The present study is aimed at providing (i) accurate data on the relationships between phytoplankton and bacteria responses to varying loading levels of nutrients such as nitrogen, phosphorus and organic matter, and (ii) insight into the denitrification process from in situ experiments. The low developpment of phytoplankton was unlikely to be accounted for by the prevailing hydrodynamic, trophic and light conditions, all indeed favouring its growth, therefore competion processes were thougth to exist between diverse plant standing stocks. Regarding the bacterial compartment, a significant increase in biomass and activity was also manifest downstream of Angoulême's sewage effluent discharges. Bacteria associated to such urban effluents were characterized by both physiological features (high ratio, i.e. 20, for leucine to thymidine incorporation rates) and morphological features (high biovolume, i.e. higher 0.2 [mu]m3) far distinct from those displayed by autochthonous bacteria (i.e. mean cellular volume fo roughly 0,15 [mu]m3 and low leucine : thymidine ratio of approx. 10). The present investigation, however, gave no evidence of signifiant relations betweeen phytoplanktonic and bacterial communities. Indeed, allochthonous supplies of organic matter were merely shown to meet bacterial carbon requirements sufficiently so that no further substantial supply by primary production (in terms of excretion or algal exudates) was needed to enable bacteria to develop. The denitrification process resulting in nitrogen losses as nitrogen gas (NH2), it was clearly pointed out that if such a denitrification process did not occur in the whole water column, its rate at the water-sediment interface might be considered as constant and independant from variations in temperature, nitrate contents and dissolved oxygen levels at the interface. Then, the annual mean rate of denitrification in the River Charente levelled off at 682 mgNO3/m2/j bejond any marked environmental influence