The Dead Zone

Closing MRGO could return life to a part of Lake Pontchartrain that is barren because of saltwater intrusion.



Decades ago, the shores of Lake Pontchartrain were dotted with numerous fishing camps, and the water itself was home to more than a hundred types of fish and a species of small clams that not only acted as a filter to clean pollutants from the water but also was instrumental in anchoring sediment to help vegetation grow along the edges of the lake. A lakefront improvement project that started in the late 1920s caused to fishing camps to move, but the ecology of the lake itself was changed dramatically when the Mississippi River Gulf Outlet (MRGO) was finished in the mid-1960s to provide a shorter route than the Mississippi River for ships traveling to the Gulf of Mexico. Since it was opened, MRGO has allowed saltwater to intrude into the lake, but consternation about the manmade channel increased exponentially after Hurricane Katrina and contributed to an influx of floodwaters from the hurricane's surge, serving more as a Mississippi River Gulf Inlet than an outlet.

It's an idea with which Michael Poirrier, an aquatic biology research professor with the University of New Orleans Estuarine Research Laboratory, won't disagree. Since the 1970's, Poirrier and his staff have researched the waters and biology of southeast Louisiana and have documented a massive 100-square-mile dead zone in Lake Pontchartrain, which he says can be directly attributed to saltwater intrusion from the MRGO via the Industrial Canal. Comprising nearly one-sixth of the lake, this large area is virtually uninhabitable by much of Pontchartrain's natural vegetation and marine life.

He says the lake would rebound in a short period of time, however, if the MRGO were closed, an action that would result in a remarkable improvement in the lake's water quality and clarity and a resurgence in the catch for recreational and commercial fishermen. "This problem is one of the easiest ones to solve," Poirrier says. "Close MRGO or stop the saltwater intrusion and the problem goes away."

"The lake has made a very strong comeback over time except on the south shore," says Carlton Dufrechou, president of the Lake Pontchartrain Basin Foundation. "The fish and crab populations are robust except for this 100-square-mile footprint leading out of the Industrial Canal. Closure of MRGO would be the most significant event to return the plumbing of the lake back to normal."

The primary culprit lies directly with saltwater flowing in from the Gulf of Mexico through the MRGO and what happens to it as it moves into the lake. Saltwater is heavier than freshwater so it flows along the bottom as a layer underneath the fresh water and is never allowed to mix with the oxygen of the atmosphere. Bottom-dwelling fauna and vegetation are then left smothered from a lack of oxygen -- or hypoxia -- which creates a virtual desert on the bottom of the lake. The condition is especially pronounced during the summer and fall.

"Because this layer of saltwater is not fixed, this hypoxia could extend beyond the documented dead zone and affect habitats lakewide, depending on weather patterns and other factors," Poirrier warns.

The layer is the most stable near the mouth of the Industrial Canal, so much so that at times it is almost a marine environment, Dufrechou says. Small oyster beds have even formed and are sometimes harvested and used to seed more traditional oyster-harvesting areas closer to the Gulf of Mexico. However, with the hypoxic and episodic nature of this saltwater, none of these oyster beds could ever mature in the lake.

The massive surge of saltwater into the lake from Hurricanes Katrina and Rita only exacerbated the problem in the short run, he says. This heavy saltwater intrusion reached nearly every corner of the 633-square-mile lake and eventually settled on the bottom, causing acute hypoxia throughout. Through normal wind and water disturbances over time, the areas furthest from the Industrial Canal should return to regular conditions. Some already have.

One of the creatures most affected by this hypoxia -- Rangia Cuneata -- turns out to be one of the most important to the overall health of Lake Pontchartrain. Most residents of southeast Louisiana are familiar with this small clam as its white shells were used widely to cover driveways before shell dredging in the lake was halted in 1990. These small clams form one of the densest populations in Lake Pontchartrain and are a primary food source for blue crabs, shrimp, Drum and even some ducks. They also provide a natural filter that -- along with restoration of the dead zone -- would process the entire quantity of the lake's water every three days and lead to enhanced water clarity and quality, Poirrier says.

Although Lake Pontchartrain's water will never resemble what you see off Key West, "It's entirely possible that at certain times of the year, you could actually see the bottom of the lake," he says.

In addition to reducing algae and suspended silts and clays in the water, research has shown that these clams remove E. coli bacteria that comes from human and animal wastes. The resulting higher water quality and clarity would have an immediate ripple effect throughout the lake's entire ecosystem, Poirrier says. The improved water clarity would spark a rapid return and expansion of aquatic grass beds along the shoreline, which are the primary nurseries for many sport fish including juvenile speckled trout, redfish, shrimp and crabs. Similar studies have shown the filtering action of clams triple the water clarity of an area, resulting in the reappearance of aquatic vegetation that had been absent for 50 years. Furthermore, the clam shells act as a stabilizer for sediment and shorelines, further enabling the resurgence of aquatic vegetation.

"The closing of MRGO would result in an almost immediate colonization of the area by these small clams," says Poirrier, who presented the information to the Army Corps of Engineers in October of last year. "Their shells would produce a more stable foundation for shoreline grass beds. Their filtering action would allow more sunlight down through the water column, and the grass beds would further expand and bring with it better fishing. The dead zone would start to recover immediately and allow for more of a diversity of things to grow. The increased health of the lake would only add to recreational activities for people."

The plan could come to fruition. Julie Morgan, a spokesperson for the Army Corps of Engineers says that while the Corps operates at the direction of the U.S. Congress, it is developing a plan to deauthorize the MRGO and is aware of the environmental issues coming from saltwater intrusion. Acts that could affect the health of the lake, she says, include "the potential construction of a MRGO closure; protection gates at [the Industrial Canal] and Seabrook; and introduction of fresh water into the estuaries east of the Mississippi River."

There is no timetable when the MRGO could be closed.

"This problem is one of the easiest ones to solve. Close - MRGO or stop the saltwater intrusion and the problem - goes away." - Michael Poirrier, University of New Orleans Estuarine - Research Laboratory - CHERYL GERBER
  • Cheryl Gerber
  • "This problem is one of the easiest ones to solve. Close MRGO or stop the saltwater intrusion and the problem goes away." Michael Poirrier, University of New Orleans Estuarine Research Laboratory

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