Urban runoff impacts on benthic communities of a receiving stream

Chemical pollutants are mobilized during rainfall episodes in urban and industrial areas. As a result, complex mixtures of pollutants may enter freshwater ecosystems, potentially exacerbating the individual pollutant effects on aquatic organisms and on ecosystems functioning. We investigated the ecological impacts of five runoff episodes on an urban stream (Girona, Spain) by assessing the effects on the structure and function of river biofilms, and on the community composition of macroinvertebrate assemblages. Each of the five rainfall events was evaluated independently using a Control (upstream) vs Impact (downstream of the urban runoff entrance) design. Urban runoff transported high concentrations of dissolved organic carbon, and a suite of organic (1,3-diphenylguanidine) and inorganic pollutants (zinc, copper). These added to other pollutants which were already present in the urban stream. Small-scale flood events following rainfall episodes caused biofilm detachment, confounding the effects of hydrological disturbance with those of urban runoff pollutants. Urban runoff caused peaks in nutrient, DOC and pollutant concentrations, consistently adding to an increase in chlorophyll-a content, phosphatase activity and bacterial densities in river biofilms. Macroinvertebrate composition variations mostly reflected temporal dynamics. Overall, pollutant impacts associated with urban runoff became masked by environmental factors, and effects on biofilm communities were most evident after events of moderate rainfall, which allowed pollutant-driven responses to emerge. These findings indicate that recurrent urban runoff episodes can increase ecological risks in urban streams, particularly under scenarios of increasing urbanisation and climate-driven extremes.