Leaf litter mediates the negative effect of road salt on forested wetland communities

Human modification of landscapes has substantially altered the quality and quantity of terrestrial subsidies to freshwater ecosystems. The same modifications frequently lead to addition of chemical contaminants to freshwater environments. Both types of environmental change can alter the abundance of species and can lead to ecological interactions that affect entire communities. We examined how variation of tree litter inputs interacts with inputs of road salt deicers, which are an increasingly common contaminant in northern latitudes. Based on studies of the effects of each factor in isolation, we hypothesized that elevated Cl^- levels would reduce copepod densities, increase algal abundance, and subsequently increase salt-tolerant consumer densities and biomass. We also hypothesized that these effects would be most pronounced in the presence of highly soluble leaf litter (e.g., Acer rubrum). We constructed experimental freshwater ponds containing assemblages of phytoplankton, periphyton, zooplankton, Physa acuta snails, and 2 species of tadpoles (Lithobates sylvaticus and Anaxyrus americanus). We used a fully factorial design, manipulating leaf litter (none, A. rubrum, or Quercus velutina) and Cl^- concentration (114, 220, 314, and 867 mg Cl/L). Road salt at the 3 lower concentrations had few significant effects. The highest Cl^- concentration reduced copepod densities and increased phytoplankton concentrations, but only in the presence of maple litter. We also observed increased rotifer densities in the highest Cl^- concentration, but only in the presence of either litter species. Our results indicate that road salt contamination can have significant effects on wetland community composition at relatively high concentrations, but these effects depend on the chemistry of allochthonous inputs.