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Salinity is a strong driver of biodiversity in aquatic ecosystems. It have been found to control the distribution and abundance of organisms at all trophic levels including plants, animals, and microbes. The effect may be direct (e.g., increasing physiological stress) or indirect (e.g., by changing the chemical properties of the environment in a way that affects its suitability as a habitat). Increases in salinity (“salinization”) can also alter rates of important chemical reactions and thus affect the cycling of major elements including carbon, nitrogen, and sulfur.
Salinization of coastal habitats is occurring at an unprecedented rate, due in large part to human activities that change the flow of freshwater (e.g., land clearance, irrigation, and drinking water withdrawal). Sea level rise associated with climate change is also a concern, and is already causing saltwater intrusion into freshwater wetlands around the world. Salinization has a negative effect on biodiversity and influences the ability of the wetland to perform important ecosystem services such as water purification, flood protection, and shoreline stabilization.
Salinization also affects the biogeochemistry of the wetlands in ways that can be globally significant. For example, wetlands play a major role in the global carbon cycle by sequestering carbon, and some scientists estimate that as much as 25 percent of global terrestrial carbon is stored in these ecosystems. Microbes in wetland soils are the foundation of this important ecosystem service. Unfortunately, changes to the hydrologic conditions of the wetland, including increases in salinity, could affect microbial activity in a variety of ways that ultimately change how much carbon is stored in these environments. In addition, salt water intrusion has been shown to dramatically affect the microorganisms involved in nitrogen cycling; these changes can hamper the ability of the wetland to remove nitrogen from polluted waters before it reaches fragile coastal ecosystems. Salinity effects on these microbial communities can also alter the rates at which the wetland releases greenhouse gases like carbon dioxide, methane, and nitrous oxide.