SALT TOXICITY. “Specific to marine environments, seagrasses are often exposed to high salt levels and short-term salinity fluctuations in the coastal and estuarine system. Increased levels of sodium (Na+) are known to be toxic, partly due to the fact that both Na+ and potassium (K+) have very similar physicochemical properties.”
http://bmcevolbiol.biomedcentral.com/articles/10.1186/1471-2148-11-8