Anthropogenic activities such as mining, water diversion, improper irrigation practices, industrial waste, and the application of road de-icing salts are increasing the concentrations of salts in fresh waters. At high concentrations, salts are toxic to many freshwater organisms. However, concurrent stressors such as the presence of invasive species might exacerbate the toxic effects of salts for freshwater organisms, and disrupt the structure and function of food webs at lower salt concentrations than expected from toxicity studies. To study the combined effects of salt pollution and invasive species, we manipulated the concentration of salt (NaCl) and the density of the invasive Asian clam in experimental freshwater ponds.
Results/Conclusions
Increasing densities of the invasive clam reduced phytoplankton abundance and reduced the abundance of a native clam species, but did not affect zooplankton abundances or the abundances of other native benthic macro-invertebrates. At high concentrations NaCl was toxic to cladoceran and copepod species, resulting in increased phytoplankton growth and increased rotifer abundance. Increasing NaCl concentrations reduced the biomass of periphyton, and the highest concentrations of NaCl (860 and 1350 mg Cl-/L) had strong toxicological effects on the native benthic invertebrates and the invasive Asian clam. However, increasing densities of the invasive clam caused the populations of benthic macro-invertebrates to decline at lower salt concentrations (250 mg Cl-/L) than NaCl alone. This indicates that multiple anthropogenic stressors intensify the negative effects that salt pollution can have on freshwater food webs.
These results highlight the need for standardized management and regulatory strategies that will protect fresh waters from continued salt pollution. We discuss potential improvements to monitoring programs and technology advancements that can be used to study the extent of salt pollution and understand its effects. We also show how salinization regulations vary among states and countries, and discuss how the failure for these regulations to include legal consequences could drive continued pollution that results in severe economic, social, and environmental consequences.