The effects of increasing chloride concentrations and temperatures on freshwater zooplankton communities
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Across the Northern Hemisphere, chloride concentrations in lakes are increasing from decades of road salt use, imperiling aquatic ecosystems. Elevated chloride concentrations threaten aquatic organisms, and zooplankton are especially sensitive. Despite the negative effects of chloride on zooplankton and aquatic communities, current water quality guidelines are based on laboratory studies that exclude indirect effects or additional stressors that may modify tolerance to chloride. Additionally, increasing lake temperatures could exacerbate the negative effects of increasing chloride concentrations on zooplankton, possibly through changes to the phytoplankton community. To determine the effects of increasing chloride concentrations and lake temperatures on freshwater plankton communities and investigate the applicability of the current Canadian Water Quality Guideline for chloride (CWQG), we conducted a mesocosm experiment. We used thirty concentrations of chloride from ambient to 1500 mg/L, ambient or warmed temperatures, and plankton from a lake typical of the Canadian Shield. We found that increasing chloride concentrations negatively affected zooplankton, even at concentrations below the CWQG. Zooplankton abundance, biomass and richness decreased with increasing chloride concentrations to an extent that would likely impair ecosystem functions in highly impacted lakes, and no zooplankton taxa were positively associated with high chloride concentrations. Increasing chloride concentrations also led to a shift in phytoplankton community composition marked by an increase in cyanobacteria, which likely contributed to the negative effects of elevated chloride on zooplankton. The effects of warming, however, were limited to increasing copepod abundance and green algae concentration, and decreasing cyanobacteria concentration. Overall, zooplankton in our experiment were highly sensitive to increasing chloride concentrations and experienced 28 – 55% declines in abundance and biomass at the CWQG, suggesting that Shield lakes are not sufficiently protected from the impacts of road salt pollution.