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Artificialization and evolution of the trophic status in a Sahelien shallow lake: Lake Guiers (Senegal) Volume 24, issue 1, Janvier-Février-Mars 2013

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Université Cheikh Anta Diop (Ucad) Département de biologie végétale BP 5005 Dakar Sénégal, IRD Département environnement et ressources 44, boulevard de Dunkerque CS 90009 F-13572 Marseille cedex 02 France

The hydrological functioning of Lake Guiers, the main freshwater reservoir of Senegal, has been totally under control since the edifications of the Diama and Manantali dams on the Senegal River in 1985 and 1987, respectively. The trophic status of the reservoir has been dramatically impacted by these huge disturbances. Physicochemical and biological parameters were monitored for four years (2002 to 2005) in order to characterize this evolution. Our results reveal an eutrophic state from 2002 to 2004, followed by a hypertrophic status in 2005. Secchi disk measurements remained below 100 cm. Water conductivity increased significantly in 2004 and 2005 compared to the previous two years. The concentrations of nutrients ranged from 0.3 to 7.8 μgP/L (PO 4 3−), 0 to 14.5 μgN/L (NO 3 ) and 1.3 to 28.9 μgN/L (NH 4 +). Chlorophyll biomasses have fluctuated annually between 5 and 50 μg/L from 2002 to 2004 and between 20 and 105 μg/L in 2005. One hundred and eleven phytoplankton species belonging to seven classes were identified during the annual cycle of 2002 to 2003 with a dominance of Cyanobacteria (25%) and Chlorophyceae (32%). The multiple and growing anthropogenic pressures to which the lake is being subjected may contribute to the continued deterioration of its water quality (eutrophication). Regular monitoring of the health status of this emblematic and strategic ecosystem is a requirement.