By Amanda Mascarelli of Nature magazine

Three coastal-restoration projects intended to rescue Louisiana's rapidly shrinking wetlands have failed to restore marsh during the past two decades.

Instead, the schemes -- which involve diverting fresh water from the Mississippi River in the hope of carrying sediment to marshes and aiding plant life -- have made these regions more vulnerable to hurricanes, according to the authors of a study published by Geophysical Research Letters.

Louisiana's coastal wetlands are up against a host of natural and anthropogenic factors. Soil compaction, geological faulting, and oil and gas drilling are causing the ground to sink. Meanwhile, dams and levees built to constrain the Mississippi have prevented it from depositing the sediments needed to rebuild wetlands.

The Coastal Wetlands Planning, Protection and Restoration Act plans to restore almost 17,000 hectares of marshland over the next two to three decades, at a cost of US$1.05 billion. About 65% of the projected costs are for freshwater-diversion projects similar to those examined by the study, says lead author Michael Kearney, a coastal scientist at the University of Maryland in College Park.

Diversion tactics

Kearney and his colleagues analysed Landsat images of three of the longest-running freshwater diversions -- the Caernarvon, Naomi and West Point a la Hache diversions -- collected between 1984 and 2009. These projects were developed in the early 1990s in an attempt to redirect water and sediments into bays and marshes to help to restore coastal wetlands.

But the researchers found that total vegetation and marsh area in the three had not grown significantly. Moreover, the regions suffered greater damage during Hurricane Katrina than surrounding areas.

For instance, in the Caernarvon diversion, Hurricane Katrina destroyed the most vegetation in zones that received the most direct freshwater flow, even though these were far from the storm's path.

Most of the new plant growth that has occurred since the diversion was built consists of algae and other floating plants rather than the deep-rooted marsh plants that hold soil in place. This, says Kearney, is due to the influx of nutrients from agricultural run-off and industrial processes.

"The amount of nutrients per acre is far in excess of what these plants can tolerate," says Kearney of the marsh-building plants.

The researchers conclude that the scientific basis for freshwater diversions is not sufficiently established, and that the emphasis on diversions as a coastal-restoration strategy should be reconsidered.

Christopher Swarzenski, a wetland biologist with the US Geological Survey, based in Baton Rouge, Louisiana, agrees with their conclusions. "There's a lot of arm-waving," says Swarzenski. "There is no science to say that [freshwater diversions] will sustain wetlands or prevent wetland loss or build wetlands, which are the three objectives."

Disregarded data

But Douglas Jerolmack, a geophysicist at the University of Pennsylvania in Philadelphia, points to a handful of studies that he says provide extensive modelling, experimental and field data on the value of diversions -- data which he says these researchers have disregarded.

For example, a 2011 study of coastal-restoration strategies involving diversions points to several success stories. One of these, originally created for flood control rather than marsh restoration, is the Wax Lake Delta in Louisiana, where more than 100 square kilometres of wetlands have been built. "We actually understand reasonably well now what it takes to create a diversion that will build marshes," says Jerolmack.

Kearney disagrees. "Even if freshwater diversions do deliver mineral sediment into the marshes -- which I highly doubt -- what would be the point if the high levels of nutrients these waters also carry essentially so damage [plant] roots that they die off?"Jerolmack does not dispute the findings by Kearney and his colleagues that the three diversions in question have not restored marsh. But, he says, "we know why the diversions that they're studying don't work -- they divert water but they don't divert sediment". For a freshwater diversion to build wetland, he says, it must siphon water from the bottom of the river, capturing sediment along with water.

Jerolmack worries that Kearney and co-workers' conclusions are "potentially damaging" to Louisiana's marsh-restoration plans. "I think there's still work to be done on understanding better the science of how we can build marshes using diversions, but we are doing that science," he says. "And we are working on translating it directly into practice."

This article is reproduced with permission from the magazine Nature. The article was first published on August 5, 2011.