Declining hydrologic function of coastal wetlands in response to saltwater intrusion

Abstract

Salinity and submergence have shaped coastal wetlands into one of the most productive and yet fragile ecotones worldwide. Sea-level rise alters both salinity and submersion regimes, threatening the future existence of these habitats. Still, the magnitude of the impacts is elusive. A current paradigm is that coastal communities will keep pace with sea-level rise through soil accretion, a process largely sustained by the interaction of plants and geomorphology. This thesis, however, rests on the assumption that submersion is the main driver of bio-geomorphic processes and does not include the effects of salinity on plant hydrologic function. Here, we show that salinity has a major influence on how tidal vegetation controls the depth of the water table - a key ecohydrological process regulating plant establishment, survival rate, and productivity. Our findings rely on long-term observations from the Florida Everglades and modeling results. They indicate that altered salinity regimes can suppress transpiration, undermining in this way the ability of tidal ecosystems to control water table movements and contrast other forms of plant stress like waterlogging. This effect is apparent in the mangroves of the Everglades' tidal fringe, where salinization has already yielded extensive decoupling between plant hydraulics and groundwater dynamics. This mechanism leads to shallow water tables and poor soil-aeration conditions, with sizable impacts on coastal wetlands' hydrologic function and elasticity.