STUDIES ON THE PHYTOPLANKTON POPULATIONS AND PHYSICOCHEMICAL CONDITIONS OF TREATED SEWAGE DISCHARGED INTO LAKE MANZALA IN EGYPT

Over a full year, the phytoplankton populations and physico-chemical c onditions of treated sewage discharged into Lake Manzala in Egypt were investigated. Sixty-seven species of algae were identified, 18 Cyanop hyta (Cyanobacteria), 19 Chlorophyta, 21 Bacillariophyta, 6 Euglenophy ta, 2 Cryptophyta and one species Pyrrhophyta. Nitzschia (6 spp.), Sce nedesmus (6 spp.), Navicula (4 spp.), Oscillatoria (4 spp.) and Euglen a (4 spp.) were the most common genera. A remarkable seasonal variatio n in species composition and standing crop of the phytoplankton popula tions was noted during the study. The total phytoplankton standing cro p appeared to be mainly dependent on the growth of certain species viz ., Oscillatoria chalybea, O. princepes, O. tenuis, Microcystis aerugin osa, Anabaena constricta (Cyanophyta), Nitzschia obtusa, Bacillaria pa radoxa, Cocconeis placentula, Cyclotella meneghiniana (Bacillariophyta ), Pandorina morum, Volvox sp. (Chlorophyta) and Phacus curvicauda (Eu glenophyta). The continuous presence of Anabaena constricta and Nitzsc hia palea was recorded in the treated sewage. The least represented al gal divisions were Pyrrhophyta and Cryptophyta, both in terms of quali ty and quantity. The data indicate that the secondary effluents were u nstable in their chemical features and grossly polluted. Therefore, th e treatment systems must treat the discharged sewage to a tertiary lev el before discharging into Lake Manzala.