Dynamic of phytoplankton size-class and photosynthetic activity in a tropical hypereutrophic lake: the Yaounde municipal lake (Cameroon)

Abstract

This study carried out within the framework of a multi-disciplinary project, aimed at highlighting the fundamental processes governing the functioning of the hypertrophic Yaounde Municipal Lake. It was based on the hypothesis that, even within a small range of lake trophic status, important variations can occur in the species composition, biomass and photosynthetic activity of the phytoplankton size-fractions. For this purpose, samples were taken at weekly intervals from November 1996 to December 1997 at a fixed set of depths in the water column. Spatial-temporal fluctuations of some physical–chemical variables, associated with biological variables such as phytoplankton size-class species composition, phytoplankton size-class biomass, phytoplankton primary production and chlorophyll-a were analysed. The water transparency was low and rarely exceeded 100 cm. Conductivity values relatively higher increased generally from the top to the lake bottom. The oxygen deficiency, and sometimes anoxia, recorded from 2.5 m depth resulted in high quantities of ammonium nitrogen. Total phosphorus and total Kjeldahl nitrogen concentrations were characteristics of hypertrophic lakes. The fertility of this biotope favoured the development of a high phytoplanktonic community with remarkable physiological adaptations to the variations of the nutritive potentials of the lake, characterized by the size-structure of these organisms. Cells of small size ( 45 μm), mainly Euglenophyta, maintained the relatively most important biomass, accounting for up to 89.85% of the total phytoplanktonic biomass. Chlorophyll-a concentrations are among the highest reported for both fresh water and sea water, being a consequence of high proportions of Chlorophyta and Euglenophyta. This led to intense phytoplanktonic photosynthetic activity which continued throughout the year, even though it was confined to the upmost first meter of the water column. Analyses pointed out the allogenic nature of the functioning of this urban lake ecosystem, due to a poor waste management on the surrounding landscape.