Effect Of Anthropogenic Pollution On Nutrient Species In An Egyptian Wetland

摘要

Lake Edku, located 30 km to the N E of Alexandria, had an area of 126 km~2, average water depth of 85 cm and a flushing rate of 36 days. The agricultural contaminated drainage waters, constituting the main water supply to this wetland, induces a lake - sea current. Wind driven contaminated seawater occasionally invades the lake. This study aims to quantify the nitrogenous and phosphorus species and silicon in Lake Edku, to compare between its present and previous nutrient data and to construct a mass balance for the reactive nutrients in this lake. The highest dissolved inorganic phosphorus (DIP) values were found at the eastern lake area, directly affected by drainage water discharges. More than half of the total phosphorus (TP) at this area was reactive. The low DIP amounts discharged into the lake during August-October have led to the decrease in the DIP content in the lake during this period. The high dissolved organic phosphorus (DOP) at the macrophyte dominated area coincided with its release from living and dead floating and submerged plants. Monthly variations of DOP showed high values from March to June, mainly from drainage water discharges and/or biological production. Lower surface percentages of particulate phosphorus (PP) were observed at the lake-sea connection and at the eastern lake area opposite to the main drains. Monthly variations of PP showed an increase in March, possibly from biological activity. The lake water was enriched with dissolved inorganic nitrogen (DIN) forms. High levels were found in the eastern lake region, directly influenced by drainage water discharges. The high DIN values at the lake-sea connection resulted from the invasion of the polluted seawater to the lake. Nitrate dominated the DIN species. It decreased markedly in summer and increased in winter. The increase in the rate of nitrate uptake and decomposition with elevation of temperature are among the factors responsible for its lower values in the warm season. Contrary to DOP, dissolved organic nitrogen (DON) constituted a high fraction of the total dissolved nitrogen (TDN). However, particulate nitrogen (PN), . similar to PP, constituted a minor fraction. The increase in DON coincided with its lower assimilation than DIN forms. The annual mean N:P ratios of the different nitrogen and phosphorus forms indicate that phosphorus could be in some occasions a limiting factor for phytoplankton. The silicon content of the lake reflects its brackish character. Apart from some elevated silicon values opposite to the drainage area, the regional silicon variations were insignificant. Its monthly variations, however, showed a significant increase during summer, when silicon dissolution from diatom frustules was aided by high temperature and the rate of drainage water discharge has increased. Comparison between nutrient contents of Lake Edku with earlier data suggests changes in the quality rather than quantity of the inflowing waters into the lake draining new reclaimed lands. The nutrient budget in Lake Edku was affected by several factors, mainly the amounts of drainage water discharges. Estimation of the nutrient balance of the lake yields amounts which guarantee a considerable nutrient stock for the future.