Bacterioplankton in the Seine River (France): impact of the Parisian urban effluent

Abstract

Bacterial abundance and biomass were studied in April, July, and October 1989 at 13 stations along 300 km of the course of the river Seine, including Paris and its suburbs. Monthly investigations were carried out at five stations downstream from Paris where the river receives the effluent of an important waste water treatment plant (Achères). As a result of an input of allochthonous bacteria from the effluent of the plant, an increase in bacterial abundance and biomass was observed below Achères (from about 5 × 10 9 to 15 × 10 9 cells L −1 and from 100 to 750 μg C L −1 ). This was followed by a rapid decrease. The allochthonous bacteria comprised a high proportion of large bacteria, which disappeared at a much higher rate than the small bacteria (0.0366 vs. 0.0125 h −1 ). Paradoxically, these large bacteria grew at a rate twice that of the smaller cells in culture experiments (0.129 vs. 0.065 h −1 in June and 0.118 vs. 0.071 h −1 in October). These large bacteria must therefore be subjected to intense losses (grazing, sedimentation, etc.). Higher rates of discharge of the river, i.e., a shorter residence time of the water masses, appeared to transmit the Achères signal farther, leading to faster transport of the bacterial populations. Key words: bacterioplankton ecology, size fractions, river ecosystem.