Salt marsh

Did You Know?

• Salt marshes occur on low-energy shorelines in temperate and high-latitudes, and in the tropics they are replaced by mangroves.
• A 2017 study mapped saltmarshes across 99 countries, with a total of 5,495,089 hectares represented in a polygon dataset.
• Formation begins as tidal flats gain elevation through sediment accretion, allowing pioneer species to colonize; filamentous blue-green algae can fix silt and clay particles.

Tidal flooding and vegetation zonation

Daily tidal flow delivers sediments, nutrients, and water to the marsh, distinguishing it from terrestrial habitats. In the upper marsh, less frequent flooding leads to variable salinity due to rainfall and evapotranspiration, creating microhabitats. Vegetation zones are determined by plants' tolerance to salinity, water level, and oxygen availability; lower marsh zones are shaped by physiological stress tolerance, while upper zones are limited by competition. In New England salt marshes, low marsh areas with high tidal flooding support a monoculture of smooth cordgrass (Spartina alterniflora). Moving landward, zones of salt hay (Spartina patens), black rush (Juncus gerardii), and the shrub Iva frutescens appear, each adapted to specific conditions.

Human impacts

Over half of the world's population lived within 60 km of the coast as of 2002, putting pressure on salt marshes. Historically viewed as coastal wastelands, marshes were reclaimed for agriculture, urban development, salt production, and recreation. A 2022 study estimated that 22% of saltmarsh loss from 1999 to 2019 was due to direct human drivers such as conversion to aquaculture, agriculture, or coastal development, while 30% of gain was also directly driven by restoration or tidal exchange modifications. Land reclamation for agriculture involved diking and grazing by livestock, altering vegetation, sedimentation, salinity, and biodiversity. In New Zealand, the cordgrass Spartina anglica was introduced in 1913 to reclaim estuary land for farming, but it spread and outcompeted native species; removal efforts are now underway.

Microbial life in salt marshes

Variable salinity, climate, nutrient levels, and anaerobic conditions create strong selective pressures on salt marsh microorganisms, which play a major 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. Chemoautotrophic bacteria such as Betaproteobacteria and Gammaproteobacteria include sulfate-reducing, sulfur-oxidizing, and ammonia-oxidizing bacteria. Up to 50% of sedimentary remineralization can be attributed to sulfate reduction, with Deltaproteobacteria (e.g., Desulfobulbus, Desulfuromonas, Desulfovibrio) being the dominant class.

Restoration and management

Salt marshes are now recognized as one of the most biologically productive habitats on earth and are protected by legislation in many countries, such as the Clean Water Act in the United States and the Habitats Directive in Europe. Interest in restoring marshes through managed retreat or reclamation has grown, though many Asian countries, including China, continue to reclaim marshland. Bakker et al. (1997) outlined two restoration options: abandoning all human interference, which is often unsuccessful due to shifted tidal cycles, 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, with pioneer-stage marshes recovering more rapidly than mature ones. Restoration can be sped up by replanting native vegetation.