The Louisiana Wetlands: An Introduction
A growing human presence interrupts the natural beauty and vastness of Louisiana's coastline. Beyond the wetlands and outside the bayous are oil platforms and it seems everywhere there are warning markers of underground oil pipes. Still, the most dramatic change these wetlands are undergoing is the inundation of saltwater and the loss of land. Since the early 1930s, it is estimated that Louisiana has lost almost 5,000 square kilometers of wetlands. In fact, some say an area the size of a tennis court disappears every thirteen seconds. The wetlands are turning into water.
A wetland can be defined in many ways but most definitions include three distinguishing features: water, either at the surface or within the root zone, soil conditions unique to this wet environment, and vegetation known as hydrophytes that is adapted to these conditions. Wetlands can be fresh or salt water and in Louisiana there are both and they're equally in danger of disappearing.
According to the United States Geological Survey (USGS) and the Louisiana Department of Fish and Wildlife (LDWF) Louisiana has about 3,560 square kilometers of fresh water wetland and 6,600 square kilometers of tidal (salt water) wetland. That's an area equivalent to two states of Rhode Island or almost 14 Isle of Mans. Wetlands include landscape features you are familiar with such as marshes, bogs, and swamps.
The wetlands I grew up with were small. You could easily walk across them and you could always see the edge. This is not the case in Louisiana. Here the wetlands stretch to the horizon. They are dotted with small fishing camps and telephone poles that sit at weird angles due to the shifting ground. Often flooded roads and abandoned houses blot the landscape. Despite this I had only to spend a single day out there accompanying two scientists on their monthly monitoring trip to Barataria Bay southwest of New Orleans, to be hooked.
We loaded our small boat early in the morning and headed to the southern most sampling station closest to the Gulf of Mexico. The plan was to work our way north collecting water samples for phytoplankton (microscopic plants), zooplankton (microscopic animals) and bacteria counts, nutrient analyses, and other water characteristics (temperature, salinity, dissolved oxygen, light attenuation). Regular monitoring is part of larger effort to understand the changes occurring along Louisiana's coastline.
On this day, we wound our way up and down narrow passages surprising fish and sneaking up on alligators. The sky was grey and ominous. Because it is so flat you can see thunderstorms well in the distance - the rain and lightening drumming the water. Twin storms merged into one and chased us all the way to our last sampling station. From there, we headed into a bayou. A bayou is a secondary extension of a larger waterway - in this case - the Mississippi River. Thick with dark green vegetation and monstrous plants, this bayou had a prehistoric and deeply wild feel. Alligator eyes rose just above the surface and brilliant blue horse flies, the size of small birds, buzzed overhead. The water was slow moving and brown, hiding whatever it was that might be waiting underneath. What the bayou's future holds is more readily detectable.
The coast of Louisiana is a deltaic system built of sediment transported from all over the United States by the Mississippi River. The Mississippi River drains 41% of the contiguous states of the US and is fed by the Missouri, Arkansas, and Ohio Rivers. The river carries this sediment load to the coast where it settles out and forms sediment lobes or land. Typically, as silt and clay clog the natural path of the river, it switches course finding a more direct and quicker route to the coast. This process is called delta switching and it occurs approximately once every one thousand years.
But as the population grew, and with the establishment of the Port of New Orleans, it became necessary to control the Mississippi River. Levees were constructed to maintain its path and the natural delta switching that occurred for thousands of years was interrupted. Gone then was the sediment distribution and ultimately the land building. Now the Gulf of Mexico can encroach, un-impeded by the River. Sometimes wetlands can keep up with sea level rise through sediment trapping and soil building but not here. These factors combine with land subsidence, the compaction of sediment, to augment the drowning of these wetlands. While subsidence can be a natural geologic process, human activities may also be the cause. Fluid (i.e. oil, gas, groundwater) extraction from underneath the marshes and/or artificial drainage systems have been blamed for high rates of subsidence in Louisiana. The final straw is shipping channels that have been cut and the pipelines that have been laid beneath the marshes. While this has increased navigation abilities it has also allowed saltwater inundation of the many freshwater marshes. For example, the cypress forests that once characterized this area are quickly vanishing because they cannot tolerate saltwater.
The disappearance of these wetlands has innumerable consequences. The coastal landscape is an important habitat for animals and migrating birds while the marsh provides an essential nursery ground for fish. According to Louisiana State University's Agricultural Center, 95% of commercial fish landings for the Gulf of Mexico depend on the coastal wetlands and 25% of all seafood consumed in the US originates from Louisiana. The bountiful harvest of oysters and crawfish (crayfish) that once drove the local economy is threatened too. Many people have relocated or constructed their houses on stilts to protect them from the encroaching water. On a national scale almost 30% of the oil and gas consumed in the US passes through ports and pipes of South Louisiana. But without these wetlands more than 48,000 km of pipelines would be exposed to open water and waves. On an even larger scale, hurricanes that have struck Louisiana have even provoked increases in global oil prices.
But I am most interested in the loss of wetlands for another reason. Wetlands are areas rich in organic matter and can be important areas for nutrient cycling. Specifically, I am interested in the cycling of nitrogen and a process called denitrification. Denitrification is bacterial process that converts usable forms of nitrogen to unusable forms. It is therefore a natural cleansing process which can remove man-made nitrogen (such as fertilisers) from the environment. As the water from the Mississippi River drains the US it not only brings sediment for land building but high concentrations of nutrients (such as nitrates and phosphates). Most of these nutrients originate from agricultural practices high in the watershed. When the nutrients reach the coast they can stimulate the growth of phytoplankton which can be good for the growing fish. But, like all things, too many nutrients and too much phytoplankton growth can lead to negative consequences. Most notably in this case, when the phytoplankton die they sink to the bottom and decompose, which leads to the water becoming oxygen depleted, killing shellfish and finfish. One striking example of this is the Dead Zone in the Gulf of Mexico. The Dead Zone is a 13,000 - 21,000 square km area of oxygen-depleted water devoid of most forms of life. While many factors contribute to its size and duration, the high concentration of nutrients entering from the Mississippi River is thought to be the main cause.
So here is our dilemma - how do we build land and yet decrease the amount of nutrients entering the Gulf of Mexico? The answer may just lie in sustaining healthy wetlands. One option is to allow more water from the Mississippi River to be directed into the wetlands. By doing this, as it flowed to the coast the river would once again distribute sediment and the wetland might act like a giant filter and remove contaminating nitrogen. Working with colleagues here at LSU I hope to improve our understanding of how much nutrient cycling can occur in these systems and how these processes might change with temperature and water flow.
Beauty-wise, Louisiana's coast rivals the icecaps of the Antarctic, the rolling waves of the Pacific and the tropical beaches of the Caribbean. But protecting these wetlands is about more than preserving a magnificent landscape. We should all feel a great sense of urgency to guard a culture, an economy, and a natural wonder teeming with life...
- Mitsch, W.J. and Gosselink, J.G. (1993). Wetlands. Van Nostrand Reinhold, New York, N.Y.
- Dunne, M. (2005). America's Wetland: Louisiana's vanishing coast. Louisiana State University Press. Baton Rouge, LA
- America's Wetland Foundation - http://www.americaswetland.com/