The different primary producers in a small African tropical reservoir during a drought: temporal changes and interactions

1. The biomass of the different primary producers, and their relative contr ibutions to the total, were measured quarterly from July 1997 to September 1998 in a tropical reservoir (Brobo, Cote-d'Ivoire). The study period was m arked by an exceptional decrease in the water level as a result of a long d rought with only one significant rainfall in June 1998. 2. In July 1997, at the beginning of the usual rainy season, the level of t he lake was normal and the ecosystem was in a moderately clear water state dominated by littoral macrophytes, mostly Potamogeton octandrus (more than 55% of the 6.4 tons of carbon of the total primary biomass). The microphyto benthos (19%) and the phytoplankton (25%) were secondary contributors, wher eas the periphyton on macrophytes (0.5%) and the epixylon (periphyton on de ad flooded trees, 0.3%) were negligible. 3. As a result of the steady decline in water level due to a lack of rain, the macrophytes were progressively stranded on the shores and had disappear ed by March 1998. From this time, microalgae (microphytobenthos and phytopl ankton) became dominant. Their combined biomass increased slowly from 2.8 t ons of carbon in July 1997 to 3.7 tons in September 1998. The microphytoben thic biomass contributed 78% to the total in March, 55% in June and 60% in September, while other contributions were mainly due to the phytoplankton. The epixylon remained negligible throughout the study ( less than or equal to 0.5% of the whole primary biomass). 4. The distribution and temporal changes in the biomass during each survey were mainly Linked to hydrology and to interactions between primary produce rs. 5. The effect of water level changes on free (planktonic) and fixed primary producers (periphytic microalgae and rooted macrophytes) is discussed.