Response of periphytic communities exposed to urban wet weather flow in artificial streams: effect of exposure duration

Urbanization causes many disturbances for natural environments. During rainfall, some polluting elements accumulated during dry weather are transported, then collected by sewerage systems. Wastewater overflows are evacuated from combined sewer overflows oil aquatic ecosystems. These << urban Wet Weather Flows >> (WWF) are characterized by composition, concentration, duration and frequency variability. Thus, to evaluate WWF effects the aquatic ecosystem must be simplified and/or controlled. In this context, periphytic assemblages of running water were exposed to an urban rain water in laboratory-designed systems. The purpose of this study was to evaluate the influence of the exposure duration, thus two times were considered (4 h and 8 h). Both structural (biofilm and algae biomass, bacterial enumerations) and functional (two extracellular enzyme activities: leucine aminopeptidase and beta-D-glucosidase) parameters were measured inintediately and 4 days later. Discharge increased bacterial abundance and activities. However, discharge had no effect on chlorophyll. The fact of doubling the exposure duration was insufficient to have a positive effect on the biofilm response.