High sensitivity of freshwater zooplankton to low chloride concentrations is unaltered by nutrient level
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Driven by decades of road salt use, freshwater salinization is prevalent across North America. Increasing chloride concentrations threaten freshwater species, and zooplankton are particularly sensitive. In Canada, the Canadian Water Quality Guidelines for the Protection of Aquatic Life (CWQG) set the chronic limit for chloride in freshwater at 120 mg/L. The ability of this guideline to protect zooplankton is questionable, given that: it is based on only five species of zooplankton, it relies on controlled laboratory studies that can miss indirect effects of contaminants on complex species interactions, and it does not account for factors that modify response to chloride. Increased food availability is one factor that has been shown to increase chloride tolerance in zooplankton species Daphnia pulex/pulicaria. It follows that differences in nutrient concentration, via their influence on phytoplankton biomass and availability of prey, could drive variation in response to chloride. Our main objectives were to understand how freshwater zooplankton respond to chloride, how nutrients could potentially modify response to chloride, and to determine if the CWQG chronic limit of 120 mg Cl-/L adequately protects zooplankton. We exposed a freshwater zooplankton community to thirty increasing increments of chloride concentrations (ambient to 1500 mg/L) for six weeks, crossed with either ambient or high nutrient treatments. Total biomass, richness and abundance of all zooplankton taxa, genera and species declined with increasing chloride, except for two rotifer genera, Keratella spp. and Lecane spp. Higher nutrients did not reduce the impact of chloride in any group, genus or species. Additionally, we observed 27.7–70.9% loss in abundance by the CWQG of 120 mg Cl-/L, suggesting that these guidelines should be revisited.