Factors limiting phytoplankton productivity in 49 shallow reservoirs of North Côte d'Ivoire (West Africa)

Several hundred ranching reservoirs are scattered in North Côte d'Ivoire, where they have an increasing economical importance as water resources during the dry season. To obtain a synoptic view of their limnological characteristics and to identify key factors limiting algal productivity, 49 of these shallow reservoirs were sampled once in 1997 at the end of the dry season. They showed low conductivity (range 40-230 µS cm -1 ) and low pH (range 5.1-7.3), and lakes deeper than 2 m were stratified. Most lakes had low nutrient concentrations (median: 0.09, 3.49 and 0.44 µM, for NO 3 -N, NH 4 -N and PO 4 -P, respectively). They were very turbid, with most of Secchi depths ranging between 0.1 and 0.4 m, and had high seston weights (median, 45 mg l-1). Many lakes had high chlorophyll concentrations (median, 106 µg l -1 ), and some of them featured algal blooms, while many lakes had primary production value <1000 mg C m -2 d -1 . Algae of size >10 µm represented about 50 percent of the chlorophyll biomass and phytoplankton production. Classification on hydrological and particle data identified five groups based on depth, turbidity and productivity. During the dry season, the initial nutrient pulse linked to the annual flood is exhausted. At this time of the year, phytoplankton production was nitrate limited in some lakes, while in some others, phosphorus was limiting. Deep sites (>2 m) were less turbid and less productive than shallow ones (<2 m). In shallow unstratified lakes showing a nutrient-rich bottom layer, easy vertical mixing allow a regular enhancement of the algal productivity. In deeper and stratified lakes, nutrient enrichment only occurs when high intensity winds induce vertical mixing, and their productivity is directly related to these episodic nutrient pulses.