…and the smallest ones too
Tiniest victims of the Gulf of Mexico oil spill may turn out to be most important
By Bob Marshall, The Times-Picayune
May 14, 2010, 7:00PM
Ellis Lucia, The Times-Picayune archive
Half of the all the life created in the nature-rich Louisiana coast, one of the world’s most productive estuaries, takes place in the thin layer of slime on top of the marshes.
To the watching world the environmental threat that BP’s oil disaster poses to the nature-rich Louisiana coast is captured in images of beautiful birds or furry creatures crippled by thick black goo. But scientists who know these estuaries best are more concerned about a less photogenic community.
The grass, microscopic algae and critters living in the wafer-thin top layer of marsh mud – called the benthic community – are the fuel that drives the whole system. If it’s covered with oil, everything above, including birds, fish and cute, furry critters, will be in trouble. And so will the humans who rely on the marsh for storm protection and seafood production.
“The top two millimeters of that marsh muck is where the action is in a coastal estuary,” said Kevin Carman, dean of the College of Basic Sciences at LSU. “That’s the base, the food that fuels the whole system. If you lose that in a large enough area it could have a disproportionate impact on the food web, and everything that depends on it: fish, shrimp, oysters, all the species that rely on the estuary.”
Microscopic creatures such as this Microarthridion littorale are a key part of the Gulf marsh ecosystem.
Half of the all the life created in the one of the world’s most productive estuaries takes place in this slimy zone just seven-hundredths of an inch thick. It’s a world too small for the human eye to detect and involves creatures few people have ever heard of, but one that looms huge for the larger critters in the system.
The production line starts with microalgae, single-celled organisms that feed and grow on nutrients deposited in the mud by decaying organic matter – primarily the local saltwater marsh grasses. Microalgae perform several key functions, including removing carbon dioxide and releasing oxygen, and leaving behind a sticky, glue-like substance that helps bind organic and mineral soils in the marsh, an important factor in fighting coastal land loss.
Most critically to the estuarine food chain, the microalgae are the forage for a vast range of equally tiny worms and crustaceans and other invertebrates, as well as the early stages of shrimp, crabs, oysters and almost all fish from menhaden to speckled trout and reds.
In fact, marine biologists estimate 97 percent of all marine species in the Gulf of Mexico depend on estuaries at some point in their life cycles, which means that benthic community has an impact far beyond the beach line.
“It’s an incredible engine for a wide range of life,” Carman said.
It’s a system that shows great resilience and continues to hum along in the face of nature’s toughest blows, from hurricanes to freezes. But scientists worry how well it would cope with the giant oil spill that could be washing ashore for weeks on end, because that has never happened before. However, they can paint a worst-case scenario from the many smaller, inland spills that have hit state’s interior coastal wetlands during the 80 years the oil industry has been here.
It’s a frightening picture.
“If the toxic components of the oil kill those invertebrates foraging on the algae, then the algae will grow out of control,” Carman said. “The analogy would be if you removed cattle grazing in a field, the grasses would just take over. Same thing here.”
The algae eventually would form a thick mat over the marsh mud, preventing sunlight and oxygen from penetrating below its surface.
“That would make it harder for anything to grow in the sediments below,” he said.
Those toxic components would dissipate in a mater of days or weeks, and the grazers might return. But if components of the oil known as asphaltenes – the thick black tars – settled on the surface, the damage could be worse.
“If the algae can’t get sunlight, they die,” said Carman. “If they die, the invertebrates have no food, and the whole web is disrupted.”
View full sizeChris Granger, The Times-Picayune archive
In spite of the containment boom at left, the bases of these lumps of marsh grass just south of Venice are black with oil. The marsh grass was photographed April 30.
The impact of an oil coating could be equally disastrous for the ecosystem if it includes the marsh grasses, which provide food and cover for many species and, critically, help hold together what little land is left.
LSU professor Irv Mendelssohn, a leading authority on coastal plant communities, said his studies on the impacts of smaller, localized oil spills show the state’s most common saltwater plants rebound fairly well after moderate coatings of the stems and leaves. In fact, research shows the best way to help these marshes in those situations is to simply leave them alone.
“If the stems and leaves are coated and die, these plants have an energy reserve which they will use to send up new shoots form the roots system, which are still alive,” he said. “But if you try to go in there and start scrubbing things or digging things, all you do is push the toxic components deeper into the mud, and that can cause more damage and kill the whole plant.”
When left alone in such conditions, Mendelssohn said, most traces of oil on the stems and leaves is usually gone within two years.
But if the oil is thick enough to coat the soil as well as the leaves and stems and seeps into the soil to affect the roots, the impact could be far longer, and much more serious.
“In that case it might be five or six years before the oil is degraded enough, because the soil would have no oxygen and no light and the organisms that can degrade the oil would not be there,” he said. “We seen examples at inland spills when the soil was soaked, and nothing really grew there for four years.”
Drawing comparisons for this event is difficult, because the inland spills studied were in areas as small as 30 acres and usually in places easy to boom off with no in the direct influences from tides and waves. This incident potentially could impact dozens of miles of broken marsh subject to high wind, wave and tide energy.
Mendelssohn said the state’s only recent experience with large marsh grass die-offs was in 2000 when more than 200,000 acres of coastal marsh were killed or severely damaged by an event researchers finally thought was caused to a historic drought.
In some areas of total die-off the land soil compacted six to seven inches. That loss of elevation put the areas below the water line, where tides and wind energy gradually turned the some of the new flats into open water.
“The underground roots and stems of these plants are like balloons that occupy volume in the soil,” Mendelssohn said. “When they die, those balloons collapse and that causes the soil surface to sink.
“In some area there was recruitment of new plants fairly quickly. In others there wasn’t, and that’s where we had land loss.
“Now, the concern with the oil spill is that if we have oil washing in and coating plants continuously for weeks or months, and we have a die-off, will there be recolonization soon enough to prevent land loss?
“We think that would be the worst case, but we just don’t know.”
The best-case scenario, Mandelssohn said, would be for BP to plug the leaks in the damaged well in the Gulf.</em>