Road salt pollution shifts urban stormwater ponds toward cyanobacterial dominance

Urban environments contend with an array of stressors, including salinization by deicing road salts. To advance understanding of how road salt pollution affects aquatic ecosystem functioning, we surveyed primary producers in 50 stormwater ponds in Brampton, Canada. We found that chloride concentrations decreased (benthic) periphytic algal biomass but had no detectable effect on the total biomass of (free‐floating) phytoplankton. However, impacts were obscured by underlying compositional shifts, as cyanobacteria generally compensated for declines of other taxa. Varying sensitivities of taxonomic groups (inferred from diagnostic pigments) revealed potential bioindicators, with the proportion of periphytic chromophytes declining most significantly and effects on the relative concentrations of green algae differing between planktonic and benthic communities. As chloride concentrations were a leading predictor of cyanobacterial dominance in our study of impaired, nutrient‐rich, urban ponds, findings reveal an emerging risk of potentially harmful organisms from the ongoing salinization of freshwater resources.