The structure and abundance of phytoplankton communities were investigated during 1997 to 1999 in Oued Mellah, a shallow brackish and hypertrophic lake, with particular regard to Anabaena aphanizomenoides dynamics. Important events of algal blooms were observed mostly by the cyanobacteria Microcystis ichthyoblabe, Anabaena aphanizomenoides and Oscillatoria chlorina and by the ichthyotoxic haptophyceae Prymnesium parvum. Anabaena aphanizomenoides proliferated during late summer after the Microcystis ichthyoblabe blooms. The percentage of Anabaena aphanizomenoides of the phytoplankton biomass varied from 88 to 94 percent during bloom periods. Maximum biomass was 146 and 120 mg fresh weight l−1 during the 1997 and 1999 summer periods, respectively. The main environmental factors leading to the ecological success of A. aphanizomenoides were high temperature (25–28ˆC), high incident light intensities (1488–1912 μ E m−2 s−1, high nutrient deficiency (0 μ g P-PO−4 l−1; 0–0.18 mg N-NO3 l−1) and decrease of alkalinity (329–494 mg HCO3 l−1). The toxicity of the Anabaena aphanizomenoides bloom was evaluated by bioassays and analyses. The lethal dose50 of the bloom sample tested in mice was 254 mgDW kg−1 body weight while toxicity (24 h LC−50) in the brine shrimp Artemia salina was 3.68 mgDW ml−1. The low microcystin content (3.28 μ g gDW−1) determined by ELISA was not consistent with the tested bioassays and is suggestive of the presence of other toxic compounds in the bloom extracts. Four toxic fractions were separated by HPLC-PDA and identified as microcystins according to their UV spectra. The production of microcystins by Anabaena aphanizomenoides bloom was confirmed by the analysis of the isolated strain which, in Z8 medium under controlled laboratory conditions, produced three variants of microcystins, two of them being similar to those produced by the natural bloom.

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