Salt marshes build themselves out of mud, grass and the rising tide
A salt marsh begins as bare tidal mud. Sticky mats of blue-green algae grab silt, pioneer plants such as glasswort trap more around their stems, and roots bind it into terraces that climb until the tide reaches them less often. The result ranks among the most productive habitats on any coast.
These wetlands occupy the upper intertidal zone, flooded regularly by salt or brackish water, and are dominated by salt-tolerant herbs, grasses and low shrubs descended from land plants. They form on sheltered, low-energy shores in temperate and high latitudes, such as estuaries and the lee of barrier islands; in the tropics, mangrove trees take their place. Examples range from the Camargue in the Rhône delta and the Mississippi delta to Morecambe Bay and the Bay of Fundy.
Tides set the layout. The lower marsh stays fairly steady in salinity because it floods daily, while the upper marsh swings with rainfall and evaporation. Lower zones are sorted by which plants can endure salt, submersion and low oxygen; higher ones by competition. In New England, smooth cordgrass monopolises the wettest ground, giving way landward to salt hay, black rush and the shrub Iva frutescens. Tall stems slow currents so sediment drops out, and a study at the mouth of the Yangtze found Spartina alterniflora may supply more than 10% of surface build-up simply through mud clinging to it.
Few plant species cope, so diversity is low, but productivity is high. Much of the cordgrass is never grazed; it dies, rots and feeds microbes that in turn feed fish and birds. Many sea fish raise their young in the sheltered channels before heading to open water, and birds nest in the tall grass, feeding on stranded fish, insects, shellfish and worms.
How much marsh exists is uncertain. A 2017 mapping effort covered 5,495,089 hectares in 43 countries and territories, at the low end of earlier guesses of 2.2–40 Mha, while a later study conservatively estimated 90,800 km2. Reclamation, pollution and rising seas have destroyed many, but recognition of their value for wildlife and carbon storage has driven restoration since the 1980s. After Winter Storm Grayson dumped sediment on a Massachusetts marsh in 2018, plots receiving 2–6 cm showed more root growth six seasons later with no loss of plant cover.
Source: Salt marsh
