Urban runoff carries contaminants from roofs, roads, and soils into streams, affecting ecosystem biodiversity and functioning, particularly during rainfall and after dry periods. To examine these effects, urban first flush samples were collected during rainfall and tested on freshwater biofilms. Also, two acute exposure experiments were conducted using typical urban runoff contaminants on a planktonic alga and biofilm communities.

Results indicate the planktonic alga as more sensitive to these contaminants, whereas freshwater biofilms show less impact. While some contaminants had minimal effects, others exhibited strong individual or combined impacts, demonstrating additive and synergistic effects that could increase runoff toxicity. Overall, urban runoff poses significant risks to river microbial biota, and these effects could be aggravated by climate change.