Impacts of urban runoff pollutants on biofilm communities

Urban runoff transports a wide array of inorganic and organic materials, such as organic matter, salt, dust, plastics and a high diversity of pollutants, which may harm freshwater ecosystems, especially during the initial runoff inputs. The aim of this study is to assess the structural and functional impacts on freshwater biofilms subjected to urban runoff pollutants. We provide a first assessment exposing natural freshwater biofilms to progressive dilution of pure first-flush urban runoff under controlled laboratory conditions. In parallel, we examined the effects of selected pollutants found in urban runoff on biofilms, testing them individually and in combination. Urban runoff contained 47 compounds from 9 different families. The most abundant were 1,3-diphenylguanidine (a tire-related additive), caffeine (a stimulant), and zinc (a metal), each reaching concentrations close to 30 μg L - 1 . Dissolved organic carbon was also high (25 mg L −1 ). Impacts of urban runoff on biofilms were strongest at 25 % dilution, and overall impacts were moderate, reflecting a balance inhibitory influence of pollutants and stimulatory effects of dissolved organic carbon and nutrients. The selected pollutants experiment revealed that biofilms were most sensitive to the mixture of pollutants and to diuron, followed by the 16PAHs and BBP. Overall, our results highlight the complex and non-linear responses of stream biofilms to urban runoff inputs, emphasizing the need to control the types and quantities of pollutants released, with particular attention to emerging contaminants. • Urban runoff carries multiple pollutants as well as organic carbon and nutrients. • Urban runoff causes both inhibitory and stimulatory effects in biofilms. • Diuron effects on biofilms occurred at environmental concentrations. • BBP and PAHs impacted biofilms only at elevated concentrations .