Phytoplankton of the Yaounde municipal lake (Cameroon): Ecological succession and populations structure
The Yaounde municipal lake (3°52'N, 11°32'E) is a shallow hypertrophic water body (Z max=4.3 m), situated downstream from Yaounde. It is an artificial lake resulting from a dam constructed in 1951 across a small river named Mingoa. Like many other water bodies in urbanised areas, this ecosystem switched rapidly from being mesotrophic to hypertrophic during the 1980's, due to the expansion of human activity and an inefficient treatment of wastewater in its catchment area. This study was carried out with the objective to analyse the ecological succession and the spatio-temporal fluctuations of the phytoplankton population structure in this shallow equatorial lake. Samples were collected at weekly intervals from November 1996 to December 1997, at a station near its central zone (depth=3.5 m), using an opaque horizontal PVC Van Dorn sampler. Sampling operations were conducted at the following levels: surface; -0.5 m; -1 m; -1.5 m and -2.5 m. Individual phytoplankton from 1% iodine acid preserved subsamples (BOURRELLY, 1990) were enumerated with an inverted microscope (Olympus CK2) at 200X magnification, using a Sedgwick-Rafter counting cell, according to UTHERMÖHL (1958). At least 400 individuals (colonies, bundles or trichomes, depending on existing life forms) were counted on each subsample. Specific biomass was then obtained using the biovolume method. To evaluate the population structure dynamics, the SHANNON and WEAVER (1949) specific diversity index method and the evenness (PIELOU, 1966) method were computerized using specific biomass, the evenness being the ratio between the real and the maximal diversity (log2 S, where S is the number of species). Also, rank-frequency diagrams were carried out in logarithmic co-ordinates according to FRONTIER (1969), using integrated samples obtained from grab samples collected over the entire water column. Cell densities were highest in the uppermost meter of the water column at the beginning of the rainy season, due to the exponential development of species such as Planktothrix mougeotii, Chlamydomonas sp., Eudorina elegans, Euglena gracilis, Gonyostomum semen, Euglena texta, Phacus helicoïdes, Closterium limneticum, Cryptomonas ovata, Peridinium cf. gutwinskii, Trachelomonas hispida var. crenulatocollis and Cyclotella meneghiniana. The specific diversity index varied 0.68 and 4.64 bits/μg, and values gradually decreased from the first uppermost meter to the bottom of the water column. Evenness varied from 0.14 to 0.84 with a somewhat similar spatio-temporal variation as the diversity index. Low values of these descriptors generally correspond to the presence of a highly dominant species. The rank-frequency diagrams were mainly S-shaped, indicating the predominance of one species which in this study was either Chlamydomonas sp or Planktothrix mougeotii. A simultaneous analysis of the ecological succession and the spatio-temporal variations of the phytoplanktonic structure revealed that the functioning of this ecosystem was essentially allogenic in nature and was strongly influenced by the supply of decayed organic matter from the Mingoa stream, and effluents from ministerial buildings and hotels located near the lake. This permanent supply of abundant biogenic elements, coupled with the low depth of the ecosystem, makes it vulnerable. Ecological succession, frequently limited to the pioneer stages (1 and 1'), reflected the briefness of the different phases of phytoplankton growth, and the consequences of disturbances regularly undergone by the phytoplankton community. The second stage was scarcely reached while the third was absent. These successions were mainly controlled by rainfall, as well as wind, which is responsible for the mixing of such a shallow water column. These results can be considered important for the Yaounde municipal lake and other water bodies close to densely populated areas in Cameroon for which there is no management plan. Urgent actions need to be carried out to rehabilitate this ecosystem that fluctuates between the eutrophic and hypertrophic status.