Impact of Water Level Fluctuation on Water Quality and Trophic State of Lake Nasser and its Khors, Egypt.

Abstract

he morophometric configuration of lakes or reservoirs plays an important role in their water quality variations. The morophometry of Lake Nasser is characterized by the presence of many dendritic side extensions known as side channels or Khors. The surface area, depth and shape of these Khors depend upon the water level upstream the Aswan High Dam (AHD). In the last decade, the water level upstream the Dam fluctuated from about 181m ASL during year 2001 to about 174 m ASL during year 2005.This Fluctuation of water level from year to year affects water quality status of the Lake and its Khors due to the change of their morophometric configuration. To study the effect of water level fluctuation, four main important Khors characterized by gentle slope and sandy bottom with different size were selected. These Khors are Kalabsha, El-Alaki, Toushka, and Sara at Km, 41, 91, 247, and 325 from AHD respectively. The main goal for the present study is to assess the quality of water in the main Khors of Lake Nasser and its main channel during periods of low and high water levels (years 2001 and 2005 respectively). This assessment explores the potential pollution hazards coming from these Khors to the whole Lake. To achieve this goal water quality data generated from the analysis of water samples collected from the Khors and main channel during high and low water levels were used to calculate the water quality and trophic state indices as well as determination of the limiting nutrient factor of the lake. The abundances of Phytoplankton were also considered. The results of the water quality index (WQI) calculations during this study showed that the drop in Lake water levels led to a decline in the water quality of the Lake and Khors from the order of good to medium. Also, the trophic state index (TSI) values revealed that the productivity of the Lake changed from mesotrophic during high water level to eutrophic during low water level. These results are consistent with high phytoplankton counts recorded during low water level period. The values of N/P ratios indicated that phosphorus controls the phytoplankton growth during high water level, while during low water level either phosphorus (P) or nitrogen (N) could be limiting phytoplankton growth. Based on the obtained results of the present study, it could be concluded that the drop in water level upstream the dam may have negative impact on the water Quality of the Khors. This impact may in turn influence the whole Lake due to water rebounce from the Khors to the main channel. Therefore, it is further concluded that khors could be considered as potential pollution sources during low water levels of the lake.