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 Letters1.
… 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.
… 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.”
…Kearney … “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 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.”