Spatial variation of phytoplankton and some physico-chemical variables during the highest flood season in Lake Nasser (Egypt)

Abstract

Species composition, abundance, and phytoplankton biomass in Lake Nasser were studied during the highest flood season in autumn 1999, and were supported by some physico-chemical environmental parameters. The nutrient salts levels were low to medium, with ranges of 0.96-9.92 g/L for nitrite, 20.14-72.14 g/L for ammonia, complete depletion -27.69 g/L for phosphate, and 5.83-9.00 g/L for silicate. A total of 71 taxa were identified, of which 46% were chlorophytes, 26% bacillariophytes and 22% cyanophytes. Phytoplankton abundance and biomass values were higher in the northern stations than in the southern, but were lower than previously reported, indicating approaching from oligomesotrophic status, with,respectively, means of 108.9 x 104 unit/L and 1.18 mg/L for the main channel, and 712 x 103 unit/L and 1.229 mg/L for the khors. Bacillariophyceae was the most important group, constituted 80% and 64% of total abundance and 61% and 51% of total biomass in the main channel and khors, respectively. Cyclotella and Melosira were the most abundant genera. Cyanophyceae second in importance, forming 11.6% and 16.8% of total abundance and 34.6% and 42.9% of total biomass in the main channel and khors, respectively. Phormidium and Anabaenopsis were the most frequent genera. Highest species diversity (2.42) and evenness (0.92) were recorded in Tushka, and highest richness (2.52) was recorded in El- Ramla. The statistical regression models performed showed that the most physico-chemical factors affecting the growth of phytoplankton abundance were pH values, dissolved phosphate and reactive silicate.As a whole, a declining in phosphorus concentration, phytoplankton abundance and biomass and development of Cyclotella and Melosira are approching the lake oligomesotrophic status.