الفهرس 
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Abstract
Zooplanktons are mostly microscopic free swimming heterotrophic animal forms that diverse morphologically and taxonomically. The importance of zooplankton in their habitat is summarized in various points (e.g. food webs, indicator of pollution and water quality). They are considered as a key position in pelagic food webs and used as useful indicator for future fisheries health. They make up an invaluable source of protein amino acids, lipids, fatty acids, minerals and enzymes, therefore they are considered as an inexpensive ingredient instead of fish meal in fish cultures. The present investigation was designed in two parts, field and laboratory: The field part was performed to study the impact of some ecological factors such as, temperature, pH, conductivity and dissolved oxygen on the density and abundance of zooplankton community in the River Nile at Assiut city. Otherwise, to study the effect of these ecological factors on the vertical distribution of zooplankton taxa in the River Nile. In the laboratory, the common cladoceran species Simocephalus vetulus was chosen as a model animal to study the effect of UV-A as an important ecological factor on biochemical, histological and morphological changes. To achieve the above goals, qualitative and quantitative samples of zooplankton were collected monthly for one year (from April 2011 till March 2012) from four different sites distributed on the River Nile at Assiut city. To study the vertical distribution of zooplankton community, monthly samples were collected from different depths. Individuals of the cladoceran S. vetulus were exposed in the laboratory to different doses of UV-A for 15, 30 and 60 minutes. The most important results of the present study could be
summarized in the following points:
•In all investigated sites, the studied ecological factors showed significant differences except for dissolved oxygen concentration.
•40 zooplankton taxa were recorded in the River Nile during the period of investigation, these taxa included 5 groups [Cladocera, Copepoda, Rotifera, Ostracoda and other group (insect larvae and aquatic mites)].
•Cladocera was the most dominant group in the four investigated sites in the River Nile and Bosmina longirostris was the most dominant cladoceran species while Daphnia longispina was the least dominant one.
•Site I showed the highest density of zooplankton community and showed the highest richness while site III showed the highest diversity of species.
•Seasonal abundance of total zooplankton showed the highest peak in winter, while the lowest peak was observed in summer and the analysis indicated that water temperature is considered as one of the most important factors affecting the distribution of zooplankton
species, and there was a negative relation between the total density and temperature.
•The total density of zooplankton recorded the highest average at the surface water level 0-0.5 m depth. This density decreased with depth and attained slight increase at 1-1.5 m depth. The same observation was noticed for the Cladocera group which showed the highest total density at the surface level and decreased at 0.5-1 m depth then returned to increase at 1-1.5 m depth where water temperature decreased and dissolved oxygen increased.
•After exposure to UV-A radiation, the intestinal cells of S. vetulus showed obvious changes at low doses for 15 minutes and with increasing the exposure time to 60 minutes, most of the intestinal cells became vacuolated, fragmented with disruption of
cellular membrane and cell contents.
•Hyperplasia and karyolysis were noticed after 60 minutes of exposure.
•High doses of UV-A caused a decrease in the total protein contents in the irradiated groups while lipid peroxidation and the activity of Catalase enzyme increased. The activity of superoxide dismutase showed an increase only after 60 minutes of exposure.
•Glutathione content in S. vetulus showed significant decrease after first periods of exposure and significant increase was observed after 60 minutes, while Nitric Oxide increased only after 15 minutes of irradiation.
•UV-A radiation induced morphological changes in the exoskeleton of S. vetulus with the appearance of irregular distances between stripes of the carapace. UV-A radiation for 60 minutes caused signs of cuticle breakdown and malformation in the spines of
carapace.