The chemical and biological characteristics of Lake Guiers (Senegal) have changed markedly since the impoundment of the Senegal River (Diama and Manantali dams) and subsequent development of irrigated agriculture in the nineteen eighties. On a longitudinal transect of 10 stations (from south to north), the environmental characteristics and the spatial variability of physicochemical variables, phytoplankton and zooplankton communities were studied. Within a marked south-north gradient, the southern stations were characterized by the highest conductivity and pH and by the lowest values of suspended solids, chlorophyll-a concentrations and phytoplankton abundance (mainly Chlorophycea, Cyanobacteria and Bacillariophycea). The spatial distribution of zooplankton showed a clear distinction between the southern zone, characterized by the presence of the rotifers Brachionus falcatus and Conochiloides sp., the cladoceran Bosmina longirostris, the cyclopoid Thermocyclops neglectus and the calanoid Pseudodiaptomus hessei. A co-inertia analysis clearly showed that environmental factors and phytoplankton drives the spatial distribution of zooplankton communities. The comparison of our data with previous studies suggests a marked change in the biological communities since the impoundment of the Senegal River, with rarefaction of P. hessei and proliferation of the cyclopoid Mesocyclops ogunnus. Several hypotheses are discussed to explain these biological changes.
IntroductionThe organization of biological communities in aquatic ecosystems is closely dependent on the variations of physical and chemical conditions linked to natural and anthropic factors (POURRIOT and MEYBECK, 1995). The zooplankton communities, very sensitive to environmental modifications, are important indicators for evaluating the ecological status of these ecosystems (MAGADZA, 1994). The presence and the relative predominance of various copepod species have been used to characterize the eutrophication level of aquatic ecosystems (PARK and MARSHALL, 2000;BONECKER et al., 2001). Herbivorous zooplankton is recognized as the main agent for the top-down control of phytoplankton, and the grazing pressure exert- ed by cladocerans and copepods on algae and cyanobacteria is sometimes an important controlling factor of harmful algal blooms (BOON et al., 1994). The impoundment of the Senegal River carried out by the OMVS (Senegal River Development Organization) in the nineteen eighties has deeply changed the northern part of Senegal. The Diama dam, located 50 km north from the mouth of the Senegal River (Fig. 1) was operated since 1985, in order to stop the marine intrusion. Indeed, before the operating of the dam, in low water situation marine intrusion went up 250 km upstream (GAC et al., 1986). The installation in 1987 of a hydropower dam at Manantali (Mali), located 1100 km upstream, also allowed control of the river flow. These two dams increased the anthropic pressure in the lower Senegal valley, allowing the development of irrigated agriculture (70,000 ha...
