In situ three grazing experiments were carried out for determining the grazing rate of zooplankton on phytoplankton species during autumn 2002, spring and summer 2003. Water samples from euphotic zone of the River Nile were inoculated with zooplankton organisms as its density in the Nile water and twice of that initial found in their natural field.
Phytoplankton communities in the different enclosures were dominated by Chlorophyceae, Bacillariophyceae and Cyanophyceae, whereas zooplankton organisms were represented by Rotifera, Cladocera, and Copepoda with the dominance of rotifers (>95%).Grazing rate reached the maximal in 2nd day of double field zooplankton enclosures during autumn and indicated that green algae and diatoms, especially Planktonema lauterbornii Schmidle, Dictosphaerium pulchellum Wood, Cyclotella operculata Kutz and Syndra ulna (Nitzsch) Ehr. were the most preferable algal cells grazed by zooplankton organisms (0.0599, 0.0174, 0.0530, 0.0371/h, respectively). Moreover, zooplankton organisms grazed to large extent on Microcystis aeruginosa Kutz (0.0504 h-1) and Merismopedia glauca (Ehr.) Nageli (0.0105/h). Contrary, the grazing rate during spring and summer seasons was obviously high on blue greens, such as Chroococcus disperses Lemm., followed by diatoms and green algae, due to the abundance of rotifers during this period. Rotifers are able to graze even on blue green algae. The results revealed that, there was no evidence of strong negative effects on phytoplankton number, whereas the grazing rate decreases with increasing zooplankton number due to the nutrient regeneration by zooplankton, which induces phytoplankton growth.Zooplankton abundance and community structure were important factors determining grazing rates in large rivers. Zooplankton density and phytoplankton can be increase two folds that found in Nile water especially in fish farms utilize River Nile water in aquaculture. Autumn season is the best time for zooplankton grazing on the different algal species inhabiting the River Nile.