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

Abstract

1. The biomass of the different primary producers, and their relative contributions to the total, were measured quarterly from July 1997 to September 1998 in a tropical reservoir (Brobo, Côte‐d'Ivoire). The study period was marked by an exceptional decrease in the water level as a result of a long drought with only one significant rainfall in June 1998. 2. In July 1997, at the beginning of the usual rainy season, the level of the 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 microphytobenthos (19%) and the phytoplankton (25%) were secondary contributors, whereas the periphyton on macrophytes (0.5%) and the epixylon (periphyton on dead 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 disappeared by March 1998. From this time, microalgae (microphytobenthos and phytoplankton) became dominant. Their combined biomass increased slowly from 2.8 tons of carbon in July 1997 to 3.7 tons in September 1998. The microphytobenthic 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 (≤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 producers. 5. The effect of water level changes on free (planktonic) and fixed primary producers (periphytic microalgae and rooted macrophytes) is discussed.