The Louisiana Environment
The Salt Marsh
by B. E. Fleury
Wetlands are a very challenging and stressful environment for both plants and animals. The year-round presence of standing water, and the resulting anaerobic conditions in the soil, require special adaptations for survival. The salt marsh is an especially stressful and difficult wetland habitat. In addition to the lack of oxygen in waterlogged soils, plants in the salt marsh have to cope with high levels of salinity. Despite having to overcome these two very challenging environmental conditions, salt marshes are among the most productive habitats anywhere in the world. A typical Louisiana salt marsh near Delacroix , La., southeast of New Orleans, had an NPP = 7,291 g/m-2/yr-1 (White et al.)
Salt marshes are widely distributed over most of the earth at middle and higher latitudes. In tropical areas, salt marshes are replaced by mangrove swamps. The rule of thumb used to distinguish between swamps and marshes is that swamps are dominated by trees, which are generally lacking in marshes.
In North America, salt marshes occur all along the coast, especially along the Gulf Coast. The entire coast of Louisiana is occupied by extensive salt marshes, which grade into brackish marshes, then into freshwater marsh in the upper Mississippi River delta. These coastal marshes depend on the input of sediment from rivers. Because of the size and volume of the Mississippi River, and its deltaic fresh and saltwater marshes, the Gulf Coast contains about 60% of the coastal marsh land of the entire United States. These coastal marshes are formed from a constant struggle between two opposing forces - subsidence or lowering of the sea floor in the Gulf of Mexico, and the raising of the marsh above sea level by the accumulation of organic peat and the deposition of new sediment.
In the past, the Mississippi River was free to go where it wanted; it wandered all over the map, changing its course entirely from time to time (read Mark Twain’s autobiographical Life on the Mississippi). In colonial times, when the Mississippi neared the Gulf it widened and slowed, forming a series of wide loops or meanders. As it slowed down, it began to drop the great load of sediment accumulated during its long journey. These sediments were deposited in the shallow offshore waters of the Gulf. Much of that sediment was trapped by the coastal marshes. After building up a vast delta, the River would eventually find a new channel, and start to build up yet another delta.
But now the river has been tamed and channeled between narrow banks by the Army Corps. When you force a fluid into a narrower channel, you increase its flow rate. Water shoots out the mouth of the River, and all that fertile topsoil from the midwest now goes hurtling out over the edge of the continental shelf. Very little is retained in the coastal marshes. And the delta, deprived of these sediments, is being steadily eroded.
Coastal marshes have also been severely damaged by extensive canals cut by the oil industry over the past several decades, during exploration and drilling for oil and natural gas. These channels gradually widen through wave action, destabilizing and gradually destroying huge tracts of marsh. Salt marshes are being lost at an alarming rate. Southern Louisiana is literally being nibbled to death by the Gulf. Forty square miles per year of Louisiana vanishes into the Gulf every year. At that rate, within 100 years all of Terrebonne Parish will have disappeared beneath the waves. New Orleans will be well on its way to being a seaport, probably another Venice!
The coastal marshes now stand as our only buffer against the tremendous forces of wind and water from the Gulf of Mexico. They are our best protection from hurricanes. The Army Corps estimates that every 2.7 mi. healthy marsh will dissipate one foot of tidal surge. The marshes are vital to Louisiana, not just as a buffer for tropical storms, but also for their bountiful seafood. Many commercial species live in these marshes, and the marshes are a vast nursery for juvenile shrimp, crabs, redfish, sea trout, and dozens of other commercially important species. Above and beyond their economic value, they are one of the most biologically productive and interesting ecosystems on the planet.
Permanently waterlogged soils are relatively poor in oxygen, which makesmarshes a very difficult place for plants to grow in. Normal soils are very porous, and these pores are usually filled with oxygen. But in waterlogged soils, water replaces the air in these pores, and the soils become anaerobic. This drastically changes soil chemistry, shifting the balance from an oxidizing environment to a reducing environment, which creates toxic sulfides and ferrous iron in the soil. Anaerobic soil conditions also slows decomposition rates. But you don't find the rapid buildup of detritus that you observe in the freshwater swamp, because the tide carries large quantities of detritus out of the system. Salt marsh ecology is dominated by the twice daily tidal flow. When the tide comes in, it brings a fresh supply of nutrients; when the tide goes out, it carries off dead and decaying organic matter. About 45% of the NPP of the salt marsh is carried off into estuaries by the tidal flow.
Salt marsh plants cope with anaerobic conditions in many different ways. They are tolerant of high levels of toxic compounds formed in this reducing environment. They preserve normal root uptake of water and nutrients, and normal root respiration, by pumping oxygen into and out of the roots. This creates a thin zone of oxygenated soil surrounding the roots (the same as the tupelo trees in freshwater swamps). In addition, salt marsh plants have a lot of aerenchyma tissue, tissues with ample air spaces. Up to about 60% of the plant bodymay be composed of aerenchyma. In the most stressful, highly anaerobic parts of the marsh, plants also rely heavily on anaerobic fermentation in their own metabolism.
But in addition to a lack of oxygen, plants also have to contend with very high levels of salt in the water and in the soil. The physiological effects of salt are similar to those of extremely arid conditions. Ironically, these plants stand in water all day, but have some of the same adaptations as desert plants (xerophytes), such as narrow leaves and sunken stomata. Both of these adaptations reduce water loss. Many salt marsh plants can also exclude salt from the roots while still taking up the water they need. Some species (like certain mangroves), can exude salt from their leaves. Marsh plants must pay a very high energetic price for growing in saline, waterlogged soils. About 80% of the carbon fixed by salt marsh plants goes into maintenance.
Very few species can survive the ecological challenge of too much salt and too little oxygen. Consequently, even though salt marsh productivity is very high, species diversity is very low. Salt marsh communities are dominated by a few species of halophytes, plants that are adapted for growth and reproduction in a saline environment. Louisiana salt marshes are dominated by two species of Spartina. In brackish marshes, Spartina patens is the dominant form, locally called cordgrass, wiregrass, marsh hay, or paille a chat tigre (hair of the tiger). In saline marshes, Spartina alterniflora (smooth cordgrass) dominates the net primary productivity of the marsh. S. alterniflora comes in two ecotypes, a tall form that grows in the deeper parts of the marsh, and a short form that grows in more elevated areas. An ecotype is a variant phenotype of the same species that is adapted to local conditions. Ecotypes like Spartina's tall and short forms seem to be due to a combination of genetic and environmental influences. It's very hard for seedlings to establish and grow in shifting sand and mud, so Spartina, like many salt marsh plants, grows and spreads by means of rhizomes (horizontal stems).
Salt marshes show a distinct zonation due to the effect of slight differences in salinity and levels of inundation. Because marshes show a strong environmental gradient, you find different species in the "high" marsh and the "low" marsh. Species that share a similar range of tolerance to salt and water levels grow together. So the salt marsh community shows an open community structure, with species distributed at random with respect to one another, according to their own particular range of environmental tolerance of salt and standing water.
The flow of energy in the marsh is dominated by the NPP of Spartina, with a small contribution from algae (about 1620 g/m-2/yr-1). The grasshopper Orchelimum, and the sap-sucking leafhopper Prokelisia graze on Spartina, along with a few other species. These herbivores are eaten in turn by other arthropods, mainly spiders and insects. There are 81 species of spiders and insects commonly found in southern salt marshes. But only about 4.6% of Spartina NPP goes into the dominant herbivores.
Over 95% of the NPP goes into an entirely different energetic pathway. Most of the plant biomass dies and decays, and most of the salt marsh energy flow passes through the detritivores in the community. The primary consumers in this detritivore food chain are bacteria and fungi. These are fed upon by protozoa, nematodes, rotifers, larval invertebrates of several species, and other predators. And these small creatures feed larger predators, like polychaete worms, snails, bivalves, shrimp, and crabs. Some of these carnivores also feed directly on the algae. There can be literally hundreds of crabs and snails per square meter in a healthy salt marsh.
There are relatively few vertebrate animals in the salt marsh. A few species of fish, like mullet, live there, and many species use the marsh as a refuge for their young. A few mammals, like muskrat and nutria, can survive in the marsh, as can about 10 species of reptiles and amphibians, including the American Alligator. The most common vertebrates in the salt marsh are birds. Only a few species of birds live exclusively in the salt marsh, birds like the clapper rail, the seaside sparrow, and the long-billed marsh wren. But many other birds feed in the marsh, including herons, egrets, wood storks, spoonbills, and ducks.
Despite this abundance of visible life, in the salt marsh it is the invisible creatures, the microscopic detritivores like bacteria and fungi, that are the dominant channel of energy flow. About 47% of Spartina NPP is ultimately lost in the respiration of the bacteria that feed on it. The salt marsh food chain is a simple one -- Spartina --> Microbes --> Animals.
Salt marsh communities, therefore, are characterized by:
1) Very high productivity
2) Very low species diversity
3) A food chain dominated by detritivores
As with the freshwater swamp, severe weather is the greatest source
of natural disturbance in salt marshes, but the effects of weather are
dwarfed by the disturbance caused by mankind. We have starved the marshes
for sediments by channeling the river, and further weakened it by carving
it into a spider web of canals. The ultimate demise of this ecosystem,
however, may be due to global warming. Some computer models of global warming
predict as much as a one meter rise in sea level over the next 100 years.
A one-meter rise in sea level may not sound especially drastic, but it
would completely wipe out 82% of our remaining coastal salt marshes.
Where to Observe the Louisiana Salt Marsh
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