Stormwater basins reduce the impact of urban runoff on the biological integrity of receiving streams: a case study from southern Quebec sites (Canada)

Urbanization alters the physical and chemical characteristics of aquatic ecosystems and affects their biota. Stormwater retention/detention basins (SWBs) are widely used to mitigate these impacts by controlling runoff and retaining contaminants; however, the quality of discharged waters remains a concern. This study evaluated the biological integrity of receiving streams upstream and downstream of stormwater effluent, comparing sites with SWBs to those lacking stormwater management infrastructure and receiving direct runoff. Diatom assemblages were used as bioindicators, and biological integrity was assessed using the Eastern Canadian Diatom Index (IDEC; Indice Diatomées de l’Est du Canada). Overall, stronger upstream–downstream declines in IDEC scores were observed at sites without SWBs, whereas most SWB-associated sites showed no significant change or improved IDEC values, indicating better protection of biological integrity. These results suggest that SWBs reduced the negative effects of stormwater effluent on receiving streams.