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.
In the Lake Oued Mellah cyanobacteria waterblooms occur periodically in late spring and summer with Microcystis ichthyoblabe as the main bloom-forming species. In 1999, a heavy waterbloom of M. ichthyoblabe occurred during May June with a maximal biomass of 298 mg/l. During this period, several bloom samples were collected. The toxicity assessment was done by mouse and brine shrimp (Artemia) bioassays. Apart from the sample collected on 15/06/1999, all the other samples were toxic by mouse bioassay. The LD50 determined by intraperitoneal injection to mice during active cyanobacterial growth and decline phases were 518 and 1924 mgDW/kg respectively. Using Artemia bioassay, the 24hLC50 varied from 6.0 to 40.6 mg/ml and the 48hLC50 ranged from 2.8 to 18.2 mg/ml. The separation and identification of microcystin variants was performed by high performance liquid chromatography-photodiode array detection. Eleven toxins were separated and preliminarily identified as microcystin variants as they exhibit a typical UV spectra like the microcystin-LR standard. The quantification of total microcystins determined by enzyme-linked immunosorbent assay showed that the contents were varied between 0.1 and 0.76 microgram/g DW.
Les eaux de l'Oued Hassar (Maroc) sont caractérisées par une forte minéralisation. La conductivité, variant entre 3470 et 7210 µS.cm-1, est due essentiellement aux fortes teneurs en chlorures. L'examen de l'Indice de Pollution Organique (IPO) montre que l'effluent d'eaux usées provenant de la ville de Mediouna constitue la principale source de pollution des eaux de l'Oued Hassar.
Concernant l'étude des diatomées épilithiques, 61 espèces et variétés de diatomées ont été inventoriées. Ces taxons sont liés particulièrement à la salinité (Pleurosira laevis, Fragilaria pulchella,…) et à la charge organique (Nitzschia palea, Gomphonema parvulum,…).
Quatre indices diatomiques ont été testés pour l'évaluation de la qualité des eaux de l'Oued Hassar : l'Indice Diatomique Pratique (IDP), l'Indice Diatomique Générique (IDG), l'Indice de Sládecek (SLA) et l'Indice Diatomique Trophique (IDT). La variation spatio-temporelle de ces indices reflète bien la dégradation de la qualité de ce cours d'eau, particulièrement en aval du rejet de la ville de Mediouna (IDP=3,86 ± 1,84 ; IDG=6,05 ± 1,95 ; SLA=4,35 ± 1,93 ; IDT=5,80 ± 1,31). Parmi ces indices testés, seul l'IDP présente une corrélation significative (0,63 ; α<5%) avec l'IPO (indice de référence) tandis que l'IDG, le SLA et l'IDT montrent des corrélations non significatives avec cet indice (0,38 ; 0,27 et -0,11 respectivement).
The Oued Mellah reservoir (33°30' N-07°20' W) is a shallow brackish lake located 25 km North of Casablanca. A study of the phytoplankton community of this lake shows that it is affected by potentially toxic blooms of Cyanobacteria and M. LOUDIKI, B. OUDRA, S. OUBRAIM, B. FAWZI, S. FADLAOUI, M. CHLAIDA
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