The effects of climate and hydrology on the trophic status of Sélingué Reservoir, Mali, West Africa
This work investigates the effects of climate and hydrology as factors controlling the trophic status of Sélingué, a monomictic reservoir in Mali, West Africa. Environmental (water transparency, hydrochemistry, nutrients) and biological (chlorophyll a concentrations) descriptors were studied on water samples collected biweekly from November 2000 to November 2001 in a station representative of the northern part of the water body. Statistical methods were used to estimate the existence and significance of breaks in the time series and environmental data were used to explain these breaks. In Sélingué Reservoir, the water column is stratified from March to May as a result of the cooling induced by the harmattan (North-East trade winds). Stratification lasts a few weeks, until the beginning of the monsoon (South-West trade winds), after which progressive warming allows vertical mixing of the lake. In harmattan or monsoon situations, the winds do not play a significant role except for storm events during the rainy season. Such events can change transiently, but markedly, the physical conditions prevailing in the whole water column. The water quality closely follows the hydrological cycle, characterized by a unique flood, which lasts a few weeks, from mid-July. Input of nutrients during the flood events is limited and local mineralization of organic matter in the hypolimnion during the stratification period seems to play a major role in the availability of dissolved inorganic nitrogen and soluble reactive phosphorus. This availability is subject to vertical mixing which is only possible when the water height is sufficiently low and the wind-induced energy is sufficiently high. Dry (dust during the harmattan period) and wet deposition (linked to rains) play a role in supplying nutrients to the lake and can be factors in the observed increased chlorophyll a concentrations. Thus, environmental factors in Sélingué change with the current hydrological conditions. Sélingué Reservoir can be regarded as oligotrophic during the high water period, but as meso-eutrophic during the low water period. Each annual flood initializes the ecological conditions in this water body. This pattern limits the risks of trophic change (which can lead to algal blooms) to short periods of very low water levels immediately preceding flood events.
Document Type: Research Article
Affiliations: Institut de Recherche pour le Développement, UR 098 Efflorescences algales, Centre de Bel Air, BP 1386, Dakar, Senegal
Publication date: September 1, 2003