The relationship between environmental variables and elemental, biochemical composition of phytoplankton species in Lake Nasser were studied. 130 phytoplankton species were recorded, the lake is dominated mainly by Cyclotella glomerata (Bacillariophyceae), Planktolyngbya limnetica, Eucapsis minuta (Cyanophyceae). Phytoplankton organic carbon content at the lake was 32%. The elevation of NO3 (365 µg/l) and CO3 (21 mg/l) at the east of the lake was concurrently with the highest proportion of inorganic C, N and algal protein content. C/N ratio decreased thus Redfield ratio indicating a high growth rate of phytoplankton with increasing protein content. C/H and O/C ratio for natural phytoplankton samples were less than one, which means unsaturated aliphatic compounds at metabolites categories of phytoplankton. RDA analysis cleared that, Cyclotella glomerata (dominant at the north area) was sensitive to flushing, tolerant to nutrient deficiency and had higher protein content. Planktolyngbya limnetica (dominant at the middle) was more sensitive to pH, phosphorus is non-limiting factor and characterized by elevation in lipid content. Eucapsis minuta (dominant at the south) was tolerant to mild light deficiency and contains a maximum value of carbohydrates and Chlorophyll a.