Calcium chloride pollution mitigates the negative effects of an invasive clam

Invasive bivalves can drastically alter freshwater ecosystems and affect ecosystem services, but they can be influenced by external factors including calcium concentrations. When a common road salt, calcium chloride (CaCl₂), enters freshwater ecosystems, it may be toxic to organisms or facilitate bivalves by serving as a calcium source. Therefore, CaCl₂ could benefit invasive mollusks tolerant to chloride that require calcium to grow. We used mesocosms to investigate the impacts of CaCl₂ (35–187 mg Ca²⁺ L⁻¹) and invasive bivalves (Asian clams, Corbicula fluminea; zebra mussels, Dreissena polymorpha) on a native lake food web. We hypothesized that invasive bivalves facilitate benthic algae because they reduce phytoplankton and excrete waste. These changes in primary producers would subsequently impact consumers. We also hypothesized that low to moderate CaCl₂ concentrations promote the survival, growth, and reproduction of native and invasive mollusks, while causing few toxic effects. If so, we hypothesized that invaded communities exposed to CaCl₂ experience stronger impacts from the invasive bivalves. We found that invasive bivalves decreased phytoplankton, which led to decreases in periphyton, zooplankton, and native clams. They caused increases in filamentous algae and isopods. While zebra mussels survived poorly in all treatments, moderate concentrations of CaCl₂ substantially reduced Asian clams, which reduced their community effects. Our highest CaCl₂ treatments also reduced zooplankton densities. Thus, while freshwater salinization from road salts poses a concern, we observed no indication that CaCl₂ road salt will benefit Asian clams and zebra mussels. However, the community-wide consequences from Asian clams at low CaCl₂ emphasize the extensive effects that invasive bivalves can have on freshwater communities and the immense concern surrounding their invasions.