Limnological and ecophysiological aspects of Aphanizomenon ovalisporum bloom in Lake Kinneret, Israel

The first appearance of Aphanizomenon ovalisporum in Lake Kinneret in August 1994 was apparently boosted by relatively high concentrations of total dissolved phosphorus (12 g P l^-1 as compared to an average of 8 g P l^-1). The increasing Aphanizomenon biomass in a lake in which phytoplankton are generally phosphate limited in summer and autumn was accompanied by high enzymatic activity of alkaline phosphatase, reaching values of 2830 nmol MU l^-1 h^-1, suggesting a great demand for phosphorus. In addition, the nitrogen requirement of the developing population of Aphanizomenon was partly provided by nitrogen fixation, as indicated by a high percentage of heterocysts. Laboratory experiments demonstrated that filtrate from an old Peridinium gatunense culture enhanced Aphanizomenon growth. Thus, it is postulated that the degradation of the massive Peridinium bloom in spring and early summer supported the development of A.ovalisporum. The high pH and alkalinity during the bloom of Aphanizomenon indicate that A.ovalisporum is probably a HCO3^- user. After 1994, akinetes of A.ovalisporum were left in sediments and the water column, and could be a source for the next year's bloom. This possibility was demonstrated by inoculation of lake water and sediments into nitrogen-depleted BG-11 medium, resulting in the dominance of A.ovalisporum.