Limnological studies on the Nozha Hydrodrome, Egypt, with special reference to the problems of pollution

Abstract

Water samples were collected in the different seasons during the period 1979–1981 from the Nozha Hydrodrome, Egypt, a small shallow lake receiving polluted Nile water, to gain information on the environmental conditions, nutrient contents, levels of trace metals and their possible variations in relation to pollution effects. The accumulation of trace metals in the lake sediments and in different tissues of the common fish, Tilapia species, was also investigated. During the study period, values for each of the parameters investigated generally gave a wide range of variability. The highest seasonal average air and water temperatures were recorded in July and the lowest values in January; the water temperature usually followed the pattern of the air temperature. The maximum seasonal average chlorosity in July is due to an increase in the rate of evaporation. The highest seasonal average for dissolved oxygen (DO) in January coincided mainly with a decrease in the rate of oxygen consumption associated with a drop in temperature and the continual mixing of the lake by wind action. The minimum regional averages of Secchi disc and chlorosity, as well as the lower regional average values of pH and DO in the supply canal reflect the turbidity and nature of the polluted feed Nile water. The seasonal changes of nutrients in the Hydrodrome are mostly influenced by their allochthonous input, and their main source is the supply of polluted Nile water, which first affects the inlet region. The high concentrations of inorganic nitrogenous and phosphate salts in the feed water are due to the discharge of untreated sewage and industrial wastes, as well as agricultural run-off into the water source of the Hydrodrome (Mahmoudia Canal). The nitrite produced in the lake is principally from nitrification of ammonia. The autochthonous supply of nitrate and phosphate is mainly from the decay of phytoplankton and subsequent release from lake sediments. The increase in silicate coincided principally with dissolution of diatom frustules. It appears that phytoplankton represents the main factor responsible for decreasing the concentrations of nutrients in the Hydrodrome. The seasonal variations of trace metals in the Hydrodrome are attributed to both external and internal effects. The former reflects the impact of man on the water source of the Hydrodrome, whereas the latter represents the events which occur within this lake. The data suggest a direct relationship between the concentrations of metals in the lake water and their concentrations in Tilapia fish. The data illustrate the ability of fish to absorb high levels of heavy metals and the ability of the lake sediments to accumulate these metals relative to those found in the lake water and the fish. In general, iron showed the highest mean values in the lake water, sediments and fish, whereas cadmium was present in the lowest concentrations. Comparison between the annual average value of each parameter in the present study and the corresponding value for the study of the heavily polluted Lake Mariut during the same period, as well as the comparison between the annual averages of nutrients and trace metals in the Hydrodrome and those in the predominantly clean Damietta branch of the Nile, provided evidence for the amount of pollution occurring in the study area.