Louisiana's disappearing WETLANDS
Tupelo Gum Pond, Source: Missouri Dept of Conservation
Louisiana’s wetlands comprise about 40% of the U.S.’s continental wetlands and include the largest contiguous wetland system in the lower 48 states. The state’s wetlands include swamps and marshes. Swamps are areas that hold water and have woody vegetation.
In many Louisiana swamps, Cypress (Taxodium spp.) and Tupelo Gum (Nyssa aquatic) trees are key speciesspecies whose presence contribute to the diversity of an ecosystem and have a critical role in maintaining the integrity of the ecological community. Marshes are areas that frequently or continually hold water and have non-woody, soft, green vegetation, including grasses and sedges. Marshes are classified according to the salt content of their water (called “salinity”) and the resulting plant and animal life that grows and thrives in that environment.
Fresh marshes contain fresh water and have the highest diversity of life. Intermediate marshes have a salt content of 0–5 ppt.ppt.: parts per thousand; the amount of a substance in 1000 units, brackish marshes, 5–15 ppt. and salt marshes have greater than 15ppt. of salinity. The plant species richness (number of species present) greatly decreases as the salt content increases. Few species can survive in the harsh environment of salt water marshes that are inundated daily with salt water tides.
Scientists may describe salinity in ppt (parts per thousand), ppm (parts per million), or as a percentage. So let's test your math. Convert the following measurements to ppt: 2.5%, 1,000ppm, and 12,000ppm. Then match the salinities with the marsh type, as previously described.
Source: Seagrass Conservation Website
Other coastal wetland habitats in Louisiana include estuariesplace where salt and fresh water meet, beaches, barrier islands and the open water of the Gulf of Mexico. The map from the EPA shows the different estuaries in the Lake Pontchartrain Basin.
Study the map of the basin. Can you determine a pattern in the location of different marsh types? What factors play a role in marsh type location?
Types of marshes. Source: U.S. Environmental Protection Agency
Forming the Wetlands
Map showing how rivers flow into the Mississippi River. Source: Wikipedia
The foundation for the coastal marshes was formed from Mississippi River BasinA basin or watershed is land area where water from precipitation drains into a body of water, such as a lake, estuary, river, or wetlands. The drainage basin includes the rivers and streams in which the water drains. sediments that were carried and deposited by the river. In draining over 40% of the U.S., the river collected huge amounts of sand, silt and clay. For over 7,000 years the river flooded, depositing these sediments and building land, a process known as accretionprocess of growth and gradual build-up. The new land created, exceeded the land lost to the natural processes of erosion, subsidenceprocess of lowering the land-surface elevation; sinking of the land, and sea-level rise. Approximately every 1000 years, the Mississippi River changed its course, creating seven major deltas in central and eastern Louisiana. Prevailing coastal currents deposited additional sediments to form western Louisiana.
Each deltaic cycle began when a distributarya branch of a river that flows away from the main stream channel, with a shorter, steeper route to the Gulf of Mexico, gradually captured the Mississippi River. As the river abandoned its old route, the fresh supply of sediment to the old lobe diminished and the area experienced compaction, subsidence and erosion. The old lobe then retreats as the Gulf advances inland. These natural deltaic cycles have been altered by the intervention of humans, as discussed later in this section.
Presented here are the Deltaic Cycles and the timeframes in which they occurred.
Deltaic cycles of the Mississippi River. Modified from: Wikipedia
Coastal wetlands disappearing
While Louisiana has 40% of the country’s wetlands, over 90% of the total coastal marsh loss in the continental U.S.’s occurs in the state. It is estimated that between 25-35 square miles of wetlands are lost each year and more than 1,000,000 acres have been lost since the turn of the century. The majority of land loss is in the Barataria and Terrebone Basins, where 10–11 square miles of land is lost each year. Louisiana’s Department of Natural Resources maintains that at current land loss rates, nearly 640,000 more acres, an area nearly the size of Rhode Island, will be under water by 2050.
Below is a map, prepared by the U.S. Geological Survey (USGS), depicting the projected land loss of southeast Louisiana. Can you find Terrebone and Barataria Bay on the map?
Map showing wetland loss in Louisiana. Modified from: LaCoast, U.S. Geologic Society
A combination of natural and human activities cause land loss. Natural causes include hurricanes, saltwater intrusion, subsidence, wave erosion and sea level rise, but human activities are most responsible for accelerated coastal land loss.
Altered Hydrology: The building of levees and flood control structures along the Mississippi River altered the hydrology and upset the balance between land lost and land gained.
Dredged canals have destroyed Louisiana wetlands. Source: U.S. Geologic Society
Raising levees along the banks of the river prevented flooding and the deposition of sediment in the coastal marshes.
The U.S. Geological Survey (USGS) maintains that, without the deposition of sediment to offset lost elevation, subsidence has accounted for 53% of land loss in Louisiana’s deltaic plain over the past century.
Multiple canals dredged in straight lines for easy navigation. Source: USGS Fact Sheet on Louisiana Coastal Wetlands
Coastal Excavation is a most destructive process that results in the rapid degradation of wetlands to open water. Land is directly removed through the construction of navigation channels, water front property with finger canals and marinas.
Extensive dredging of canals—over 10,000 miles—also occurred with oil and gas exploration, which peaked in the 1960s to 1980s. The channels serve as conduits through which salt water enters interior marshes and weakens and eventually destroys freshwater vegetation. Marsh soil disintegrates without plants to hold it in place, thus hastening the conversion of wetlands to open water.
Close-up of a nutria. Source: Wikipedia
Invasive Species: Even the best of intentions can turn out BAD! Invasive species are nonnative animals or plants that adversely affect the environment and/or economy of the region they have invaded. Nutria are an invasive species. They were originally imported in the 1930s to Avery Island from South America by the McIihenny family, the makers of Tabasco sauce, for the purpose of fur farming.
Nutria soon escaped the island and have since populated the coastal lands of south Louisiana and other areas of the nation, destroying thousands of acres of wetlands. How? They eat, and eat, and eat, and reproduce, and reproduce, and reproduce!
These semiaquatic rodents devour the stems of vegetation, killing plants, which hastens erosion of the wetlands. The problem is so severe that multi-million dollar eradication programs are underway.
Nutria look like beavers, but have long tails like rats. Source: Public Domain Clipart
Subsidence is the lowering of the land surface elevation or the sinking of land. It occurs naturally as the soft river sediments settle and are compacted. However, human activities—such as the extraction of groundwater, oil, and natural gas in a coastal area—may cause subsidence. Subsidence may occur gradually or rapidly, depending on the cause, and is usually a local or regional phenomenon. Normally, river deltas slowly increase in size due to sediment deposition.
Survey marker showing subsidence of land over 200 years. Source: National Oceanic & Atmospheric Administration
However, in the last 200–300 years, human intervention has disrupted this natural process causing an imbalance in sediment deposition and subsidence resulting in widespread erosion of the Mississippi River delta. According to LaCoast (USGS), “With levees, land subsidence of 4-4.3 feet per century for the deltaic plaina plain made by the deposition of silt at the mouth of a river and 1.3-2 feet per century for the chenier plainsFrench word that refers to beach ridges—made of mostly sand, clay, and mud—that are backed by large marshes and coastal grasslands in western Louisiana is not balanced by sediment from the river.” Coastal subsidence and sea level rise create conditions conducive to flooding, the intrusion of salt water into fresh water habitats, and coastal erosion.
Sea level rise is occurring along most of the U.S. coast and around the world as well. A global sea level rise of 0.1-0.25 m was recorded over the last century, but along the Mid-Atlantic and Gulf Coasts, sea level rose more than 5-6 inches above the global average due to the subsidence of coastal lands.
Sea level changes: estimated (gray), recorded (red) and predicted (blue). Source: Environmental Protection Agency
Predictions on sea level rise in the next century vary. According to the The International Panel on Climate Change, the global average sea level will rise between 0.6 and 2 feet in the next century. Climate change models predict higher temperatures and water expands when heated. The rise in sea levels is due to the expanding ocean water and the melting of mountain glaciers, small ice caps, and portions of Greenland and the Antarctic ice sheets.
A rise in sea level would flood hundreds of square miles of low-lying coastal areas. The economic and cultural impact would be great. The tourist business along coastal beaches, the fishing industry, wildlife habitats, the protective barrier islands, and residential areas would all be negatively affected.
Study the graph to answer these questions. Why is the red line narrower between 1950 and 2000? Why does the blue line for future projections widen as it nears 2100?
A typical salt marsh. Spartina alterniflora, in the front of the picture, is a perrenial grass commonly found in salt marshes. It may grow up to 1.5m (ft) and has a hollow stem. Notice Black rush, or Juncus roemerianus, in the rear of the picture. Source: Loyola
Salt Water Intrusion: Both human activities and natural forces create conditions that allow for saltwater intrusion. The dredging and excavationa cavity or hole created by cutting, digging, or scooping of navigation channels and oil field canals in the wetlands produce open channels through which saltwater can easily infiltrate deep into freshwater or low saline environments. The animals and plants found in salt marshes are different than those found in fresh marshes.
Exposed roots of the marsh plants. Source: University of Maryland Center for Environmental Science, Credit: Adrian Jones
Freshwater vegetation cannot tolerate these saltier conditions and die. As the plant roots are no longer there to hold soil in place, erosion occurs. Tidal action, storms, subsidence, and sea level rise can hasten the erosion process.
Hurricanes are a way of life to the people of south Louisiana and neighboring Gulf Coast communities. From June until the end of November, the public tunes into daily weather reports for updates on the latest tropical storms brewing in the Caribbean or the Atlantic Ocean. Hurricanes are powerful, spiraling storm systems that generate waves, strong currents, and storm surges that change coastlines, destroy buildings and existing infrastructure, and take lives.
Source: U.S. Geological Society, Marine and Coastal Geology Program
Of the most severe catastrophes in our nation’s history, hurricanes account for two-thirds of the insured property losses. These data were collected before Hurricane Katrina hit our shores. Hurricane Katrina was the costliest hurricane in U.S. history, with $75 billion in estimated property damages and $200 billion in economic damage.
Katrina, a category 5 hurricane, with sustained winds of more than 250 kilometers per hour. Source: NASA
Hurricane Katrina came ashore in Plaquemines Parish, Louisiana, on August 29, 2005. It was the largest storm of its strength to make landfall in the United States in recorded history. The Pontchartrain Basin lost 40 square miles in one day, more than what was lost in the entire previous decade. In parts of Slidell, Louisiana, and the Mississippi Gulf Coast, homeowners couldn't find their homes or belongings because they had been swept away by the 24-foot storm surge.
The eye of Hurricane Katrina passed directly over the Chandeleur Islands, causing heavy damage. This chain of barrier islandsnarrow strips of land (sand and sediment) that run parallel to the mainland and are separated from the mainland by a shallow bay or sound is located just north of the current delta and west of Breton Sound (see figure). Initial land loss estimates indicated that the Chandeleur islands lost about half of their land area. In a little over a decade, five hurricanes have impacted these islands. Grand Isle, another barrier island found in southeast Louisiana, was also heavily affected by Katrina's storm surge and high winds. In addition to eroding the coastline, widespread structural damage to most buildings on the island occurred.
Although many barrier islands are popular tourist destinations and have been heavily developed, they are fragile, constantly changing ecosystemsAn ecosystem consist of all the living organisms in a particular environment along with the nonliving components in the environment with which the organisms interact. Barrier islands serve as important storm buffers, protecting the mainland from wave action and storm surges.
Wetland loss: What's at risk?
Many people think of wetlands as mosquito-infested, mucky, wet land that is of little value. In fact, over a century ago, the president of the American Health Association endorsed a national campaign to eliminate wetlands. Today, we understand that Louisiana’s coast is an economically vibrant and valuable area that serves many functions for the state and the nation as a whole. The estimated value of coastal wetlands in the state is between $86,040/acre/year - $143,400/acre/year (Coast 2050). As the wetlands disappear, we face very real threats to the environment, culture and economy of our state. A study by the Waldemar Nelson Company (2003) estimated that coastal erosion will directly impact four distinct areas: 1) Oil and natural gas production, 2) Transportation and navigation, 3) Commercial fishing, and 4) Recreational activities.
Wetlands House Oil & Gas Infrastructure
Louisiana is home to the natural gas and oil industry with a value exceeding $16 billion a year. 18% of U.S. oil and 24% of natural gas originates and is processed or transported through Louisiana coastal wetlands. One fourth of the nation’s energy supply depends on the support facilities in south Louisiana.
As wetlands and barrier islands disappear, the wells, pipelines, ports and roads that make the energy industry possible will be more exposed to open water, wave action, storm surges and water traffic. This includes thousands of miles of pipeline that will be increasing vulnerable to rupture, resulting in oil spills and gas leaks. Over $103 billion in assets will be at risk and the transfer of petroleum products within the state and to the rest of the nation will be disrupted.
At risk pipelines include those that carry oil and natural gas from platforms in the Gulf of Mexico to inland refineries. The Pipeline Safety Trust estimates that there are over 130 pipeline companies in the state that are responsible for more than 68,000 miles of pipelines. These pipelines carry various materials, including natural gas, gasoline, and jet fuel. Every major roadway, railroad, and navigable waterway in the state is crossed by a pipeline. The market value of interstate pipelines in Louisiana is over $2.4 billion. This value translates into 5,000 full-time jobs with an annual payroll of approximately $250 million (mean salary of $50,000).
Where is the greatest concentration of pipelines found?
Wetlands are Vital to Transportation & Navigation
Louisiana ranks first in the nation in total shipping tonnage, with 500 million tons of waterborne cargo passing through Louisiana's system of deep-draft ports and navigational channels. If present land loss rates continue, more than 155 miles of waterways and several of the ports will be exposed to open water within 50 years.
Source: Rebuilding the Port of New Orleans, U.S. Army Corps
Wetlands are Critical to Wildlife & Fish
Wetlands are ecologically significant with abundant fisheries and wildlife. It’s one of the largest habitats in the world for migratory waterfowl and 95 percent of all marine species in the Gulf of Mexico spend all or part of their lifecycle there. 66% of Gulf of Mexico sport and commercial fish species are dependent on coastal wetlands for their survival. Louisiana’s wetlands are home to some species that are on the endangered or threatened list.
Louisiana’s commercial fisheries are the most bountiful of the lower 48 states, as the source of more than 30 percent of the nation’s commercial catch. It is first in the annual harvest of oysters, shrimp, crabs, crawfish, red snapper, wild catfish, sea trout and mullet. Sources estimate that by 2050, the annual loss in commercial fisheries due to eroding wetlands will be nearly $550 million.
In general, wetlands purify water by trapping sedimentSediment can interfere with fish spawning, which is the production or depositing of eggs and excess nutrients and pollutants. This is especially important when the wetlands are associated with ground water supply or bodies of water used by humans for recreational uses.
Wetlands Act as Storm Buffers & Flood Protection
Wetlands and barrier islands provide protection from the strong winds and storm surges of hurricanes. It is estimated that every 2.7 miles of wetlands reduces a storm surge by one foot. As erosion continues, and wetlands turn to open water, their ability to decrease approaching storm surges decreases and the risk of catastrophic loss of life and property from hurricanes is greatly increased. Data from past hurricanes indicates that the loss of every one-mile strip of wetlands along the coast, results in an estimated $5,752,816 average annual increase in property damage. In addition, between 60 and 70% of Louisiana’s population, over 2 million people, live within 50 miles of the coast and will become increasingly vulnerable without adequate coastal protection from the wetlands.
A 12 foot storm surge from a hurricane is passing through 30 miles of marsh in the Breton Sound Basin before hitting New Orleans. How much is the surge reduced by the marsh? What is the height of the storm surge as it encroaches on the city?
Wetlands act like a sponge, soaking up and storing extra runoff water after a storm and then releasing it slowly into an aquifer or nearby stream or lake. The size and type of wetland affects the degree of protection provided. Without wetlands to temporarily store storm waters, flooding would be more prevalent.
Check out a cool 7-minute presentation at nola.com that summarizes the issues discussed in this section, The Rise and Disappearance of Southeast Louisiana by Dan Swenson.