Several ecosystem services provided by salt marshes are appreciated by society, and some protective measures are in place. The regular rise and fall of water in salt marshes, either daily with tides or seasonally with rainfall, enhances at least six valued functions:
Denitrification improves water quality. The sediments of tidal marshes are well suited to denitrification, which occurs most rapidly at oxic-anoxic interfaces. The first step, nitrification, occurs near soil-water or root-soil interfaces or along pores where oxygen enters the soil at low tide. The second step requires anoxic conditions and proceeds rapidly where moisture is sufficient for bacteria to respire and remove oxygen. In this step, nitrate is reduced to nitrogen gas in a series of microbially mediated steps. The rise and fall of tide waters ensures that oxic and anoxic conditions coexist.
Carbon sequestration slows greenhouse warming. The high net primary productivity of salt marshes creates high potential for carbon storage and the anoxic soils slow decomposition, so carbon can accumulate as peat. Large standing crops of roots, rhizomes, and litter are fractionated by a diversity of invertebrates and microorganisms and incorporated into soil. Rates are potentially highest at cooler latitudes, where decomposition is slowed by low temperatures. Sea-level rise is also a key factor; as coastal water levels become deeper, decomposition slows. Sedimentation also buries organic matter, making it less likely to decompose. With sea level rising a millimeter or more per year, on average, salt marsh vegetation can build new rooting zones above dead roots and rhizomes of past decades. Along the USA Gulf of Mexico, the ability of salt marshes to keep up with rising sea level is attributed to root and rhizome accumulation, not just sedimentation. If decomposition proceeds anaerobically to states that produce methane, however, not only is carbon storage reversed, but carbon is also released in a form that contributes more to global warming than carbon dioxide.
Fin- and shellfish have commercial value. Tidal marshes are valued for their nursery function, meaning that the young of many fishes, crabs, and shrimp make use of estuarine waters as 'rearing grounds'. In the USA, it is estimated that some 60% of commercial species spend at least part of their life cycle in estuaries. Several attributes of salt marshes contribute to the food-web-support function, including high productivity of both algae and vascular plants, detritus production and export to shallow water-feeding areas, refuge from deepwater predators, plant canopy cover as a refuge from predatory birds, warmer temperatures that can accelerate growth, and potential to escape disease-causing organisms and parasites that might have narrower salinity tolerance.
Forage is used to feed livestock. In Europe and Asia, graziers move cattle, horses, sheep, or goats onto the marsh plain during low tides. It is common to see ponies tethered to stakes in Puccinellia-dominated salt marshes of UK. The temporary availability (between tides) allows recovery between use and, potentially, high-quality forage and salt for livestock.
Recreational opportunities and esthetics are appreciated by people who live near or visit coastal areas. By virtue of their low-growing vegetation and locations between open water and urban areas, salt marshes attract both wildlife and people. The combination provides high value for birdwatchers, hikers, joggers, and artists. Where there is flat topography above and near the salt marsh, the needs of elderly and disabled visitors can be accommodated along with hikers, school children, and those seeking a refuge from city life. Of particular interest is the ever-changing view, as tides rise and fall along marine coasts, and as water levels change with season in inland systems. Visitor centers have been constructed near many urban salt marshes. Ecotourism then adds economic value to the local municipality as well as the larger region.
Shorelines are anchored by salt marsh vegetation. Recent damages from hurricanes and tsunamis have called attention to the protection that wetland vegetation provides to coastal lands, and especially high-cost real estate. Water flow is slowed by stems and leaves of salt marsh plants, and their roots and rhizomes bind inflowing sediments. Mucilage produced by biofilms (algae, fungi, and bacteria) can then cement particles until new plant growth anchors the substrate. The stems of vascular plants are often coated with biofilms, particularly those of tuft-forming cyanobac-teria, such that the total surface area available for sediment-trapping and anchoring is greatly enhanced. Floating mats of green macroalgae (Ulva, Enteromorpha) also collect sediments and, when they move to the wrack line and join other debris, add to accretion at the upper marsh plain boundary.
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