Detecting changes in the benthic invertebrate community in response to increasing chloride in streams in Toronto, Canada

Salinization of streams is a growing ecological concern in urban areas. Winter application of deicers to roads (road salt) in cold climates strongly influences stream water chemistry. Total salt use and associated stream Cl– concentrations have increased in North American cities over the past few decades such that current elevated Cl– levels represent an evolutionarily novel stressor to freshwater biological communities. We investigated changes in stream Cl– concentrations between 2002 and 2012 in Toronto, Canada, and related Cl– concentrations to corresponding changes in benthic macroinvertebrates over the same period. Median stream Cl– concentrations in 2012 were almost 2× higher than the median concentrations in 2002, despite the mild winter in 2012. We used the strong relationship between field conductivity and laboratory Cl– concentrations (R2 = 0.93) to model summer Cl– values for 51 benthic macroinvertebrate (BMI) sampling sites in 2002 and 2012. We investigated BMI community shifts with respect to Cl– with conventional metrics and Threshold Indicator Taxa ANalysis (TITAN). Conventional metrics failed to show a coherent pattern of the change between 2002 and 2012, but TITAN results showed that the number of taxa sensitive to Cl– decreased and the number of taxa tolerant of Cl– increased between 2002 and 2012. We identified a BMI community threshold in 2002 with a range of ∼50 to 90 mg/L of Cl–, the level at which the community demonstrated the most taxa changes. The 50 to 90 mg/L threshold is below the Canadian Water Quality Guideline of 120 mg/L for chronic exposure to Cl–. Thus, Cl– may be having nonlethal effects on the BMI community.