SALT MARSHES AND MUD FLATS

J. M. Teal, Woods Hole Oceanographic Institution, Some marshes, on coasts with little elevation Rochester, MA, USA change, have their highest parts flooded only sea-

Copyright & 2001 Elsevier Ltd. sonally by the equinoctial tides. Other marshes occur in areas with small lunar tides where flooding is This article is reproduced from the 1st edition of Encyclopedia of Ocean predominantly wind-driven, such as the marshes in Sciences, volume 5, pp 2490–2495, & 2001, Elsevier Ltd. the lagoons along the Texas coast of the United States. They are flooded irregularly and, between flooding, the salinity is greatly raised by evaporation Structure in the hot, dry climate. The salinity in some of the Salt marshes are vegetated mud flats. They are above higher areas becomes so high that no rooted plants mean sea level in the intertidal area where higher survive. These are salt flats, high enough in the tidal plants (angiosperms) grow. Sea grasses are an ex- regime for higher plants to grow, but so salty that ception to the generalization about higher plants only salt-resistant algae can grow there. The weather because they live below low tide levels. Mud flats are further affects salinity within marshes and mud flats. vegetated by algae. Weather that changes the temperature of coastal waters or varying atmospheric pressure can change sea level by 10 cm over periods of weeks to months, Geomorphology and therefore affect the areas of the marsh that are Salt marshes and mud flats are made of soft sedi- subjected to tidal inundation. ments deposited along the coast in areas protected Sea level changes gradually. It has been rising since from ocean surf or strong currents. These are long- the retreat of the continental glaciers. The rate of rise term depositional areas intermittently subject to may be increasing with global warming. For the last erosion and export of particles. Salt marsh sediments 10 000 years or so, marshes have been able to keep are held in place by plant roots and rhizomes up with sea level rise by accumulating sediment, both (underground stems). Consequently, marshes are re- through deposition of mud and sand and through sistant to erosion by all but the strongest storms. accumulation of peat. The peat comes from the Algal mats and animal burrows bind mud flat sedi- underground parts of marsh plants that decay slowly ments, although, even when protected along tidal in the anoxic marsh sediments. The result of these creeks within a salt marsh, mud flats are more easily processes is illustrated in Figure 1, in which the eroded than the adjacent salt marsh plain. basement sediment is overlain by the accumulated Salinity in a marsh or mud flat, reported in parts marsh sediment. Keeping up with sea level rise cre- per thousand (ppt), can range from about 40 ppt ates a marsh plain that is relatively flat; the elevation down to 5 ppt. The interaction of the tides and determined by water level rather than by the geo- weather, the salinity of the coastal ocean, and the logical processes that determined the original, base- elevation of the marsh plain control salinity on a ment sediment surface on which the marsh marsh or mud flat. Parts of the marsh with strong, developed. Tidal creeks, which carry the tidal waters regular tides (1 m or more) are flooded twice a day, on and off the marsh, dissect the flat marsh plain. and salinity is close to that of the coastal ocean. Heavy rain at low tide can temporarily make the Organisms surface of the sediment almost fresh. Salinity may vary seasonally if a marsh is located in an estuary The duration of flooding and the salinities of the where the river volume changes over the year. Sal- sediments and tidal waters control the mix of higher inity varies within a marsh with subtle changes in vegetation. Competitive interactions between plants surface elevation. Higher marshes at sites with and interactions between plants and animals further regular tides have variation between spring and neap determine plant distributions. Duration of flooding tides that result in some areas being flooded every duration controls how saturated the sediments will day while other, higher, areas are flooded less fre- be, which in turn controls how oxygenated or re- quently. At higher elevations flooding may occur on duced the sediments are. The roots of higher plants only a few days each spring tide, while at the highest must have oxygen to survive, although many can elevations flooding may occur only a few times a survive short periods of anoxia. Air penetrates into year. the creekbank sediments as they drain at low tide.

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