¹Faculty of Sciences Semlalia, Department of Biology, Laboratory of Phycology PO Box 2390, 40000 Marrakech, Morocco
²Faculty of Sciences El Jadida, Department of Biology, PO Box 20, El Jadida 24000, Morocco
³CIIMAR - Center of Marine and Environmental Research, R. Campo Alegre, 823 - 4000 PORTO, Portugal
⁴Faculty of Sciences Ben M'sik, Department of Biology, Laboratory of Ecology, PO Box 7955 Sidi Othmane, Casablanca, Morocco

^*Corresponding author: Fax. : 212 44 43 74 12, E-mail sabour.brahim@ucd.ac.ma

Abstract

  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 - 28° C), high incident light intensities (1488 - 1912 µE m-2 s^-1), high nutrient deficiency (0 µg P-PO₄ 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 mg_DW kg^-1 body weight while toxicity (24 h LC₅₀) in the brine shrimp Artemia salina was 3.68 mg_DW ml^-1. The low microcystin content (3.28 µg g_DW^-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.