by Tom Moorman, Ph.D.
The explosion and subsequent sinking of the Deepwater Horizon drilling rig triggered the largest environmental catastrophe in the history of the United States. The blown-out well, located about 40 miles southeast of the mouth of the Mississippi River in more than 5,000 feet of water, spewed tens of millions of gallons of crude oil into the Gulf of Mexico. Species and habitats at risk—some superabundant, others regrettably rare—range from poorly studied deepwater coral reefs to sea turtles, marine mammals, and highly productive wetlands vital to waterfowl and other wetland-dependent migratory birds.
As of press time in early August, BP had successfully staunched the flow of oil with an experimental cap and a "static kill," as efforts to drill relief wells neared completion. But even after the well is permanently sealed, the vast quantities of oil and chemical dispersants applied to break down and sink the oil may present unprecedented future environmental challenges.
Over eons, the muddy, sediment-laden waters of the Mississippi River built a vast area of coastal deltaic marshes interspersed with hundreds of thousands of ponds that provide rich habitat for multitudes of wetland-dependent birds and other wildlife. These wetlands are the heart and soul of this region's commercial and recreational fisheries and the foundation of its culture and the livelihoods of local residents. For waterfowl and millions of other migratory birds, the Gulf Coast marshes are at the end of a vast funnel. Ducks, geese, and many other bird species raised in the Arctic, boreal forest, and prairies fill the funnel each fall as they migrate down the Mississippi and Central flyways. Louisiana's expansive coastal marshes host the majority of these birds—up to 9.2 million ducks and geese in some years. Sadly, since the 1930s over 1.2 million acres of Louisiana's coastal marshes have been lost to saltwater intrusion, erosion, and subsidence. Moreover, vast expanses of marsh have been degraded by the same forces, further diminishing the region's ability to support waterfowl and other wildlife.
The primary cause of Louisiana's enormous wetland losses has been the construction of levees along the lower Mississippi River to control flooding and improve waterborne navigation and commerce. Today, these levees reach almost to the Gulf, robbing the marshes of the sediment-laden waters that created and once sustained them. Unfortunately, the oil spill may increase the risk to wetlands along the Gulf Coast, especially in Louisiana.
While the stakes are exceedingly high, great uncertainty exists about the oil's immediate and long-term effects on wetlands and waterfowl. Essentially, the oil spill is an unplanned large-scale experiment in one of North America's richest and most diverse wetland areas, adding a new dimension of risk to an ecosystem already losing approximately 20,000 acres of wetlands annually. But one thing is certain. This region's coastal wetlands and the fish, wildlife, and people who depend on them cannot sustain such losses much longer.
What Will Happen to the Oil?
As of press time in early August, an estimated 200 million gallons of crude oil had escaped into the Gulf of Mexico. What will become of all this oil? Seeps of oil occur naturally in the Gulf, and the relatively small amounts of oil these seeps release are generally not a serious threat to the marine environment. Certain species of bacteria consume the hydrocarbons in oil and eventually break them down into byproducts such as water and carbon dioxide. The downside is that these bacteria consume oxygen in the process.
Marine scientists fear the sheer volume of oil in the Gulf may cause an explosion of bacteria that will consume lots of oil, but in doing so also consume lots of dissolved oxygen, creating "dead zones" devoid of life. Oxygen depletion could also occur in coastal wetlands and shallow inshore waters, killing fish, shrimp, crabs, clams, and oyster beds. Monitoring is ongoing, and only time will tell to what extent this may happen and what effect it may have on the Gulf's marine life and productive fisheries.
Photo: Dale Humburg, DU
It's difficult to describe the wetlands of the Gulf Coast to those who have not had the good fortune to see them firsthand. The countless waterfowl, shorebirds, wading birds, and other wetland wildlife make dawn in a duck blind in coastal Louisiana a spectacular testimony to the rich abundance and diversity of migratory birds wintering in the marsh. It's one of the finest waterfowling experiences to be found in North America.
The Gulf Coast is also one of the most diverse ecosystems on the planet. Marsh plant communities change with the degree of saltwater influence from the Gulf. Varying levels of salinity create a mosaic of different wetland types, ranging from marine to freshwater. While waterfowl use these different marsh types disproportionately, the birds ultimately use all of the marsh. And no marsh type is completely independent of the others.
|Uncertainty abounds, but the U.S. Fish and Wildlife Service, with input from Ducks Unlimited and others, is working to assess potential oil-spill impacts on scaup, redheads, and other waterfowl.
Salt marsh forms the outer fringes of the coast, where land dissolves into the open waters of the Gulf. While these high-salinity wetlands are of relatively low value to waterfowl, clapper rails and seaside sparrows find them much to their liking. Inland of the salt marsh is brackish marsh, where salinity is about seven to 18 parts per thousand—an amount easily detected by the old "taste test." Brackish marshes, with their moderate salinity, also tend to be somewhat less important to waterfowl. Along the Gulf, however, "less important" does not mean "unimportant." Shallow-water areas in brackish marshes often support large, dense beds of wigeon grass, a favorite food of gadwalls, wigeon, pintails, and green-winged teal. In years when bumper crops of wigeon grass are present, these dabbler species heavily use brackish marsh habitats.
But the waterfowl habitat value of brackish marsh pales in comparison to that of intermediate marsh, where salinities range from about one to seven parts per thousand, and to freshwater marsh, where salinity only occurs in background levels of about 0.5 parts per thousand—about the same as you may find in a glass of tap water. These Gulf Coast marshes support larger numbers and a greater diversity of wintering waterfowl than anywhere else in North America. They host the majority of this continent's gadwalls and resident mottled ducks as well as significant numbers of blue-winged teal, green-winged teal, shovelers, pintails, wigeon, ring-necked ducks, and canvasbacks. Most of these ducks rely on the dense beds of submersed aquatic vegetation that grow in freshwater and intermediate-salinity ponds interspersed throughout the marsh. Several species of pondweed, southern naiad, delta duck potato, and annual plants like wild millet—as well as an abundant supply of small invertebrates—provide the food energy that ducks need to survive winter and fatten up for spring migration.
So far, the oil spill has mainly affected barrier islands, beaches, and salt marsh perimeter vegetation not directly important to waterfowl, but vital to colonial beach-nesting birds such as roseate spoonbills and brown pelicans, and to several species of terns, herons, and egrets. These salt marshes have served as a natural barrier that so far has protected interior intermediate and freshwater marshes from oil. If this trend holds, large-scale impacts to most waterfowl and their habitats may be averted.
Remarkably, at press time various government sources were reporting that nearly 75 percent of the spilled oil was gone—either skimmed, burned, evaporated, or widely dispersed into the Gulf. Hence, while risk of storm damage is always present on the Gulf Coast, it appears that the likelihood of wetland losses from oil damage is diminishing. Good news for a system that has already lost nearly 40 percent of its historic wetlands.
For millennia, the Mighty Mississippi deposited countless tons of fertile sediment where the river meets the Gulf, creating a vast, productive marsh once covering some 3 million acres. Today, fewer than 2 million acres are left, and these remaining marshes are not considered adequate to support waterfowl populations at desired management levels. Nevertheless, extensive high-quality waterfowl habitat remains, especially in shallow intermediate- salinity and freshwater ponds scattered throughout the marsh. These ponds, under appropriate growing conditions, support dense beds of submersed aquatic plants, including various species of pond weed, southern naiads, and many others—offering in effect a salad bar for ducks.
These plants also provide habitat for a host of aquatic insects and crustaceans, rich sources of protein ducks need to complete their winter molt and prepare for spring migration. Moreover, beds of submersed vegetation serve as nursery habitat for a variety of commercially important fisheries, including white and brown shrimp and blue crabs. Submersed aquatic plants are a key part of the foundation that supports the region's enormous biological productivity.
Photo: Dale Humburg, DU
But uncertainties remain about how the oil spill will affect scaup, redheads, and other waterfowl that largely use offshore habitats. Deeper brackish bays and lakes, such as Lake Pontchartrain and Lake Borgne, regularly winter approximately 500,000 lesser scaup. In addition, large numbers of scaup—in some years more than 1 million birds—winter in the Gulf off the southeast Louisiana coast. To the east, sizeable concentrations of scaup occur in Mississippi Sound, Mobile Bay, Tampa Bay, and Charlotte Harbor—all areas that were vulnerable but appear to have mostly been spared from oil impacts. Questions remain, however, regarding how the oil spill will affect small marine clams, believed to be the staple food of wintering scaup. If the abundance of clams declines significantly in oil-impacted waters, scaup may be forced to migrate to other areas—certainly not ideal, but something the birds are well adapted to do. In a more troubling possibility, filter-feeding clams could absorb toxins derived from oil or chemical dispersants. If consumed in large enough quantities (by scaup feeding on clams) these toxins could directly kill or weaken birds or cause reproductive problems for breeding females in the future. Given that scaup numbers are already at depressed levels, any large-scale impacts to the population would be devastating.
In contrast, redhead numbers are currently healthy and more than 85 percent of the population winters in the Laguna Madre of Texas and Mexico, far to the west of the area currently impacted by the oil spill. But about 10,000 redheads winter near the Chandeleur Islands just off the southeast Louisiana coast, which have already been heavily impacted by oil. Another 80,000 redheads winter in the Big Bend of the Florida Gulf Coast from Panama City to Cedar Key—an area that appears to have been spared from most oil impacts. Redheads feed largely on shoalgrass and other submersed aquatic plants. If this vegetation is killed or contaminated by oil, redheads wintering in these areas would be subject to the same biological threats as scaup. Uncertainty abounds, but the U.S. Fish and Wildlife Service, with input from Ducks Unlimited and others, is working to assess potential oil-spill impacts on scaup, redheads, and other waterfowl.
|Regardless of whether the oil spill worsens ongoing habitat losses, the need for long-term conservation strategies to sustain the region's wetlands and waterfowl populations has never been more urgent.
While the Gulf oil spill is an almost incomprehensible environmental catastrophe, this region's fragile coastal wetlands were already in grave danger before this disaster unfolded. Louisiana has already lost more than 1.2 million acres of its most productive coastal marsh. In addition, rice agriculture in south Texas and Louisiana, which provides important foraging habitat for wintering ducks and geese, has declined by roughly 50 percent in recent decades. Regardless of whether the oil spill worsens ongoing habitat losses, the need for long-term conservation strategies to sustain the region's wetlands and waterfowl populations has never been more urgent.
In March, Ducks Unlimited completed a 10-year strategic plan outlining the direct programs, public policy, and scientific research needed to advance our conservation goals in the Gulf Coast region. In cooperation with a host of conservation partners and with support from DU members across North America, we must increase our efforts to restore and sustain coastal wetlands for waterfowl and all who value them as part of our waterfowling heritage. DU was working hard to restore Gulf Coast wetlands prior to the oil spill, and we will remain there as long as it takes to ensure these vital habitats—and the region—can support healthy waterfowl populations today and in the future.
For the latest news and analysis about the impacts of the Gulf oil spill on wetlands and waterfowl, visit the DU website at www.ducks.org.
Dr. Tom Moorman is director of conservation planning in DU's southern region and leader of DU's Gulf Coast Response Team.