Persistent application of de-icing agents (mainly NaCl) on roads and other paved surfaces in winter has resulted in increased chloride concentrations in lakes and streams. Although essential to human safety, the use of road salt is inadvertently contaminating freshwater ecosystems and there is mounting evidence of impaired community structure and ecosystem function. Zooplankton are key components of lake food webs, transferring energy from primary producers to higher trophic levels, including fish, yet we have limited understanding of the effects of increasing chloride on zooplankton community structure across diverse habitats. Food availability and water hardness can influence chloride toxicity, indicating that regional variation related to surficial geology, land-use, and habitat connectivity may influence the vulnerability of local zooplankton communities to chloride toxicity. To determine the effect of nutrients (i.e., food quantity), water temperature, and history of exposure to chloride, we conducted field mesocosm experiments where we exposed zooplankton communities to an NaCl gradient, crossed with either nutrient, temperature, NaCl exposure treatments.
Results/Conclusions
We found that the effect of chloride varied among lakes with differing salt exposure, but was less affected by our nutrient, water hardness, or water temperature treatments. When the lake of origin had a low chloride concentration (<1mgCl/L), salt additions increased the abundance of littoral taxa and decreased abundance of pelagic cladocerans. When the zooplankton community originated from a lake intersected by a winter road (~20mgCl/L), there was no detectable effect. In contrast, zooplankton response was similar in experiments conducted in water with low Ca concentration (3 mgCa/L) compared to high Ca concentration (30 mgCa/L). In both treatments, total abundance declined as chloride concentration increased from ~ 5 to 400 mgCl/L and richness declined as chloride concentration increased from ~5 to 500 mgCl/L. Abundance and richness remained uniformly low for chloride concentrations ranging from 500 to 1500 mgCl/L. In both experiments, copepods were the most sensitive, followed by cladocerans, then rotifers. There was no detectable effect of increased nutrients or increased temperature on the response of zooplankton to increasing salt. Further study is needed to elucidate factors driving differences in regional responses to salinity.