Living in any habitat comes with hurdles that make it harder for plants and animals to thrive. We call these hurdles “stress”. Coastal wetlands demonstrate several kinds of stresses to both plants and animals. Through many years of evolution, plants and animals have adapted to living with these stresses, also called being “stress tolerant”. Adaptations can be in physical structure changes or on the smaller scale (cellular). Some stresses that come with living in coastal wetlands include salinity (the amount of salt or ions in the water), inundation (flooding at least above the ground, sometimes even higher than the whole plant), and hypoxia (low dissolved oxygen in the water). 
Salt water intrusion has been increased by dredging navigation channels among other impacts. Saltwater intrusion makes fresh bodies of water more saline than they usually are. The problem with this is that the plants that live in such places are adapted to live in fresh water and generally cannot deal with increases in salinity more than 1 or 2 parts per thousand (ppt). For reference, the Gulf of Mexico’s average salinity is approximately 36ppt. Some plants, though, can live in full-strength sea water. For example, the black mangrove (Avicennia germinans) has several adaptations that let it keep its cells safe from high salinity. Like smooth cordgrass (Spartina alterniflora), black mangroves excrete salt onto their leaves to get it out of their systems. Some fish have similar adaptations in their gills that allow them to keep their internal salt concentrations at safe levels.
 Bradford, Nick. “Stressed Wetlands.” NEEF, 10 May 2016, http://www.neefusa.org/nature/land/stressed-wetlands.
 Gilman, Sharon. “Plant Adaptations.” ci.coastal.edu/~sgilman/778Plants.htm.
Featured image is of A. germinans from Wikimedia commons, courtesy of Judy Gallagher