Daniel Corbin

Zooplankton Variability in 49 Shallow Tropical Reservoirs of Ivory Coast (West Africa)

Zooplankton samples were collected from 49 small reservoirs of northern Ivory Coast in April 1997. Thirty taxa were identified, including 20 rotifers, 3 copepods and 6 cladocerans. The number of taxa per lake ranged between 12 to 22 and decreased with the total abundance of zooplankton. Copepods dominated standing biomass. Coinertia analysis suggested the role of seston food abundance, oxygen depletion and turbidity for zooplankton abundance and community structure. Rotifers, and particularly Brachionus angularis, Polyarthra and Filinia, were more abundant than copepods in the most eutrophic, turbid and deoxygenated reservoirs. The role of oxygen as a determinant of community structure is probably linked to the specific tolerance of taxa, but turbidity role could not be evaluated with certainty in the absence of information on visual predators.

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-α concentrations ranged from 5.0 to 27.4 μg l -1 in mesotrophic Brobo reservoir and from 15.1 to 123.0 μg 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 μm equivalent spherical diameter, ESD), were observed during the flood. Slow growing/slow sinking species, with relatively large cells (> 15 μm ESD, mainly Cyanobacteria and Chlorophyceae), were present during the dry season. Small species (< 10 μm 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 I, 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.