Freshwater ecosystems worldwide are threatened by salinization caused by human activities. Sea level rise from climate change is salinizing coastal freshwater ecosystems. Natural resource mining, agricultural practices, and the use of road deicing salts are leading to increased salinity of inland fresh waters. Lakes, streams, and wetlands have all been affected. Most freshwater organisms have a distant evolutionary history with high salinity, which makes it critically important to assess how salinization will alter ecological communities and the quality of fresh waters. Here, we briefly review the ecologically impacts of salinization in lakes, streams, and wetlands. Spanning multiple trophic levels, we review the species-, community, and ecosystem-level responses to salinization. In addition to ecological impacts, recent discoveries have documented evolutionary responses to freshwater salinization. We report new research suggesting that evolutionary responses to salinization can potentially reduce the magnitude of ecological impacts.
Results/Conclusions
From biofilms to fish, freshwater salinization triggers negative responses from species at all trophic levels. Species-level impacts are generally sub-lethal, leading to reductions in growth and reproduction, which can be magnified by natural stressors like predation. Community-level impacts of salinization include cascading effects leading to algal blooms and reduced biodiversity. Enhanced recruitment of salt-tolerant host species such as mosquitoes might have implications for disease transmission. At the ecosystem level, elevated salinity can alter nutrient and energy flow. Contaminated wetlands will likely export more greenhouse gases, streams will export more nitrogen and carbon, and lakes will encounter altered hydrology and oxygen dynamics leading to greater phosphorus release from sediments. We found that rapid evolutionary responses were able to reduce some of the ecological impacts of salinization, such as cascading effects that lead to algal blooms. Through mechanisms of tolerance and cross-tolerance, freshwater zooplankton were able to reduce the intensity of algal blooms, but this was highly dependent on salt concentration and salt type. Ultimately, salinization will change the structure and function of our freshwater ecosystems. Proactive policies and mitigation strategies are needed to curb the salinization of surface and ground waters to prevent loss of valuable ecosystem services.