Fast and slow responses of coastal freshwater ecosystems to salt water intrusion events

Abstract:

For many mid-latitude coastal regions across the globe, climate change models predict increased drought and hurricane intensity. Severe droughts and wind surges can increase the upstream extent of brackish water and the peak salinity of coastal freshwater marshes, swamps, and tidal creeks across the world. The spatial extent of these saltwater intrusion events is increasing through time as a result of both gradual sea level rise and the construction of artificial drainage networks that enhance hydrologic connections between the land and sea. Saltwater intrusion leads to dramatic changes the chemistry of freshwater wetlands. Seawater has high salinity and high alkalinity waters that are rich in base cations and low in organic matter while the majority of freshwater coastal wetlands are acidic with high connectrations of colored organic matter. The resultant mixing of widely divergent waters leads to rapid changes in solute chemistry. Cations in seasalts (i.e., Mg²⁺ and Na²⁺) can displace soil bound ammonium (NH₄⁺) and lead to flocculation of DOM through cation bridging. These fast changes dramatically change the magnitude and the stoichiometery of element exchange between land and sea. Slower changes also accumulate as sea salt enrichment of soils leads gradually to the alkalinization and sulfidation of wetland sediments and soils. Because of drought and event loading of marine salts we expect significant landward migration of brackish waters and a loss of typical coastal freshwater wetland vegetation from much of the coastal plain well in advance of significant sea level rise. The consequences of salt water intrusion for the large standing stocks of organic matter currently stored in coastal freshwater wetlands is less well understood.