Variation of water quality and phytoplankton along different zones of Aswan High Dam Reservoir
Egyptian Journal of Aquatic Biology and Fisheries
his study was carried out at Aswan High Dam Reservoir and focused on the spatial variation of water quality parameters and phytoplankton composition. Water samples were collected from eighteen sampling stations, where six stations for each of the different zones were selected to represent the lacustrine, transition and riverine zones of the reservoir during 2007 and 2008. The results of the environmental factors showed wide variations in their concentrations along the different zones of the
... nt zones of the reservoir. In this study, chlorophyll a concentration was lower in the lacustrine zone than in the riverine, although turbidity and secchi depth values were all optimal for light availability. This result was influenced by higher abundances in the reservoir phytoplankton assemblages where riverine conditions predominate, by species tolerant to turbulent conditions and typical high mineral turbidity. Calculation of the trophic state index showed that riverine and transition zones are classified as eutrophic, while lacustrine zone is mesotrophic. Phytoplankton composition recorded thirty nine species belonging to Chlorophyceae (19 species), Cyanophyceae (10 species), Bacillariophyceae (7 species), and rare groups including Dinophyceae (2 species) and Eugelnophceae (1 species). Cyanophyceae were the most encountered group in the lacustrine and transition zones, while Bacillariophyceae were most encountered group in the riverine zone. This is due to that Cyanophyceae prefer the steady, transparent and low nutrients water (lacustrine), while Bacillariophyceae prefer flowing, turbid and nutrient-rich water (riverine). Statistical analysis showed that certain environmental factors do affect the phytoplankton growth. The results of the one-way ANOVA revealed that the different environmental factors (DO, water temperature, secchi depth, water velocity, turbidity, nitrate, phosphorus, magnesium, total dissolved solids, and trophic state index) and biotic factors (such as chlorophyll a and phytoplankton groups) were significantly different at the three zones of Aswan High Dam Reservoir (p<0.05). This result supports the speculation of variation in water quality and phytoplankton along the different zones of the reservoir. T Talaat A. Salem 88 other beneficial uses. They are considered as hybrid systems between rivers and lakes (Collier et al., 1998) , because they exhibit a progressive transformation from lotic systems (riverine) to lentic or lake systems (lacustrine). Aswan High Dam Reservoir which was formed in the 1960s by the construction of the High Dam in order to control the annual floods of the Nile River is considered the strategic water bank for Egypt. It extends for about 350 km along Egyptian land, and 150 km in the Sudan with storage capacity of 162 billion cubic meters between levels of 83 to 182 meters above sea level. Building the High Dam across the Nile River, and impounding water behind it, caused profound changes in the limnological regime of the waterbody. These included chemical and physical changes, that in turn affected the flora and fauna of the regulated waterbody (Reynolds, 1997) . There are considerable increases or decreases in the values of many physico-chemical and biological parameters (e.g., minerals, composition and number of planktonic organisms), which is in contrast to undisturbed reservoir zones (Stanford and Ward, 2001) . Reservoirs exhibit a large degree of spatial heterogeneity in phytoplankton productivity and biomass as a result of longitudinal gradients in basin morphology, water residence time, flow velocity, suspended solids, and availability of light and nutrients. A typical reservoir commonly has three distinguishable zones along its longitudinal axis Wetzel, 1990 ). 1. The riverine zone which is characterized by higher flow, shorter water residence time, and higher levels of available nutrients, suspended solids, and light extinction relative to downstream portions. Abiogenic turbidity will often limit light penetration, thereby limiting the thickness of the photic layer. A real primary productivity is often light limited. 2. The transition zone, characterized by high phytoplankton productivity and biomass. This occurs in conjunction with increasing breadth of the basin, decreasing flow velocity, increased water residence time, sedimentation of silt and clay particles from near-surface waters, and increased light penetration. The transition zone can be considered as the most fertile region of a reservoir, because both light and nutrients are available for algal photosynthesis. 3. The lacustrine zone which occurs nearest the dam and usually has the longest water residence time. It also exhibits lower concentrations of dissolved nutrients and suspended abiogenic particles, higher water transparency, and a deeper photic layer. However, the volumetric phytoplankton productivity of the photic zone is reduced, often by nutrient limitation, during most of the growing season, and is supported mostly by in situ nutrient cycling rather than by advected nutrients. Each of these zones has distinct biotic, physical, and chemical characteristics.