Low marsh

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Low marsh is a tidal marsh zone located below the Mean Highwater Mark (MHM). Based on elevation, frequency of submersion, soil characteristics, vegetation, microbial community, and other metrics, salt marshes can be divided to into three distinct areas: low marsh, middle marsh/high marsh, and the upland zone. [1] Low marsh is characterized as being flooded daily with each high tide, while remaining exposed during low tides.

Contents

Flora and fauna

Tall-form Spartina alterniflora (Smooth Cordgrass) is the dominant vegetative species in low marsh areas. S. alterniflora is a native marsh species that is adapted to salt marsh habitat and found along the eastern seaboard of North America, along the coast of Washington, and along the Gulf of Mexico. [2] This tall, warm-season grass grows in frequently inundated and areas with high salinity. [2] This species provides shelter and cover for Fiddler crabs (Uca pugnax), ribbed mussels (Geukensia demissa), waterfowl, wading birds, shorebirds, muskrats, and commercially important fish and shellfish. [2] S. alterniflora contributes to the fight against shoreline erosion by providing soil stabilization and improves water quality by filtering toxic material, such as heavy metals, from the water column. [2]

Ecosystem services

Salt marshes are very productive ecosystems and provide many ecosystem services including carbon cycling, [3] [4] nitrogen cycling, [1] [5] and coastal flood protection. [4] [6]

Flood protection

Coastal infrastructure is vulnerable to flooding from sea level rise, storms, and land subsidence. [7] Salt marshes help to mitigate effects of flooding by directly buffering coastlines and dissipating waves. [7] They are some of the many types of natural coastal landforms that are widely recognized as barriers to waves and tidal flows. [7] Marsh vegetation causes wave attenuation and may account for up to 60 percent of wave reduction. [7] Marsh plants also improve soil stability, which decreases soil erosion. [7]

Biogeochemical ecosystem services

Salt marshes and other coastal wetland ecosystems play an important role in the global biogeochemical cycle, especially in the carbon and nitrogen cycles. [1] [8] [5]

Carbon cycle

Coastal wetlands, such as salt marshes, can sequester carbon at a rate up to ten times that of a mature tropical forest. [9] Through photosynthesis, marsh vegetation capture large amounts of carbon dioxide from the atmosphere. [9] This carbon is stored in plant tissues and soil for hundreds or thousands of years. [9] Coastal salt marshes can sequester about 210 grams of carbon per meter squared per year, [3] which is 2-5 times more carbon per equivalent area than tropical forests. [9]

Eh potential, the energetic favorability of a reaction, is the lowest in low marsh. [8] Eh potential indicates the potential for carbon loss via oxidation into the atmosphere as carbon dioxide. [8] Therefore, the low marsh may have the lowest carbon dioxide emissions compared to other parts of the marsh platform.

Nitrogen cycle

Both nitrification and denitrification occur in salt marshes. In nitrification, ammonium is oxidized to nitrite, then nitrite is oxidized to nitrate. In denitrification, organic matter is oxidized using nitrate as a terminal electron acceptor. Denitrification is highest in the low marsh. [5] Nitrogen recycling is the lowest in the low marsh. [1]

Threats to low marsh

Sea-level rise

Future health and persistence of coastal wetlands remains very uncertain. [4] [6] [10] Coastal wetlands, such as salt marshes, with low elevation gradients are the ecosystems that will be first affected by and have to adapt to increased sea level rise(SLR). [11] Areal loss has been predicted for salt marshes with low and/or declining sediment supply. [10] Depending on the Intergovernmental Panel on Climate Change (IPCC) RCP scenario, 60-91 percent of salt marshes in a meta-analysis study will not be able to keep up with future rates of SLR. [10] In this same study, 8 out of 9 marshes with the highest rates of local SLR were already not keeping pace with SLR and are experiencing an average loss of 3.9 millimeters per year. [10] Increased thermal expansion and increased water supplied to oceans due to higher global temperatures associated with anthropogenic carbon dioxide emissions have caused the rates of SLR along the United States’ Atlantic coast to range from 0.6 to 4 millimeters per year as of the year 2021. [12] Rates of SLR are expected to only increase in the future as the magnitude of greenhouse gases in our atmosphere increases. [12] [10] SLR will cause low marsh to "drown" and be converted to open water. [5]

Human disturbance and interference

Coastal development, such as roads and houses, prevents salt marshes from migrating inland away from the coast as sea level rises. [10] In the past, salt marshes have migrated inland as a response to sea level from glaciation. [10] Land directly above marshes is slowly converted to high marsh due to increased salt water inundation due to SLR. [13] Upland vegetation is replaced by halophyte marsh species as a result from increased soil salinity and moisture. [13] This occurs as low marsh closest to the tidal creek is converted to open water. [13] The boundaries of marsh vegetation zones shift inland. [13] This allows for the same square footage of each marsh zone to remain the same. However, when infrastructure is located directly upland to a marsh, that marsh is physically blocked from migration. [13] The boundary between the low marsh and the high marsh continues to shift inland, but the upland region of the marsh has no land to convert to marsh area. This results in the upland and eventually high marsh zones to be lost.

In addition to coastal development blocking upland marsh migration, it also increases runoff into the marsh. [14] Increases of impervious surfaces in coastal development nearby marshes increases the amount the rainfall runoff and surface water that may enter marshes. [14] Runoff carries pollutants, including but not limited to fertilizers, sediment, waste, and litter, as well as freshwater into marshes. [14] The specific ecological effects and their magnitudes vary depending on the concentrations, frequency, and chemical makeup of runoff pollutants. Furthermore, marshes may be drained, dredged, and filled to make the land available for coastal development and agriculture. [15] Marshes are also commonly ditched and drained for mosquito and other pest control. [15]

Invasive species

As with many different ecosystems, salt marshes are susceptible to invasions by non-native species. [16] Phragmites australis (Common Reed), is a perennial, aggressive wetland grass that grows in dense stands over 10 feet tall and is a common invader of salt marshes. [17] Phragmites rapidly colonizes near areas and can out compete replace native marsh vegetation. [17] This invasive species provides little to no food or shelter to salt marsh wildlife. [17]

Wrack, while not an invasive species in the traditional sense, also can destroy native salt marsh vegetation.[ citation needed ] Wrack is deposited material composed mostly of dead marsh vegetation.[ citation needed ] If it accumulates over vegetation for sufficient period of time, it will block sunlight and smother the plants underneath.[ citation needed ]

Sesarma reticulatum (Purple marsh crabs) are a native marsh species, although fishing and crabbing can remove their predators from the ecosystem. [18] When purple marsh crab populations remain unchecked, they will "mow down" S. alternilfora and increase the number of burrows in the soil. [18] Burrows decrease soil stability and make the soil more likely to erode. [18]

Conservation and management

There are many worldwide organizations like The Nature Conservancy, Environment Protection Agency, Buzzards Bay Coalition, Association to Preserve Cape Cod, and Department of Environmental Management that participate in salt marsh restoration, protection, and management.

Common restoration strategies include recovery of tidal exchange, recovery of sediment characteristics, reconstruction of soil level, conversion of dredged sediment to salt marsh, control of invasive species, and restricting boating and other water vehicles. [19] The method(s) employed in a salt marsh depend on the specific marsh itself and local area. [19] For example, a salt marsh in the Bay of Fundy, Canada may be restored by expanding the tidal channel through constructing a culvert, while a salt marsh in Northwest Europe may be restored by removing dikes. [19]

In order to protect salt marshes, governments create reserves such as Shifting Lots Preserve in Plymouth, MA, Estero Marsh Preserve, and Clive Runnells Family Mad Island Marsh. These are areas where resources and land are managed and restricted.

Related Research Articles

<span class="mw-page-title-main">Marsh gas</span>

Marsh gas, also known as swamp gas or bog gas, is a mixture primarily of methane and smaller amounts of hydrogen sulfide, carbon dioxide, and trace phosphine that is produced naturally within some geographical marshes, swamps, and bogs.

<span class="mw-page-title-main">Estuary</span> Partially enclosed coastal body of brackish water

An estuary is a partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone between river environments and maritime environments and are an example of an ecotone. Estuaries are subject both to marine influences such as tides, waves, and the influx of saline water, and to fluvial influences such as flows of freshwater and sediment. The mixing of seawater and freshwater provides high levels of nutrients both in the water column and in sediment, making estuaries among the most productive natural habitats in the world.

<span class="mw-page-title-main">Wetland</span> Land area that is permanently, or seasonally saturated with water

A wetland is a distinct ecosystem that is flooded or saturated by water, either permanently or seasonally. Flooding results in oxygen-free (anoxic) processes prevailing, especially in the soils. The primary factor that distinguishes wetlands from terrestrial land forms or water bodies is the characteristic vegetation of aquatic plants, adapted to the unique anoxic hydric soils. Wetlands are considered among the most biologically diverse of all ecosystems, serving as home to a wide range of plant and animal species. Methods for assessing wetland functions, wetland ecological health, and general wetland condition have been developed for many regions of the world. These methods have contributed to wetland conservation partly by raising public awareness of the functions some wetlands provide.

<span class="mw-page-title-main">Marsh</span> Wetland that is dominated by herbaceous rather than woody plant species

A marsh is a wetland that is dominated by herbaceous rather than woody plant species. Marshes can often be found at the edges of lakes and streams, where they form a transition between the aquatic and terrestrial ecosystems. They are often dominated by grasses, rushes or reeds. If woody plants are present they tend to be low-growing shrubs, and the marsh is sometimes called a carr. This form of vegetation is what differentiates marshes from other types of wetland such as swamps, which are dominated by trees, and mires, which are wetlands that have accumulated deposits of acidic peat.

<span class="mw-page-title-main">Salt marsh</span> Coastal ecosystem between land and open saltwater that is regularly flooded

A salt marsh, saltmarsh or salting, also known as a coastal salt marsh or a tidal marsh, is a coastal ecosystem in the upper coastal intertidal zone between land and open saltwater or brackish water that is regularly flooded by the tides. It is dominated by dense stands of salt-tolerant plants such as herbs, grasses, or low shrubs. These plants are terrestrial in origin and are essential to the stability of the salt marsh in trapping and binding sediments. Salt marshes play a large role in the aquatic food web and the delivery of nutrients to coastal waters. They also support terrestrial animals and provide coastal protection.

<span class="mw-page-title-main">Tidal creek</span> Inlet or estuary that is affected by ebb and flow of ocean tides

A tidal creek or tidal channel is a narrow inlet or estuary that is affected by the ebb and flow of ocean tides. Thus, it has variable salinity and electrical conductivity over the tidal cycle, and flushes salts from inland soils. Tidal creeks are characterized by slow water velocity, resulting in buildup of fine, organic sediment in wetlands. Creeks may often be a dry to muddy channel with little or no flow at low tide, but with significant depth of water at high tide. Due to the temporal variability of water quality parameters within the tidally influenced zone, there are unique biota associated with tidal creeks which are often specialised to such zones. Nutrients and organic matter are delivered downstream to habitats normally lacking these, while the creeks also provide access to inland habitat for salt-water organisms.

<span class="mw-page-title-main">Tidal marsh</span> Marsh subject to tidal change in water

A tidal marsh is a marsh found along rivers, coasts and estuaries which floods and drains by the tidal movement of the adjacent estuary, sea or ocean. Tidal marshes experience many overlapping persistent cycles, including diurnal and semi-diurnal tides, day-night temperature fluctuations, spring-neap tides, seasonal vegetation growth and decay, upland runoff, decadal climate variations, and centennial to millennial trends in sea level and climate. Tidal marshes are formed in areas that are sheltered from waves, in upper slops of intertidal, and where water is fresh or saline. They are also impacted by transient disturbances such as hurricanes, floods, storms, and upland fires.

<span class="mw-page-title-main">Aquatic ecosystem</span> Ecosystem in a body of water

An aquatic ecosystem is an ecosystem formed by surrounding a body of water, in contrast to land-based terrestrial ecosystems. Aquatic ecosystems contain communities of organisms that are dependent on each other and on their environment. The two main types of aquatic ecosystems are marine ecosystems and freshwater ecosystems. Freshwater ecosystems may be lentic ; lotic ; and wetlands.

<i>Phragmites australis</i> Species of grass

Phragmites australis, known as the common reed, is a species of plant. It is a broadly distributed wetland grass that can grow up to 20 feet tall.

<span class="mw-page-title-main">Brackish marsh</span> Marsh with brackish level of salinity

Brackish marshes develop from salt marshes where a significant freshwater influx dilutes the seawater to brackish levels of salinity. This commonly happens upstream from salt marshes by estuaries of coastal rivers or near the mouths of coastal rivers with heavy freshwater discharges in the conditions of low tidal ranges.

<span class="mw-page-title-main">Freshwater marsh</span> Non-tidal, non-forested marsh wetland that contains fresh water

A freshwater marsh is a non-tidal, non-forested marsh wetland that contains fresh water, and is continuously or frequently flooded. Freshwater marshes primarily consist of sedges, grasses, and emergent plants. Freshwater marshes are usually found near the mouths of rivers, along lakes, and are present in areas with low drainage like abandoned oxbow lakes. It is the counterpart to the salt marsh, an upper coastal intertidal zone of bio-habitat, which is regularly flushed with sea water.

<span class="mw-page-title-main">High marsh</span>

High marsh is a tidal marsh zone located above the Mean Highwater Mark (MHW) which, in contrast to the low marsh zone, is inundated infrequently during periods of extreme high tide and storm surge associated with coastal storms. This zone is impacted by spring tides, which is a bi-monthly lunar occurrence where the high marsh experiences higher inundation levels. The high marsh is the intermittent zone between the low marsh and the uplands, an entirely terrestrial area rarely flooded during events of extreme tidal action caused by severe coastal storms. The high marsh is distinguished from the low marsh by its sandy soil and higher elevation. The elevation of the high marsh allows this zone to be covered by the high tide for no more than an hour a day. With the soil exposed to air for long periods of time, evaporation occurs, leading to high salinity levels, up to four times that of sea water. Areas of extremely high salinity prohibit plant growth altogether. These barren sandy areas are known as "salt pans". Some cordgrass plants do survive here, but are stunted and do not reach their full size.

<span class="mw-page-title-main">Salt pannes and pools</span> Water retaining depressions located within salt and brackish marshes

Salt pannes and pools are water retaining depressions located within salt and brackish marshes. Pools tend to retain water during the summer months between high tides, whereas pannes generally do not. Salt pannes generally start when a mat of organic debris is deposited upon existing vegetation, killing it. This creates a slight depression in the surrounding vegetation which retains water for varying periods of time. Upon successive cycles of inundation and evaporation the panne develops an increased salinity greater than that of the larger body of water. This increased salinity dictates the type of flora and fauna able to grow within the panne. Salt pools are also secondary formations, though the exact mechanism(s) of formation are not well understood; some have predicted they will increase in size and abundance in the future due to rising sea levels.

<span class="mw-page-title-main">Blue carbon</span> Carbon stored in coastal and marine ecosystems, such as salt marshes, seagrasses, and mangroves

Blue carbon refers to organic carbon that is captured and stored by the world's oceanic and coastal ecosystems, mostly by algae, seagrasses, macroalgae, mangroves, salt marshes and other plants in coastal wetlands. The term "blue carbon" was coined in 2009 to highlight the contribution of coastal vegetated ecosystems to climate change mitigation. Because oceans cover 70% of the planet, there is increasing industry interest in developing blue carbon potential. Research is ongoing, and while in some cases it has been found that these types of ecosystems remove far more carbon per area than terrestrial forests, the effectiveness of blue carbon as a carbon dioxide removal solution remains highly contested.

<span class="mw-page-title-main">Inland salt marsh</span>

An inland salt marsh is a saltwater marsh located away from the coast. It is formed and maintained in areas when evapotranspiration exceeds precipitation and/or when sodium- and chloride-laden groundwater is released from natural brine aquifers. Its vegetation is dominated by halophytic plant communities.

<span class="mw-page-title-main">Salt marsh die-off</span> Ecological disaster in low-elevation salt marshes

Salt marsh die-off is a term that has been used in the US and UK to describe the death of salt marsh cordgrass leading to subsequent degradation of habitat, specifically in the low marsh zones of salt marshes on the coasts of the Western Atlantic. Cordgrass normally anchors sediment in salt marshes; its loss leads to decreased substrate hardness, increased erosion, and collapse of creek banks into the water, ultimately resulting in decreased marsh health and productivity.

<span class="mw-page-title-main">Climate change in Delaware</span> Climate change in the US state of Delaware

Climate change in Delaware encompasses the effects of climate change, attributed to man-made increases in atmospheric carbon dioxide, in the U.S. state of Delaware.

<span class="mw-page-title-main">Climate change in Virginia</span> Climate change in the US state of Virginia

Climate change in Virginia encompasses the effects of climate change, attributed to man-made increases in atmospheric carbon dioxide, in the U.S. state of Virginia.

<span class="mw-page-title-main">Sedimentation enhancing strategy</span>

Sedimentation enhancing strategies are environmental management projects aiming to restore and facilitate land-building processes in deltas. Sediment availability and deposition are important because deltas naturally subside and therefore need sediment accumulation to maintain their elevation, particularly considering increasing rates of sea-level rise. Sedimentation enhancing strategies aim to increase sedimentation on the delta plain primarily by restoring the exchange of water and sediments between rivers and low-lying delta plains. Sedimentation enhancing strategies can be applied to encourage land elevation gain to offset sea-level rise. Interest in sedimentation enhancing strategies has recently increased due to their ability to raise land elevation, which is important for the long-term sustainability of deltas.

The marsh organ is a collection of plastic pipes attached to a wooden framework that is placed in marshes to measure the effects of inundation time and flood frequency on the productivity of marsh vegetation. The information is used for scientific research purposes.

References

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