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This work investigates the ecological modifications induced by the annual water level change as a consequence of hydrological regime and power production in Selingue, a monomictic reservoir in Mali (West Africa). High waters occur from November, after the flood, while low waters occur from June, at the end of the dry season. Decrease of water level is linked to environmental factors (marked hydrological pattern, both for river flow and rain) and to human management of the reservoir during the dry season (irrigation, high hydropower demand). Difference in elevation between the high water and the low water phases is 9 m. To assess the ecological impact of such water level variations, environmental and biological descriptors were studied on water sampled biweekly from November 2000 to November 2001 in stations representative of the north part of this water body. As a consequence of high water and of regional meteorology (NE trade winds), the water column in Selingue is stratified from March to May. Stratification can act as a trap for nutrients in the hypolimnion, preventing the euphotic epilimnion to be realimented in dissolved P and N mineral components. This process lasts until the beginning of the monsoon (SW trade winds) when progressive warming and energetic inputs through storms allow vertical mixing. The sink-phase is then replaced by a spring-phase when the water column is not anymore stratified and when the water level is low enough to allow wind-induced resuspension and vertical mixing. At the end of the low water phase, phytoplankton blooms are observed, allowed by the proximity of the productive euphotic layer and the deep mineralization layer and their possible mixing. Water level is also important for fisheries, since fishes are “diluted” in high water (i.e., more difficult to catch with the artisanal tools operated by the local fishermen) but are “concentrated” in low water (i.e., more easily over fished in the minor riverbed, where most fishes are sheltered at the end of the dry season). Irrigation was until now a secondary purpose of the Selingue project, behind power production. Selingue is now of secondary importance for power production in Mali (behind Manantali) and the planned extension of irrigated areas in the Office du Niger is about to seriously increase the need of water from Selingue, the only impoundment upstream. During a hydrological cycle, the only conditions possibly leading to ecological unbalance are met at the end of the low water period. As long as the water level is high enough to allow vertical stratification and vertical separation of the productive euphotic layer from the sink in the bottom layer, Selingue trophic status varies from oligotrophy to mesotrophy. The low water phase is very sensitive, and wise management rules should limit its duration. But the projects leading to a more intensive and time-shifted use of Selingue water could have as consequences an early dewatering of the reservoir and an early shift toward eutrophic conditions. Therefore, there is a need to define and to apply multipurpose rules of water level management that could help to minimize or to prevent these ecological consequences. |
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