Changes in the structure and metabolic activities of periphytic communities in a stream receiving treated sewage from a waste stabilization pond

In order to determine the impact of the treated water discharge of a waste stabilization pond on the periphyton in the stream receptor, the structure and the metabolic activities of the periphytic communities were studied above and below the outflow. Only slight changes were observed in the bacterial community. The Diatom abundance and biomass and the relative abundance of the different species differed between upstream and downstream stations. The heterotrophic index was generally lower in the upstream control station than in the downstream stations, whereas periphytic chlorophyll a concentrations decreased downstream of the stabilization pond. This implied an increase in heterotrophic microorganisms and/or an adaptation by autochthonous periphytic microorganisms. Photosynthetic carbon fixation decreased beyond the pond outflow maybe because of the disappearance of sensitive species, while the appearance of other resistant species led to an increase in productivity (autotrophic assimilation/chl a concentrations). Finally, potential chemoheterotrophic activity increased in the downstream stations, while the potential photoheterotrophic activity decreased. On the whole, the pond effluent preferentially favoured the development of chemoheterotrophic microorganisms to the detriment of photosynthetic microorganisms.