Trophic coupling between bacterial and phytoplanktonic compartments in shallow tropical reservoirs (Ivory Coast, West Africa)

Biomass and production of bacterial and phytoplanktonic communities were measured during diurnal cycles at different stations in 3 shallow tropical reservoirs (Ivory Coast). Investigations were conducted in 1995 during 2 typical hydrological seasons (dry season in March and following rainy season in December). Bacterial production in the plankton ranged from 1.2 to 26.2 mu g C l super(-1) h super(-1) and bacterial biomass ranged from 11 to 163 mu g C l super(-1). A slope of 0.625 (n = 93) for the regression of log-transformed bacterial biomass versus log-transformed production suggests that the bacteria were strongly controlled by bottom-up processes. Ratios between net primary production and bacterial production averaged 67% (range 38 to 140%), indicating that the reservoirs studied can be considered as meso-eutrophic ecosystems. Average bacterial carbon demand corresponded to 97% of the net primary production, suggesting that the biological systems studied are based on autotrophic metabolism. These relationships are the result of a close metabolic coupling between bacterioplankton and phytoplankton, with a large fraction of primary production routed through heterotrophic bacteria and the microbial loop.