Louisiana revival: Eco-engineering on a giant scale

STANDING knee-deep in the waters of Bay Jimmy in south-eastern Louisiana, Daniel Deocampo pours a bucket of clay mixed with seawater into the marsh. Oil swirls around our legs and the air reeks of burnt petroleum. With oil from BP's Deepwater Horizon spill sitting in the bay, the burning question is - remove the oil or wait for nature to take its course?

R. Eugene Turner, a coastal ecologist at Louisiana State University (LSU) in Baton Rouge, says that interfering with nature could backfire. Even just wiping oil off leaves, he says, spreads the toxic residue around. But Deocampo, a sedimentary geochemist at Georgia State University, Atlanta, and his colleague Kuk-Jeong Chin say leaving the oil will doom the marsh, and are testing ways to give oil-munching bacteria a turboboost. "If we do nothing, all the plants will die," Chin says. "Nature will take too long."

The disagreement underscores a debate that has been raging over marsh restoration for decades. Even before the spill, the marshes were disappearing at an alarming rate - the consequence of the dams, levees and canals built to provide shipping channels and protect New Orleans from flooding. Without intervention Louisiana's bayous could become open water within 50 years.

As a result, support has been growing for a suite of projects to resurrect the marshes (see map). At one end of the spectrum are those who advocate minimal intervention and letting nature take its course. At the other, the call is for yet more engineering: new, hardier breeds of grasses, seeding from the air and artificial reefs to shore up the sinking sediment.

Ultimately, the marshes are vanishing because sediment is in short supply. Wind and waves constantly erode the shoreline, and dams and levees hold back sediment flowing down the Mississippi. What's more, a network of canals dredged by the oil and gas industry carry saltwater inland, killing freshwater marshes. Add to all this rising sea levels and the largest oil spill in US history and the situation is desperate. Without its marshes, Louisiana's thriving seafood industry would crumble and the state's coast would lose its natural defences against the powerful storms that blow in from the Gulf of Mexico.

Turner advocates small-scale intervention: filling in thousands of kilometres of abandoned canals with the dredged sediment that is still piled up alongside them. He also favours helping sediment flow to the marshes. Historically, when the Mississippi's waters ran high, "crevasses" appeared in the river banks and carried sediment into the deltas. Dams and levees now prevent this, so Turner suggests punching holes in the river's embankment to spur the process. "So many marsh restoration ideas assume we can do better than nature," says Turner. "I think that's pretty arrogant."

Others say we need to think big. "Over the years, we've done all kinds of patchwork projects," says Harry Roberts, a retired sedimentary geologist at LSU. "They're not long-term solutions." Roberts has calculated that 18 to 24 billion tonnes of sediment will be needed to maintain the delta as the sea level rises in the next century (Nature Geoscience, DOI: 10.1038/NGEO553). Small crevasses can never meet that demand, he says.

For researchers like Roberts the question has become not how to restore Louisiana's lost Eden but how to create a new, improved one. As part of that, in April, a $23 million project kicked off to pipe mud more than 6 kilometres from Cote Blanche Bay to Vermilion Bay's Marsh Island Wildlife Refuge. The aim is to recreate 160 hectares of marsh.

Once the soil is in place, you need grasses - and not just any grasses. Two decades ago, Michael Materne, a wetland plant specialist at LSU's AgCenter, collected varieties of the native smooth cord grass (Spartina alterniflora) from across the US, searching for the hardiest ones. Today, Materne's Vermilion variety, named after the Louisiana parish it originated in, is the only one used in re-vegetation projects. To increase genetic diversity in the restored ecosystems, it will be joined next year by up to six more varieties that he has cross-bred.