TROPHIC COUPLING BETWEEN BACTERIAL AND PHYTOPLANKTONIC COMPARTMENTS IN SHALLOW TROPICAL RESERVOIRS (IVORY-COAST, WEST-AFRICA)

Biomass and production of bacterial and phytoplanktonic communities we re measured during diurnal cycles at different stations in 3 shallow t ropical reservoirs (Ivory Coast). Investigations were conducted in 199 5 during 2 typical hydrological seasons (dry season in March and follo wing rainy season in December). Bacterial production in the plankton r anged from 1.2 to 26.2 mu g C l(-1) h(-1) and bacterial biomass ranged from 11 to 163 mu g C l(-1) A slope of 0.625 (n = 93) for the regress ion of log-transformed bacterial biomass versus log-transformed produc tion suggests that the bacteria were strongly controlled by bottom-up processes. Ratios between net primary production and bacterial product ion averaged 67% (range 38 to 140%), indicating that the reservoirs st udied can be considered as meso-eutrophic ecosystems. Average bacteria l carbon demand corresponded to 97% of the net primary pro duction, su ggesting that the biological systems studied are based on autotrophic metabolism. These relationships are the result of a close metabolic co upling between bacterioplankton and phytoplankton, with a large fracti on of primary production routed through heterotrophic bacteria and the microbial loop.