Road salt alters chironomid assemblage structure and impairs diversity in coastal urban lakes

Lakes in urban areas face increasing salinization from the widespread application of road de‐icing salts. Chironomid (Diptera: Chironomidae) larvae are common bioindicators and important lower‐trophic‐level organisms, but their response to road salt in urban lakes is not well‐characterized. Here, we examine chironomid remains from the surface sediments of 34 urban lakes in Halifax, Nova Scotia, Canada, to characterize community response along a road‐salt impact gradient (6–250 mg Cl − L −1 ). We ask if chloride significantly affects chironomid assemblages, if so at what concentrations, and if we can identify indicator taxa. A forward‐stepped redundancy analysis using a suite of environmental data from municipal monitoring identified chloride, aluminum, and depth as significant drivers of chironomid assemblage structure ( p < 0.001), and chloride was significantly related to Shannon diversity ( p = 0.005) and rarefied richness ( p = 0.03). Threshold Indicator Taxa Analysis suggested most community change occurred at 78.5 mg Cl − L −1 , and we constructed binomial models for the five taxa identified by this analysis as indicators of chloride (all p < 0.05). Species response curves using presence‐absence data identified 16 taxa responding to chloride, and spatial analyses indicated that environmental variables and chloride explained variation in chironomid assemblages independent of spatial structuring. We show that chironomid assemblages are sensitive to road‐salt, which could facilitate long‐term reconstructions, and that increased salinization impairs chironomid diversity in urban lakes. Our study contributes to a body of research showing adverse biological effects of chloride far below the current Canadian water quality guidelines (120 mg Cl − L −1 ).