High salt marsh dieback, or salt marsh browning, is the primary force in

salt marsh A salt marsh or saltmarsh, 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 domin ...

degradation in the high marsh. The general effect is that the plants in the marsh die off and brown, leaving dead

organic matter Organic matter, organic material, or natural organic matter refers to the large source of carbon-based compounds found within natural and engineered, terrestrial, and aquatic environments. It is matter composed of organic compounds that have c ...

, and ultimately open

sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand an ...

. Without strong plant roots holding the sediment, these open areas of land

erode Erode () is a city in the Indian state of Tamil Nadu. Erode is the seventh largest urban agglomeration in the state, after Chennai, Coimbatore, Madurai, Tiruchirapalli, Tiruppur and Salem. It is also the administrative headquarters of the ...

, causing the salt marsh to retreat back to the mainland.Webb E.C., Mendelssohn I.A., Wilsey B.J. (1995) "Causes for vegetation dieback in a Louisiana salt marsh: a bioassay approach". ''

Aquatic Botany ''Aquatic Botany'' ("An International Scientific Journal dealing with Applied and Fundamental Research on Submerged, Floating and Emergent Plants in Marine and Freshwater Ecosystems") is a peer-reviewed scientific journal covering research on struc ...

'' 51: 281–289 Dieback zones lack their main producers, such as the salt marsh cord grass, or ''

Spartina alterniflora ''Sporobolus alterniflorus'', or synonymously known as ''Spartina alterniflora'', the smooth cordgrass, saltmarsh cordgrass, or salt-water cordgrass, is a perennial deciduous grass which is found in intertidal wetlands, especially estuarine salt ...

'', and ultimately become completely unproductive.

Overview of hypotheses

Scientists have studied salt marsh dieback for decades, and they still argue about its causes. One of the main ideas suggests that salt marsh dieback is caused by waterlogging in ''S. alterniflora'' from increased submersion within the tides, increased sediment, and

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...

deficiency. Other scientists have researched the possibility of increased soil salinity and decreased soil water as the causes for dieback.

Importance of salt marshes

Salt marshes are important in preserving the brooding and nursery habitats of shellfish, fish, and insects.

Waterlogging hypothesis

Waterlogging is the result of too much water in a plant's

root In vascular plants, the roots are the organs of a plant that are modified to provide anchorage for the plant and take in water and nutrients into the plant body, which allows plants to grow taller and faster. They are most often below the sur ...

system and the surrounding soil, and usually occurs in the inland areas of the marsh. With the increase of surface water, waterlogged soils contain many reduced molecules, which can induce the accumulation of sulfide and other toxic compounds.Mendelssohn I.A., McKee K.L. (1988) "''Spartina alterniflora'' die-back in Louisiana: Time-course investigation of soil waterlogging effects". ''Journal of Ecology'' 76: 509–521 Current studies suggest that increased waterlogging is caused by

sea level rise Globally, sea levels are rising due to human-caused climate change. Between 1901 and 2018, the globally averaged sea level rose by , or 1–2 mm per year on average.IPCC, 2019Summary for Policymakers InIPCC Special Report on the Ocean and Cry ...

, a possible effect of

global warming In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to E ...

, which has many natural and anthropologic causes of its own.Mendelssohn I.A., Kuhn N.L. (2003) "Sediment subsidy: effects on soil-plant responses in a rapidly submerging coastal salt marsh". ''Ecological Engineering'' 21: 115–128

Reduced aerobic respiration

Salt marsh dieback results in the death of marsh-specific plants and the erosion of the landscape. One of the causes of waterlogging is the reduced

aerobic respiration Cellular respiration is the process by which biological fuels are oxidised in the presence of an inorganic electron acceptor such as oxygen to produce large amounts of energy, to drive the bulk production of ATP. Cellular respiration may be des ...

by the roots of ''S. alterniflora''. It occurs mainly in the inland zones, though the streamside plants show partial anaerobic respiration. Aerobic respiration takes sugars and oxygen to create carbon dioxide, water, and energy. As aerobic respiration decreases, the plants become oxygen deficient, since the roots are unable to produce enough oxygen in the reduced soil conditions. Decreased oxygen uptake can also decrease plant productivity.Mendelssohn I.A., McKee K.L., Patrick, W.H. (1981) "Oxygen deficiency in ''Spartina alterniflora'' roots: Metabolic adaptation to anoxia". ''Science'' 214: 439–441 To gain energy, these plants then go through a process of alcoholic

fermentation Fermentation is a metabolic process that produces chemical changes in organic substrates through the action of enzymes. In biochemistry, it is narrowly defined as the extraction of energy from carbohydrates in the absence of oxygen. In food ...

(Mendelssohn et al. 1981). This fermentation process has an end product of carbon in the form of ethanol, which is diffused from the roots. Therefore, the plants are unable to use the diffused carbon, so the available plant energy decreases.

Increased soil sulfide

Another product of waterlogging is the increase of

sulfide Sulfide (British English also sulphide) is an inorganic anion of sulfur with the chemical formula S2− or a compound containing one or more S2− ions. Solutions of sulfide salts are corrosive. ''Sulfide'' also refers to chemical compounds lar ...

in soil. The increase of sulfide is caused by

anaerobic Anaerobic means "living, active, occurring, or existing in the absence of free oxygen", as opposed to aerobic which means "living, active, or occurring only in the presence of oxygen." Anaerobic may also refer to: * Anaerobic adhesive, a bonding a ...

and aerobic bacteria, which are mainly seen in reduced soils. Increased sulfide has been shown to inhibit NH₄-N (ammoniacal nitrogen, an ammonium salt) uptake within the plant. NH₄-N is the most available form of

nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...

within the soil and it is a limiting nutrient in ''S. alterniflora'' productivity. A higher concentration of NH₄-N in the soil may show that the plant's uptake of NH₄-N has decreased, leaving excess molecules in the soil. In addition, reduced soils can cause plant nitrification to decrease, leading to a greater lack of NH₄-N uptake.

Possible solutions

Some scientists have found solutions to this problem. Mendelssohn and Kuhn set up an experiment with plants and soils in a Louisiana salt marsh in 2003. They found that when sediment deposits are increased within an unhealthy salt marsh area, the plants and soils are in better conditions. The experiment showed that the plants with the greater sediment levels had more plant cover, with higher plants and a greater bulk density. The surface elevation increased with the increase of sediment, therefore reducing flooding. The roots could respire aerobically, so they did not have to rely on fermentation for energy. The plants with more sediment also showed a decrease in sulfide and NH₄-N concentrations in the soil. Mendelssohn postulates that since the concentration of NH₄-N decreased after the addition of sediment, more of the nitrogen was used by the plants.

Salinity hypothesis

A second hypothesis of salt marsh dieback focuses on increased salinity and lack of soil water being the main causes of salt marsh dieback. Some scientists see this hypothesis as relevant, since global warming suggests that increased global temperatures may lead to increased

evaporation Evaporation is a type of vaporization that occurs on the surface of a liquid as it changes into the gas phase. High concentration of the evaporating substance in the surrounding gas significantly slows down evaporation, such as when humidi ...

and

transpiration Transpiration is the process of water movement through a plant and its evaporation from aerial parts, such as leaves, stems and flowers. Water is necessary for plants but only a small amount of water taken up by the roots is used for growth a ...

. Brown and Pezeshki devised an experiment in which many ''S. alterniflora'' individuals were put under situations of increased salinity, increased water stress, and then a combined treatment. They found that those plants that experienced the combined treatment exhibited an increase in water stress, where plants are unable to get a sufficient amount of water from the soil, a decrease in photosynthetic activity, and ultimately death (Brown & Pezeshki 2007).

References

{{Reflist Dieback Environmental soil science