Salt marshes are saline (typically at or above seawater, >34 gl-1) ecosystems with characteristic geomorphology (sedimentary environments, fine soil texture, and relatively flat topography), herbaceous vegetation, and diverse invertebrates and birds. They occur along shores in estuaries, lagoons, forelands (open areas), and barrier islands in marine environments, and in shallow inland sinks where salts accumulate. They are not found where waves, currents, or streamflow create strong erosive forces. Salt (which stresses most species) severely limits the pool of plant species that can colonize saline sediments, and wetness typically confines the vegetation to herbaceous species, although some species are long-lived 'subshrubs'. Given a near-surface water table, most shrubs and trees cannot establish their extensive root systems.

Plants of tidal marshes are usually able to colonize sediment above mean high water during neap tides (MHWN = average higher high-tide level during lower-amplitude neap tides, which alternate with the broader-amplitude spring tides). Sediment stabilization by halophytes initiates salt marsh formation. Plants not only slow water flow and allow sediments to settle out, but also their roots help hold sediments in place. Gradual accretion around plant shoots can further elevate the shoreline, allowing development of a marsh plain and transition to upland. This process can reverse, with tides eroding accumulated sediments. When sedimentation is outweighed by erosion, salt marshes retreat.

The overriding physiochemical influence is salt, which comes from marine waters, from exposed or uplifted marine sediments, or from evaporation of low-salinity water in arid-region sinks. Salt marshes along coasts typically have tidal influence (Figure 1), although many nontidal lagoons have saline shores that support salt marsh vegetation. Salt marshes in inland settings occur

Figure 1 A tidal marsh in San Quintin Bay seen from the air. Image by the Pacific Estuarine Research Lab.

in shallow sinks (e.g., around the Great Salt Lake, Utah, USA). The salts that contribute to salinity are primarily those of four cations (sodium, potassium, magnesium, calcium) and three anions (carbonates, sulfates, and chlorides); the relative proportions differ widely among soils of inland salt marshes, but sodium chloride is the predominant salt of seawater.

Tidal regimes differ around the globe, but most tidal marshes experience two daily high tides of slightly different magnitude, while some have the same high and low tides from day to day. Levels alternate weekly as neap and spring amplitudes, with the amplitudes readily predicted given gravitational forces between the Earth, the Moon, and the Sun (astronomic tides). Forces vary in relation to global position and coastal morphology; in southern California, mean astronomic tidal range is 3 m, while in the Bay of Fundy it is 16 m. The influence of seasonal low- and high-pressure systems on water-level oscillations (atmospheric tides) also vary greatly. For example, in Western Australia's Swan River Estuary, atmospheric tides outweigh astronomic tides. In the Gulf of Mexico, astronomic tides are minimal because of limited seawater connection with the Atlantic Ocean. Water levels within the Gulf vary only a few centimeters except during storms and seiches.

In tidal systems, marsh vegetation generally ranges from MHWN to the highest astronomic tide. Depending on tidal amplitude and the slope of the shore, salt marshes can be very narrow or kilometers wide. Strong wave action limits the lower salt marsh boundary, but a sheltered area can extend the lower boundary below MHWN.

Animal diversity is high, especially among the benthic and epibenthic invertebrates and the arthropods in the soil or plant canopies. Species that complete their life cycles within salt marshes either tolerate changing salinity and inundation regimes or avoid them by moving elsewhere or reducing contact. Globally, salt marshes are known to support large populations of migratory birds in addition to resident birds, insects, spiders, snails, crabs, and fin and shellfish. Indeed, foraging is the most visible activity in salt marshes.

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