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 2x 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 (R-2 = 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 similar to 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.