. ovalisporum bloom in 1994 had no serious direct impact on water quality, the continued presence of a potentially toxic cyanobacterium in Lake Kinneret, a major national water supply source, is a cause for serious concern.
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