Phosphorus-nitrogen loading and trend of fish catch as index of lake Mariut eutrophication.

Lake Mariut was considered in the past to represent an important source of fish production in Alexandria especially during the periods which were characterized by low catch from its marine fisheries. The growing population in Alexandria and the expanding industries around the lake were accompanied with increased quantities of domestic and industrial wastes discharged annually into the lake proper. These quantities were increased from 192 million cubic meters in 1974 to about 300 million cubic meters in 1992. Consequently, the water quality in this basin was highly deteriorated and changed to become markedly eutrophic. High input of nutrients through sewage and industrial wastes has considerably increased the phosphorus and nitrogen load in the lake proper. Phosphorus and nitrogen loading in this basin were estimated in 1992 to be 60.2 gm/m3 P/y and 79.8 gm/m3 N/y. According to the nutrient concept given by Vollenweider (1974), these two figures have provided an evidence that the lake proper was strongly changed from oligotrophic to polytrophic phase. The trend of its total fish catch as well as the trend of different fish categories catch during the period 1970-1992 also confirms its eutrophication. This is in addition to bad changes that took place with respect to environmental conditions due to the continuous inflow of domestic and industrial wastes from different sources into this part of Mariut lake. Marginal increments of the lake proper total catch were decreased from 20.6% in 1970 to -28.8% in 1992, while the marginal increments of Tilapia sp., Clarias sp, Anguilla sp. and Mugil sp. were decreased from 21.4%, 14.3%, 29.8%, and 28.9% respectively in 1970 to -29.1%, -25.3%, -43.0%, and -36.8% in 1992. Based on these two indices of the lake proper eutrophication, the expected improvement of its environmental conditions will take several years after treating all inland discharges flowing into the lake and/or diverting all relevant outfalls away from this basin.