What can save Gulf's fragile coastal wetlands? Salt water, perhaps.
Saltwater marshes on the Gulf Coast are far more resilient than freshwater marshes, new research finds. The results could reframe how scientists work to stop the chronic erosion of coastal wetlands in the Gulf.
Now comes a fresh ingredient for buttressing some of the region's fragile coastal wetlands: Salt water.
That's the bumper-sticker implication of new research looking at the effects of 2005's hurricanes Katrina and Rita on the region's coastal marshes.
The results suggest that salt marshes are far more resilient to the scouring action of storms' waves than their fresh-water counterparts. And they imply that projects to divert Mississippi River water and sediment to rebuild fresh-water marshlands in the delta may be doing more long-term harm than good.
"The introduction of fresh water to marshes as part of restoration efforts may ... weaken existing wetlands, rendering them vulnerable to hurricanes," concludes the team reporting the results in the most recent issue of Proceedings of the National Academy of Sciences.
To counteract that, it may be necessary to allow newly created or augmented freshwater marshes to morph into a saltwater marsh, according to Duncan FitzGerald, a marine geologist at Boston University who has studied the delta region extensively.
"What we want to consider is whether we want those regions that are eventually built to be saltwater communities or freshwater communities," he says.
Dr. FitzGerald and two Boston University colleagues, as well as scientists from the University of New Orleans's Ponchartrain Institute for Environmental Sciences, as well as researchers with the US Army Corps of Engineers comprise the research.
The work touches on an vital issue in the delta region: how best to preserve and enhance wetlands that often are called the region's first line of defense against hurricane storm surges. Specialists trace the gradual disappearance of the Mississippi Delta and its wetlands in no small part to a century of levee-building along the Mississippi to control floods.
Those floods once delivered silt deep into the delta to replace sediment lost to erosion and subsidence. Today, that sediment heads straight into the Gulf of Mexico.
To counteract that, conservation managers have established several diversion projects -- controlled releases of Mississippi River water at strategic points along its route through the delta.
This latest study was triggered by post-Katrina pontoon-plane trips FitzGerald and a colleague took to gather insights on the storm's effect on wetlands.
Flying over a broad expanse of marsh that emptied into Louisiana's Breton Sound, east of the Mississippi's "birds foot" delta, they noticed some striking features.
As the aircraft buzzed the shoreline, the duo spotted two- to three-foot-high walls of what they called "peat balls" – bunches of uprooted marsh grass with large wads of peat still attached to their roots.
The plane flew back and forth over the wetlands as the scientists hunted for the peat balls' source. "What really struck me was that some regions of the marsh were torn up, while other regions seemed very resilient and hadn't changed at all," FitzGerald says.
The yin-and-yang regions were separated by a relatively narrow stretch of high ground that once served as the banks of a Mississippi tributary that emptied into the Gulf.
Freshwater marshes dominate the landscape west of the rise, sustained by the Caernarvon Diversion project. They sustained the most damage. On the other side of the rise, saltwater marshes dominated and sustained far less damage.
The plant-survival pattern the team saw there held for coastal plains and deltas along the shoreline west of the Mississippi as well, the team says. Indeed, in some cases, the salt marshes took the brunt of the hurricane's surge, yet the freshwater marshes sustained far more damage.
The key, the team found, was in the depth of roots from the plants in each type of marsh. There was little difference in the soil types in each – both consist of fine silts.
The denser, deeper roots of the saltwater grasses required significantly higher stresses to shear off the upper layer of sediment than did the shallower, more sparse roots of the plants in the freshwater marshes.
One reason for the difference in root depth: The freshwater plants seemed less able to thrive if their roots penetrated into oxygen-deprived soils, which sit below a relatively thin layer of surface soils, according to the team.
But the nutrient-rich diversion water from the river also played a role, the team suspects. With more nutrients available at or near the soil surface, the freshwater plants have no incentive to send roots deep into the underlying soil.
This mechanism for weakening diversion-fed freshwater wetlands dovetails with research Eugene Turner, a zoologist at Louisiana State University, has conducted in the region. As for allowing a freshwater marsh to toughen up by steering it toward a saltier future, "salinity control is not a documented restoration strategy along this coast," he writes in an email exchange.
At the least, he says, the results reinforce the need for a rigorous effort to monitor the wetlands that diversion projects aim to help.
Even then, he adds, with confirmation, the new results represent "a deal breaker" for diversion projects as wetlands restoration tools in the region.