Salt marsh

Did You Know?

• Salt marshes occur on low-energy shorelines in temperate and high-latitudes and are replaced by mangroves in the tropics and sub-tropics.
• The formation of a salt marsh begins as tidal flats gain elevation by sediment accretion, allowing vegetation to colonize the exposed surface.
• Mats of filamentous blue-green algae can fix silt and clay particles to their sticky sheaths, increasing erosion resistance of sediments.

Tidal flooding and vegetation zonation

Daily tidal flow distinguishes coastal salt marshes from terrestrial habitats, delivering sediments, nutrients, and water. At higher elevations in the upper marsh, tidal inflow is much less, leading to lower and more variable salinity due to rainfall and evapotranspiration. Lower marsh zones have fairly constant salinity from everyday tidal flooding. Vegetation is differentiated by tolerance to salinity, water table levels, and submersion. Upper marsh zones limit species through competition and lack of habitat protection, while lower marsh zones are determined by physiological stresses such as salinity, submergence, and low oxygen. In the New England salt marsh, strong tidal influences produce distinct zonation: low marsh areas with high flooding are dominated by a monoculture of smooth cordgrass (Spartina alterniflora); landward zones feature salt hay (Spartina patens), black rush (Juncus gerardii), and the shrub Iva frutescens, each adapted to different conditions.

Human impacts

Over half of the world's population lived within 60 km of the coastline as of 2002, making coasts highly vulnerable to human activities. Salt marshes were historically viewed as coastal wastelands and were lost through land reclamation for agriculture, urban development, salt production, and recreation. Indirect effects such as nitrogen loading also cause dieback in the high marsh and die-off in the low marsh. A 2022 study estimated that 22% of saltmarsh loss from 1999 to 2019 was due to direct human drivers like conversion to aquaculture, agriculture, or coastal development, while 30% of saltmarsh gain over the same period resulted from direct drivers such as restoration or modifications promoting tidal exchange. Reclamation for agriculture often involved building dikes and grazing livestock. In New Zealand, the cordgrass Spartina anglica was introduced from England in 1913 to reclaim estuary land for farming, leading to increased sedimentation and spread into other estuaries, outcompeting native plants and animals. Removal efforts are now underway.

Microbial life in salt marshes

Variable salinity, climate, nutrient levels, and anaerobic conditions in salt marshes exert strong selective pressures on microorganisms, which play the main role in nutrient cycling and biogeochemical processing. The microbial community has not been found to change drastically due to human impacts, though research continues. Salt marshes host chemo(litho)autotrophs, heterotrophs, and photoautotrophs, contributing to sulfate reduction, nitrification, decomposition, and rhizosphere interactions. Chemoautotrophs, such as Betaproteobacteria and Gammaproteobacteria, include sulfate-reducing bacteria (SRB), sulfur-oxidizing bacteria (SOB), and ammonia-oxidizing bacteria (AOB). Up to 50% of sedimentary remineralization can be attributed to sulfate reduction, with the dominant class being Deltaproteobacteria, including genera Desulfobulbus, Desulfuromonas, and Desulfovibrio.

Restoration and management

Salt marshes are now recognized as one of the most biologically productive habitats on earth, comparable to tropical rainforests, and are protected by legislation in many countries, such as the Clean Water Act in the United States and the Habitats Directive in Europe. However, many Asian countries, including China, continue to reclaim marshland due to intense coastal development. Bakker et al. (1997) outlined two restoration options: abandoning all human interference, which is often unsuccessful because vegetation struggles to revert and tidal cycles are altered; or restoring the habitat to its natural state at the original or a replacement site. Under natural conditions, recovery can take 2–10 years or longer, depending on the disturbance and marsh maturity. Pioneer-stage marshes recover more rapidly than mature ones, and restoration can be sped up by replanting native vegetation.