Environmental conditions and phytoplankton assemblages in two shallow reservoirs of Ivory Coast (West Africa)

Phytoplankton biomass, productivity and assemblages were studied during the main hydrological seasons in two shallow reservoirs in Ivory Coast (West Africa). In this area, the hydrological pattern begins with the flood from July to October, that is the main change in environmental condition. The flood is followed by a dry period from November to March; then the rainy season resumes from April with numerous and intense storms. Chlorophyll-a concentrations ranged from 5.0 to 27.4 mug l(-1) in mesotrophic Brobo reservoir and from 15.1 to 123.0 mug l(-1) in eutrophic Nambengue reservoir. Despite their different trophic states, phytoplankton production varied between 0.2 and 1.8 g C m(-2) d(-1) in both reservoirs. Both Chlorophyceae and Cyanobacteria largely dominated the phytoplankton communities. Opportunist species (mainly Chlorophyceae), with medium sized cells (10-15 mum equivalent spherical diameter, ESD), were observed during the flood. Slow growing/slow sinking species, with relatively large cells (> 15 pm ESD, mainly Cyanobacteria and Chlorophyceae), were present during the dry season. Small species (< 10 mum ESD), dominated by Chlorophyceae and Cyanobacteria were abundant during the rainy season. At this late stage of the hydrological year, N-2 fixing Cyanobacteria like Anabaena and Anabaenopsis were also present. Eight functional associations (H 1, J, K, L-M, N, P, S 1, X 1) from the thirty-one proposed by REYNOLDs et al. (2002) were outlined in the two systems surveyed; these functional groups were representative of warm, shallow, turbid, nutrient-enriched and frequently mixed habitats. A large part of the biomass was composed of cells hardly matching the needs of superior consumers, in size as well as in shape. These conditions could explain the inefficient transfer of biomass within the pelagic food web, resulting in trophic dead end and contributing to the eutrophication of the sites.