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Experiments were carried out to determine the effects of some environmental factors causing the phenomenon of algal blooming in the upper lake of Wadi El-Raiyan under laboratory conditions. Microcystis aeruginosa and M. flos aquae were isolated from Wadi El-Raiyan Lakes. Mono-clonal culture of each species was kept in 100-ml BG11 media in the laboratory for 5 days. The algal growth was measured by the determination of chlorophyll a and cell count to understand the succession patterns of both species in relation to the tested environmental conditions. The best photoperiod cycle was found to be (14:10 hrs) and the growth of the two species increased with increasing light intensities up to 45µ E m-2 s-1. Salinity and the growth of tested species exhibited a significant inverse relationship, increased salinity inhibiting algal growth. Water pH range from 7 to11 was suitable for growth of both species, while a pH below or above this range caused a significant decrease in growth. High nitrogen sources entering into Wadi El-Raiyan Lake may be one of the reasons for the blooming of the tested microalgae. The toxic effects of the bloom-forming species impact the economic fish resources of the lake leading to fish kills.

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