Studies on the phytoplankton populations and physico-chemical conditions of treated sewage discharged into Lake Manzala in Egypt.

Over a full year, the phytoplankton populations and physico-chemical conditions of treated sewage discharged into Lake Manzala in Egypt were investigated. Sixty-seven species of algae were identified, 18 Cyanophyta (Cyanobacteria), 19 Chlorophyta, 21 Bacillariophyta, 6 Euglenophyta, 2 Cryptophyta and one species Pyrrhophyta. Nitzschia (6 spp.), Scenedesmus (6 spp.), Navicula (4 spp.), Oscillatoria (4 spp.) and Euglena (4 spp.) were the most common genera. A remarkable seasonal variation in species composition and standing crop of the phytoplankton populations was noted during the study. The total phytoplankton standing crop appeared to be mainly dependent on the growth of certain species viz., Oscillatoria chalybea, O. princepes, O. tenuis, Microcystis aeruginosa, Anabaena constricta (Cyanophyta), Nitzschia obtusa, Bacillaria paradoxa, Cocconeis placentula, Cyclotella meneghiniana (Bacillariophyta), Pandorina morum, Volvox sp. (Chlorophyta) and Phacus curvicauda (Euglenophyta). The continuous presence of Anabaena constricta and Nitzschia palea was recorded in the treated sewage. The least represented algal divisions were Pyrrhophyta and Cryptophyta, both in terms of quality and quantity. The data indicate that the secondary effluents were unstable in their chemical features and grossly polluted. Therefore, the treatment systems must treat the discharged sewage to a tertiary level before discharging into Lake Manzala.

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 assessment 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 phytoplankton populations. Marked seasonal and local variations were observed for the physical and chemical characteristics of water. 157 species of algae were identified, 59 Chlorophyta, 37 Bacillariophyta, 30 Cyanophyta (Cyanobacteria), 28 Euglenophyta, one Pyrrhophyta and 2 Cryptophyta. Distribution and abundance of these algal divisions were found to differ at different sampling stations. Qualitative and quantitative growth of each algal division displayed great seasonal variations. The phytoplankton standing crop was mainly due to the contribution of Bacillariophyta whereas the species composition is dependent mainly on Chlorophyta. A great parallelism was noted between the quality of water samples based upon the chemical and physical investigations and their quality based upon the biological indices. Compound eutrophication index indicated that the nature of the investigated water ranged between eutrophic and hypereutrophic conditions. Diversity index values indicated that 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.