Caractérisation d'un Système Lagunaire en Zone Tropicale: Cas du lac Nokoué (Bénin)

Abstract

This survey permitted to make hydrological balance of the lake Nokoué and to know quality physico-chemical of the waters of the lake in order to offer the basis elements for a future ecological modeling of the system. The results of hydrological balance show that the average of the flow of the soft waters received by the lake in period of high waters is 216m3/s essentially brought by the Ouémé stream, the longest stream of Benin that crosses the country from the north to the south. This value falls to 49m3/s in dry season with 86% of the flow brought by the Sô river in dry season. In dry season, the Sô at Ganvié has the most important rate of organic load (53% DBO). For the phosphorus in dry season, the highest rate is gotten at the level of strongest rate is gotten at the level of Ouémé stream at Totchè (42%). In the period of high waters (from August to October) a dilution effect of the contribution is observed. In term of nutrient loads brought by purification systems, the results obtained are 360kg/j-P and 840 kg/j - N for Cotonou and Calavi and 150 kg/j- P and 700 kg/j-N for Ganvié. The contributions of "Acadjas " give 2,3 - 2,9 kg/j P and 29 - 36 N kg/j. The main results obtained for the physico-chemical for characterization are presented as follows: * a very remarkable saline intrusion in dry season, Sô at Ganvié the value passes from 0g/L in period of high waters to 2,5g/L in dry season, * the average pH varies between 6,6 and 7,5 for the waters of the bottom and between 6,9 and 7,8 for the waters of surface of the lake. The pH varies very little between the bottom of the lake and its surface, * the values of nitrogen and phosphorus nutrients are extensively high than the limited values indicated by the system of classification by the Organization of Cooperation and Economic Development (OECD). The waters of the lake are therefore eutropheses. We notice a seasonal evolution of the system. According to the factors of control hyacinths of waters, it is concluded that nutrients control the development of hyacinths when the salinity doesn't inhibit the growth of hyacinths anymore; that means of July to January