Role des bacteries phototrophes anoxygeniques dans une partie stratifiee d'une lagune tropicale (Lagune Ebrie, Cote d'Ivoire)

In the deepest parts of the estuarine region of Ebrie Lagoon (Abidjan Africa, (5 degree N, 4 degree W) stratified conditions have been observed during the rainy season (May to Nov.), owing to salinity differences between oxic and anoxic layers. At the interface, 3-4 m depth, phototrophic bacteria (Chromatium, Chlorobium and Pelodictyon ) developed, forming a brown layer due to their different pigments. Their production has been estimated at 41% of the total photosynthetic production during high stratification (Sep. 1981) and their biomass was used by the copepod population for 40 to 60% of its diet. During the dry season, the water column became homogeneous and oxygen was detected to 6m depth. The brown layer of phototrophic bacteria disappeared, as well as sulfate reducers, from the hypolimnion, whereas aerobic heterotrophic bacterial activity and phytoplankton production increased. Thus, They recycled organic and mineral compounds that accumulated in the hypolimnion during stratification. During this stratification, a part of organic and mineral compounds was recycled, by means of anaerobic processes and phototrophic bacteria, thereby making a contribution to the C budget and the food chain which developed in this lagoon.