CHANGES IN THE STRUCTURE AND METABOLIC-ACTIVITIES OF PERIPHYTIC COMMUNITIES IN A STREAM RECEIVING TREATED SEWAGE FROM A WASTE STABILIZATIONPOND

In order to determine the impact of the treated water discharge of a w aste stabilization pond on the periphyton in the stream receptor, the structure and the metabolic activities of the periphytic communities w ere studied above and below the outflow. Only slight changes were obse rved in the bacterial community. The Diatom abundance and biomass and the relative abundance of the different species differed between upstr eam and downstream stations. The heterotrophic index was generally low er in the upstream control station than in the downstream stations, wh ereas periphytic chlorophyll a concentrations decreased downstream of the stabilization pond. This implied an increase in heterotrophic micr oorganisms and/or an adaptation by autochthonous periphytic microorgan isms. Photosynthetic carbon fixation decreased beyond the pond outflow maybe because of the disappearance of sensitive species, while the ap pearance of other resistant species led to an increase iii productivit y (autotrophic assimilation/chl a concentrations). Finally, potential chemoheterotrophic activity increased in the downstream stations, whil e the potential photoheterotrophic activity decreased. On the whole, t he pond effluent preferentially favoured the development of chemoheter otrophic microorganisms to the detriment of photosynthetic microorgani sms. (C) 1998 Elsevier Science Ltd. All rights reserved.