by Tom Moorman, Ph.D.
Hurricanes Katrina and Rita have been devastating for residents along the Gulf Coast. As vital recovery work for people and their homes and businesses proceeds, biologists are assessing the impacts the storms had on Louisiana's coastal marshes that provide vital waterfowl wintering habitat for several million ducks and geese and untold numbers of other wetland birds and wildlife.
Historically, hurricanes—along with fire—played an important role in the complex ecology of coastal wetlands. Healthy marshes consist of dense stands of grasses and other emergent wetland vegetation interspersed with small ponds, lakes, and bayous. If unchecked, however, marsh vegetation can choke out open-water areas over time. This natural process, known as succession, eventually forms "closed marsh" with little value for waterfowl and other wildlife. In the past, hurricanes periodically set back succession in Louisiana's coastal marshes by breaking up dense stands of vegetation and scouring new ponds and lakes. Additionally, accompanying storm surges pushed salt water inland into fresh and intermediate-salinity marshes where plants are intolerant of high salinity levels. Lengthy exposure to salt water kills salt-intolerant vegetation, creating more open-water areas. Whether caused by wind and wave action or changes in salinity, the new ponds and lakes created by hurricanes were prime habitat for waterfowl, as wild celery, widgeon grass, delta duck potato, and other prime duck foods flourished in the shallow, open water.
Unfortunately, people have dramatically altered the hydrology of Louisiana's coastal marshes, increasing the vulnerability of both coastal communities and waterfowl habitats to the effects of intense storms such as Katrina and Rita. Robbed of nourishing sediments from the Mississippi River and fragmented by numerous shipping canals, much of Louisiana's remaining coastal marsh is rapidly eroding. Consisting of large expanses of open water interspersed with small islands of vegetation, this "broken marsh" is the opposite condition of healthy coastal wetlands.
The impacts of hurricane winds and storm surges on broken, unhealthy marshes can be devastating. Early estimates from the USGS National Wetlands Research Center suggest that Katrina converted nearly 30 square miles of marsh east and southeast of New Orleans to open water. It's unclear at this time whether these losses will be permanent, or if flooding in these areas will eventually subside. However, it is very likely that substantial marsh vegetation was swept away as the storm struck some of the most vulnerable broken marsh areas in coastal Louisiana.
Another area where long-term impacts from Katrina are apparent is in the Chandeleur Islands located east and southeast of New Orleans. Aerial photographs taken by USGS personnel clearly show these barrier islands have been reduced in size by almost 50 percent. Important nesting habitat and rookeries used by brown pelicans, black skimmers, and several other species of waterbirds on the islands were severely eroded and likely destroyed by Katrina. In addition, the hurricane appears to have devastated extensive shoalgrass beds surrounding the islands, which were a critical food source for as many as 25,000 redheads and smaller numbers of lesser scaup. With the loss of protection once provided by the islands, a substantial portion of these shoalgrass beds will probably never recover.
Hurricanes Katrina and Rita will also have significant short-term consequences as saltwater storm surges from the hurricanes affected interior marshes far inland. Some of the largest concentrations of wintering waterfowl in North America gather on these marshes, where the birds feed on submersed aquatic vegetation and seeds of emergent plants. But storm surges likely uprooted and destroyed much of this beneficial vegetation, and most of the rest was likely killed by high salinity in the days and weeks following the storms.
Barring any additional saltwater flooding, vegetation in these areas will recover during the next growing season, but this winter, waterfowl will find considerably less food in Louisiana's coastal marshes than in a typical year. As a result, it's possible that some species like mallards, teal, and pintails will spend more time in southwest Louisiana's rice fields. Other ducks, particularly gadwalls and divers that rely on marsh habitats, may redistribute to other wintering areas in the Mississippi Alluvial Valley, Texas coast, Mexico, and Latin America.
Fortunately, waterfowl are highly mobile, an adaptation that serves them well given the highly dynamic nature of the wetland habitats upon which they depend. This adaptation is one reason Ducks Unlimited works to conserve wetlands in a number of historically important wintering areas across North America so that waterfowl populations will never be limited by winter habitat conditions.
Dr. Tom Moorman is director of conservation planning at DU's Southern Regional Office in Jackson, Mississippi.