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Effects of the Gulf Spill on coastal plants
Story by Melissa Gaskill
THE 22-FOOT FIBERGLASS BOAT slid over a line of bright-orange boom, some of the more than 30,000 feet of surface and absorbent material surrounding marsh-covered Coffee Island, just off the coast of Alabama. Jeff DeQuattro, coastal project manager for The Nature Conservancy, peered into brackish water, carefully avoiding bags of oyster shells just beneath the surface, part of the organization's ambitious project to restore important oyster reef habitat in Mobile Bay. Oyster reefs form the foundation of many of the Gulf's productive reefs and marshes, and their loss contributed to Mobile Bay losing 85 percent of its historic marsh and seagrass habitat, says Judy Haner, marine program director for TNC Alabama.
This early August day, however, DeQuattro focused not on the shells but on the shore of the barrier island. Earlier in the week, a Shoreline Cleanup Assessment Team spotted tar balls there. Sure enough, DeQuattro found thousands of pea-sized oil stains and tar balls, several hundred feet of absorbent boom in the marsh, and under it, solid lines of oil and oil-soaked debris. He and a TNC intern, working in full Tyvek suits, rubber gloves and boots despite the heat, filled 12 large bags with oil-contaminated boom and debris.
Oil from the Deepwater Horizon blowout last April came ashore all along the northern Gulf Coast for months. Booms designed to prevent it from doing so often failed and, in cases like Coffee Island, even made things worse.
While its beaches bore the brunt of the oil spill, Alabama's marshes weren't affected as much as those in Louisiana. However, for these extremely productive ecosystems designed by nature to efficiently trap whatever washes through them – be it sand, silt, seaweed, nutrients or oil – time will tell the extent of the damage in even the leastaffected areas, and many experts say this spring will test their chances of recovery.
Salt marshes that line the northern Gulf Coast were among the major coastal habitat affected by oil, says Patrick Biber, associate professor of marine botany at the University of Southern Mississippi's Gulf Coast Research Lab. Those marshes can be categorized as saline, brackish, intermediate or freshwater, based on plant species, frequency of fire and salinity. They are dominated by Spartina and Juncus plant species (see sidebar), with numerous secondary species and constantly changing boundaries. Marsh systems serve as nurseries for many fish species in the Gulf, including many commercially important ones. Largely thanks to its extensive wetlands, the Gulf of Mexico produces 1.3 billion pounds of seafood each year, according to Harte Research Institute at Texas A&M Corpus Christi. In fact, it produces more finfish, shrimp and shellfish than the south and mid-Atlantic, Chesapeake Bay and New England combined.
In addition to providing vital habitat for a wide variety of species, marshes and other wetlands also filter sediment and pollution from the water. This service is particularly critical in the Gulf, where the entire landmass between the Rocky and Appalachian Mountains, or 40 percent of the continental U.S., drains – along with soil, fertilizers and other chemicals from cities, roads and agriculture. Plants also convert much of what ends up in the marsh into biomass. Wetlands sequester carbon dioxide, both in soil and plants, and the fish and other species that depend on marsh habitats prefer specific plant communities.
In the past decade, saltwater inflow from five major hurricanes wiped out an estimated 500 square miles of marshland, says Malcolm Vidrine, professor of biology at Louisiana State University Eunice. In addition, levee construction, logging and canals cut through Louisiana's marshes have caused the loss of about 25 square miles of marshland a year since 1930 – a total of some 2,300 square miles, or 90 percent of marsh loss in the U.S., according to Vidrine.
The 5,300 square miles or so that remain still account for roughly 30 percent of the nation's coastal marsh. An oil spill clearly was the last thing this struggling coastline needed.
Oil affects plants both physically and chemically. If oil coats foliage, it can cause reduced transpiration, leading to temperature stress, and can reduce the plant's ability to photosynthesize. Many marsh plants can tolerate oil on their leaves, even several times, says Andy Nyman, a Louisiana State University professor in wetland and wildlife ecology who conducted studies with smooth cordgrass (Spartina alterniflora) some years ago. Oiling caused the plants to lose their leaves, but the stems made more leaves. However, Nyman says that after repeated oiling, plants may exhaust their resources and be unable to replace dead leaves.
Juncus and Spartina species differ in their sensitivity to oil, according to Biber, and these differences could result in changes to the composition of coastal plant communities. Juncus species are also slowgrowing, which means that faster-growing Spartina species could colonize areas where oil affected Juncus. Spartina habitat tends to be more open and less dense than Juncus, so this would mean a potential change in habitat values. Since animals that prefer one community type might not move to a different one, some organisms could suffer.
Most dominant marsh grasses reproduce mainly through vegetative growth. When oil coats soil, it can keep leaves and shoots from regenerating and damage root membranes, which can affect a plant's ability to tolerate salt. Without this ability, plants won't last long in a salt marsh.
Efforts to clean marshes actually can make things worse – one reason that Jeff DeQuattro says they did not attempt to clean the marsh affected at Coffee Island. Foot traffic can drive hydrocarbons into the soil, where they degrade more slowly. Cleanup efforts can disturb and compact soil and vegetation, impeding root growth and plants' ability to take in water and nutrients. Oil kills plants by preventing air from reaching roots or by poisoning them or limiting their reproduction with their toxins. Theoretically, oil also could act on microbial communities within the soil to reduce plant energy and slow growth.
Paul Sammarco, a professor at the Louisiana Universities Marine Consortium, says that oil can persist in the environment for years, even decades, and continue to depress the physiological condition and activity of microalgae in the sediment. Oil also can change the structure of the community of organisms that live on the bottom of the ocean. Chemical dispersants such as those used on the spill can increase the oil's toxicity, Sammarco says.
When marsh plants die, their root systems no longer hold the soil together, resulting in loss of soil elevation and creating open water in what had been marsh. Loss of this habitat ripples throughout the food chain.
Even where plants survive, stress might prevent them from building up as much marsh as they would if healthy, LSU professor Nyman adds. "Where plants can't keep up with subsidence and sea level rise, you end up with more flooding stress, and in a few years plants just drown out. In that case, oil would prove the straw that broke the marsh's back. It's going to be hard to know, even down the road, because picking out the effects of just the oil is not going to be straightforward."
MARSHES CAN BE CATEGORIZED as saline, brackish, intermediate or freshwater, says Andy Nyman, Louisiana State University associate professor in wetland and wildlife ecology. Plant species, varying flood inundation, frequency of fire and salinity define each of these. Saltmarsh cordgrass (Spartina alterniflora) dominates Gulf saline marsh, and saltmeadow cordgrass (Spartina patens) is most common in marshes that are brackish and intermediate.
Around the Mississippi-Alabama border, where tides fluctuate less than three feet, black needle rush (Juncus roemerianus) makes up 90 percent of emergent marsh vegetation, with smooth cordgrass along the water's edge.
Smooth cordgrass, big cordgrass (Spartina cynosuroides) and reed (Phragmites australis) dominate the Mississippi River delta in Louisiana. The rest of Louisiana, Texas and Florida marshes divide about fifty-fifty between Juncus and Spartina species.
West of the Mississippi River delta to the north Texas coast, saltmeadow cordgrass and Gulf cordgrass (Spartina spartinae) make up chenier or oak plains. South Texas and south Florida also contain mangrove ecosystems, one of the most endangered on the planet, cleared by humans for shrimp farms, rice paddies, and, more commonly on the Gulf coast, residential and commercial development.
Gulf coastal estuaries, where fresh water from rivers and streams mixes with saltwater, contain five species of the marine flowering seagrass plants, says David Newstead, an environmental scientist with the Coastal Bend Bays and Estuaries Program in Corpus Christi, Texas. Seagrass is important habitat for birds, finfish and shellfish, and it is a primary food for sea turtles and manatees. Its photosynthesis makes it an important part of the nutrient cycle.
Most of what scientists know about oil's effects apply to oil floating on the surface, not below it, and this spill poured out nearly a mile below the surface. Scientists also don't know much about the potential impact of oil dispersed in the water by chemical or natural processes.
Oil first arrived at Grand Bay National Estuarine Research Reserve, on the Alabama-Mississippi border, on June 12, 2010, says Dave Ruple, reserve manager. Reserve staff began monitoring a week after the Deepwater Horizon explosion for baseline data, and more than 15 miles of boom were deployed to protect the reserve. But Ruple knows all too well that booms have limitations. "They work well when the water is calm, but oil easily goes over booms when it is choppy," he says. "And we don't know what's under the surface. The loss of larvae and plankton from oil in the estuaries may be seen down the road."
The overall effect of that oil remains to be seen, agrees Thomas S. Bianchi, oceanography professor at Texas A&M University. "Even if marshes return, this oil is not magically gone. Where sprouts come up, the underlying root structure is probably not affected, but it's hard to imagine that some oil didn't make its way down. Mousse [residual emulsified oil] covered plants and killed surface vegetation, and as it decomposed, oil went further down in the mud. Is it still there in the form of smaller molecules? Do organisms that live there have it in their tissues? If organisms pass hydrocarbons up the food chain, that could be a problem for commercially important species like oysters and crabs. No way you put that much oil in the basin and have it just go away.
"Yes, bacterial communities have evolved in this area of the U.S. over millions of years from natural oil seep that releases oil from the bottom of the Gulf to its surface and shorelines, and they eat this stuff, so then it's in the bacteria.
If marshes are the lifeblood of the coast, plants are the lifeblood of the marshes. These plant species are tough, adapted to deal with a naturally harsh environment. They can handle limited amounts of oil without serious effects, Biber says, as long as other stressors are minimal. But along this coast, minimal would hardly describe the other stress placed on marshes.
"Spring will be the test of whether colonization and re-colonization happens successfully in areas exposed to oil," Biber predicts. "Plants may have been oiled and died back, but as long as the root structure is intact and nothing else is causing stress, they will come back this spring."
For scientists, reserve managers and other dedicated workers on the Gulf coast, spring can't come soon enough.
Melissa Gaskill is an Austin-based science writer who spent time last summer visiting scientists, research labs, preserves, parks and beaches in Louisiana, Mississippi and Alabama to get a first-hand look at the effects of the Deepwater Horizon spill.