Abstract A 1 year qualitative and quantitative evaluation of phytoplankton and chlorophyll-a, as well as some physicochemical parameters, was recorded in a shallow tropical lake in Cameroon: the Yaounde Municipal Lake. Physicochemical measurements also were regularly made in its main tributary (Mingoa Stream). These analyses aimed to assess the lake's trophic status and to propose measures for controlling its degradation process. The Secchi disk transparency was low and rarely exceeded 100 cm. Conductivity was higher near the lake bottom. The oxygen deficiency, and sometimes anoxia, recorded from a 2.5 m depth leads to high quantities of ammonium-nitrogen. The total phosphorus concentrations varied from 80–2290 µg P L−1 and the total Kjeldhal nitrogen concentrations fluctuated between 3 and 15 mg L−1. Upstream to the lake, in the Mingoa Stream, total phosphorus concentrations ranged from 0.6–3.8 mg P L−1 and total Kjeldhal nitrogen concentrations ranged from 10–22 mg L−1. There are up to 102 phytoplankton-specific taxa, with Euglenophyta and Chlorophyta particularly more diversified. The phytoplankton biomass and chlorophyll-a concentrations reached 225 µg mL−1 and 566 mg m−3, respectively. The analyses pointed out the allogenic nature of the functioning of this ecosystem as a result of bad waste management in the surrounding landscape. Urgent actions need to be undertaken in order to rehabilitate this lake, which rapidly shifted to a hypertrophic status.