Yaounde Municipal Lake supply by untreated domestic waste water, and effect on assimilation capacity of the ecosystem

The spatio-temporal variations of some physico-chemical parameters and chlorophyll a were carried out on the weekly basis, from November 1996 to December 1997, in the Yaounde municipal lake and in the Mingoa stream (upstream and downstream the lake). The results obtained show that in this shallow lake (Zmax = 4.3 m), the eutrophic zone is slightly above one meter. The waters are averagely mineralised (electric conductivity attends 435 µS/cm). The first meter of the water column is well oxygenated and whereas an important deficiency in dissolved oxygen and at times the anoxia is often recorded below 1.5 m depth. The consequence is a production of high quantities of ammoniacal nitrogen (average tenor at 2.5 m depth = 8.9 ± 0.46 mg NH4 + /l). The tenor of phosphorous varies from 80 to 2290 µgP/l. Chlorophyll a concentrations are fairly high and fluctuated between 4.11 and 566 mg.m -3 . These different physico-chemicals parameters coupled with chlorophyll a concentrations explain eutrophic to hypereutrophic status of the lake. In the Mingoa, at the lake upstream position, phosphorus and ammoniacal nitrogen tenors varied respectively from 600 to 3800 µgP/l and from 5.87 to 20 mgNH4 + /l. For these nutrients, the higher ratio of the tenors between lake upstream and downstream shows a reduction rate of 6.3 to 78.45 %. This lake constitutes consequently a natural lagunage for the Mingoa's waters. Wastewater from "Grand-Messa" SIC building estate drained by this stream, is the major cause of the accelerated eutrophication process of the Yaounde municipal lake, beside influents from ministerial edifices and hotels situated near the lake.