EFFECT OF TREATED SEWAGE ON THE WATER-QUALITY AND PHYTOPLANKTON POPULATIONS OF LAKE MANZALA (EGYPT) WITH EMPHASIS ON BIOLOGICAL ASSESSMENT OF WATER-QUALITY

The effect of treated sewage on the quality of water and phytoplankton populations of Lake Manzala was studied with emphasis on use of algae to monitor water pollution as part of a search for a biological asses sment of water quality. Lake Manzala is situated at the northern part of the Nile-delta, Egypt. Disposal of treated sewage into Lake Manzala appeared to have differential effects on water quality and phytoplank ton populations. Marked seasonal and local variations were observed fo r the physical and chemical characteristics of water. 157 species of a lgae were identified, 59 Chlorophyta, 37 Bacillariophyta, 30 Cyanophyt a (Cyanobacteria), 28 Euglenophyta, one Pyrrhophyta and 2 Cryptophyta. Distribution and abundance of these algal divisions were found to dif fer at different sampling stations. Qualitative and quantitative growt h of each algal division displayed great seasonal variations. The phyt oplankton standing crop was mainly due to the contribution of Bacillar iophyta whereas the species composition is dependent mainly on Chlorop hyta. A great parallelism was noted between the quality of water sampl es based upon the chemical and physical investigations and their quali ty based upon the biological indices. Compound eutrophication index in dicated that the nature of the investigated water ranged between eutro phic and hypereutrophic conditions. Diversity index values indicated t hat the water in the study area was of a moderate level of pollution. Saprobic index and saprobic quotient revealed the presence of beta- to alpha- mesosaprobic forms of algae.