Coastal salt marshes and reedbeds are mosaics of salt-tolerant grasses and low, typically succulent shrubs. structured by strong gradients of salinity and tidal influence. Salts may approach hypersaline levels near the limit of high spring tides, especially in the tropics. As well as larger plants, algal mats and phytoplankton contribute to productivity, while freshwater run-off and tides bring organic material and nutrients. Bacteria and fungi decompose biomass in oxygen-poor subsoils, and support a range of crustaceans, worms, snails and small fish. Shorebirds breed and forage in saltmarshes, with migratory species dispersing plants and animals.