Biogenic elements impact on chlorophyll a of two common diatoms isolated from Lake Qarun (Egypt) in laboratory bioassay

Abstract

Lake Qarun is a protected saline lagoon in the northern part of the El-Fayoum depression (Middle Egypt). The lake is bordered by agricultural land to the south and desert to the north. The main source of water for the lake is from drainage of irrigated land, which it receives through two major drains: Batts and El Wadi. As the only outflow for the water is evaporation, salinity is continually increasing. Today, Lake Qarun is slightly more saline than seawater (about 40 g/L). Nilotic aquatic fauna has mostly disappeared, being replaced by many marine species. Lake Qarun is of national importance as a valuable resource of fish and as a tourist site. Lake Qarun holds large numbers of waterfowl in winter. The increasing salinity of the lake is constantly changing its ecology, and is likely to become too high for many life forms to continue to exist. Laboratory bioassay experiments were conducted to assess growth of the dominant species Nitzschia longissima W. Smith, estimated by chlorophyll a, with different strengths of agricultural drainage water from El Batts and El Wadi. Enrichment levels ranging from 25% to 100% of raw drainage water pronouncedly enhanced the growth of N. longissima. Investigations on the impact of biogenic elements, e.g., nitrogen (N), phosphorus (P) and nitrogen−phosphorus combinations (N and P) on chlorophyll a of N. longissima and Amphiprora paludosa W. Smith were estimated. Optimum treatments, at which highest chlorophyll a yields were sustained, were determined. Generally, the addition of N salts and the N-P combinations had significantly stimulated chlorophyll a yield of N. longissima, rather than of A. paludosa. Conservation plans should be constructed entailing the division of the lake into several segments, where salinity, in at least some parts of the lake (near sources of fresh water), could be maintained at favorable levels, while other parts would be sacrificed. Mitigation efforts to date should be focused on nitrogen reduction, because nitrogen is expected to be the limiting nutrient, by treating water entering the lake through the two major drains, Batts and El-Wadi.