Salt marsh dieback

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Salt marsh dieback results in the death of marsh-specific plants and the erosion of the landscape.

Salt marsh dieback, or salt marsh browning, is the primary force in salt marsh degradation. The general effect is that the plants in the marsh die off and brown, leaving dead organic matter, and ultimately open sediment. Without strong plant roots holding the sediment, these open areas of land erode, 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 51: 281-289] . Dieback zones lack their main producers, such as the salt marsh cord grass, or "Spartina alterniflora", and ultimately become completely unproductive [Bertness M, Silliman B.R., Jefferies R. (2004) Salt marshes under siege. American Scientist 92(1): 54] .

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 deficiency [Goodman P.J, Williams W.T. (1961) Investigations into ‘die-back’ in "Spartina townsendii" agg. Journal of Ecology 49(2): 391-398] . Other scientists have researched the possibility of increased soil salinity and decreased soil water as the causes for dieback [Brown C.E., Pezeshki S.R. (2007) Threshhold for recovery in the marsh halophyte "Spartina alterniflora" grown under the combined effects of salinity and soil drying. Journal of Plant Physiology 164: 274-282] . Although these two hypotheses state that different factors are the main causes of salt marsh dieback, it is important that scientists research all possibilities for salt marsh dieback so ecologists can preserve these unique ecosystems.

Importance of salt marshes

Salt marshes are incredibly important in preserving the brooding and nursery habitats of shellfish, fish, and insects, and it is necessary that current scientists work on ways to save these environments. These habitats, in addition to existing as nurseries, are huge parts of the biodiversity of the earth. If salt marshes continue to decline, so will the diversity of marine species, birds, and mammals that feed in the salt marsh. In addition, salt marshes must be preserved for the lives of future generations, so that everyone may see the beauty of the salt marsh.

Waterlogging hypothesis

Waterlogging is the result of too much water in a plant’s root 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, a possible effect of global warming, 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 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 (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 [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] .

Increased soil sulfide

Another product of waterlogging is the increase of sulfide in soil. The increase of sulfide is caused by anaerobic and aerobic bacteria, which are mainly seen in reduced soils [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] .

Increased sulfide has been shown to inhibit NH₄-N (ammoniacal nitrogen, an ammonium salt) uptake within the plant [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] . NH₄-N is the most available form of nitrogen 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 [Webb E.C., Mendelssohn I.A., Wilsey B.J. (1995) Causes for vegetation dieback in a Louisiana salt marsh: a bioassay approach. Aquatic Botany 51: 281-289] .

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 [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] .

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 and transpiration.

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).

Comparison of hypotheses

Both of these hypotheses have experiments to back up their claims. The hypothesis of waterlogging does have a long history, but new ideas can always show different ways of thinking about the subject, and may ultimately be the proven theory. It is always important to produce new ideas for saving specific ecosystems, but it is also important to know when to start working on the disappearing wetlands.

References