Is it land or is it sea? Salt marshes the border between two worlds.
Salt marshes are the transitional environment between the marine and terrestrial. They’re typically, made of peat or alluvial sediments, and are inundated and exposed with the tides. They’re found globally, but the majority are found within temperate regions, in areas that receive limited wave action, like estuaries, deltas or behind barrier islands. Within more tropical climates, they are replaced by mangrove forests.
Salt marshes develop in between the Mean High Water of Spring tides (MHWS) and the Mean High Water of Neap tides (MHWN), which is the area that receives the highest rate of accumulation of settling sediments. It’s believed that the settled sediment is first stabilised by micro-algae, such as Diatoms, which secretes a mucus, partly binding the sediment. Once the buildup of sediment has reached the point where the upper limits of the intertidal zone is no longer submerged with every tide; pioneer species can inhabit. The establishment of these species (eg. Salicornia spp or Spartina spp), increase the buildu
p and depositing of sediments. They do this by decreasing wave energy, by creating drag, as well as trapping and stabilising sediment with their roots and rhizomes. This continuous build up and stabilisation of sediments allows for more complex species, which are less tolerant to inundation and require stable sediments to establish and grow. These more complex species will out compete the pioneer species the more stable the conditions get. This process is known as succession. Upon a salt marsh, different bands of floral species can be seen. Pioneer species being found closer to the sea and more complex species being found moving towards land. These bands are caused by succession. The edges of these bands are normally determined by biological and mechanical factors such as: wave exposure, inundation, sedimentation and sediment quality. The geology and conditions between these zones also differ.
As plants die, most of the plant mass remains within the salt marsh due to the low wave action and becomes locked within the sediment. However, with the decomposition of the plant matter, as well as the inundation by the tides, oxygen levels decline, turning the sediment hypoxic. This sediment hypoxia is also a reason for the bands of flora.
The importance of salt marshes
Salt marshes are important habitats. They serve as coastal protection, guarding the coast from the extreme waves of the open ocean, as well as preventing extensive coastal erosion. Salt marshes are also inhabited by a large variety of flora and fauna, many of which are endemic to salt marshes. They are also essential for sustaining healthy fisheries: providing refuge, nursery grounds and essential food for many fish species. Within America, they provide for more than 75% of important fishery species, which includes shrimp, blue crabs and many fish. But perhaps the most important quality of a salt marshes is its ability to store carbon.
The salt marsh carbon sink
Salt marshes have the ability to store large amounts of carbon, making them important as global carbon sinks. The salt marsh plants absorb large amounts of carbon dioxide from the atmosphere for primary production, fixing the carbon within the plant itself. As the plant dies it becomes trapped within the sediment along with the carbon. Salt marshes are thought to store 18 to 1,713 g Cm-2yr-1, but this only accounts for the surface productivity. Within most salt marshes the below ground production is many times higher. Making them extremely effective carbon sinks, as well as valuable environments.
What’s happening to salt marshes?
Human activity has caused harm to the salt marsh environment, leading to a decrease of salt marsh habitat as well as the health of the marsh. The most imminent thread is coastal development. With increasing population, there is a greater demand for housing and real estate, and coastal areas are in high demand. Due to the serenity, great view and access to the ocean, salt marshes are being filled to allow for development, destroying large areas of habitats and altering soil types as well as elevati
on. Another problem that salt marshes face, is the pollution from urban and farmland runoff. Rain water washes into the marsh from the land carrying fertiliser, septic material and farm waste like herbicides. These pollutants can change the function of natural ecosystems, especially large quantities of nitrogen which can course eutrophication. Non-native or invasive species that have been introduced by humans are also having negative effects on the marsh. Without natural predators or grazers, invasive species ,such as Phragmites australis, can out compete the indigenous wildlife, causing disruptions within the food web and the overall function of the salt marsh environment.
Hope for the future?
Due to recent findings and studies, governments have come to realise how important these habitats are and have looked to protect them. They have turned many salt marshes into reserves, protecting them against any further urban development. Governments have also looked to encourage the growth of salt marshes and have introduced legislation into place to limit the run-off of nearby farm land and urban areas.