Peuplement phytoplanctonique et qualité des eaux en milieu lacustre anthropisé: Cas du lac municipal de Yaoundé (Cameroun)

Abstract

An annual qualitative and quantitative evaluation of phytoplankton and chlorophylla, as well as of some physico-chemical parameters were recorded in the Yaounde Municipal Lake. Physico-chemical measurements were also regularly done in its main tributary (Mingoa stream). The study aimed at demonstrating the cultural eutrophication of this lake. The discharge of untreated urban effluents into the lake seriously threatens his health state. Indeed, the water transparency is globally low and exceptionally exceeds 1m. Water conductivity is higher near the lake bottom, reaching 408 μS cm^-1. The strong deficiency in oxygen contents recorded from 2.5m depth leads to the production of high quantities of ammonium nitrogen. The total phosphorus concentrations vary from 80 to 1650 μgP l-1, and the total Kjeldhal nitrogen concentrations fluctuate between 3 and 15 mgNH₄ + l^-1.  Upstream, the Mingoa exhibits total phosphorus concentrations ranging from 600 to 3800 μgP l^-1, and total Kjeldhal nitrogen concentrations ranging from 10 to 22 mgNH4 + l^-1. Conditions prevailing in this biotope favour a massive growth of phytoplankton, dominated by Euglenophyta and Chlorophyta. Both spatio-temporal analysis of phosphorus, chlorophyll a, and the composition of the  phytoplanktonic community, stress the anthropogenic impact on the ecosystem. The progression of this hydrosystem towards a dystrophic state is going on, and implies an urgent need for its restoration, followed by a rehabilitation of the “Grand Messa” sewage treatment plant, and the elimination process of solid materials in the tributary, upstream.

Key words: sewage, lake, phytoplankton, eutrophication, central Africa.