Post-Katrina Study Shows Strength of Salt Marshes

Andrea Mustain LiveScience.com 26 Jul 10;

Hurricanes Katrina and Rita sent devastating waves through Louisiana's coastal plains in 2005, washing away hundreds of square miles of the state's wetlands.

Yet post-hurricane surveys of the Breton Sound, a large area of marshes along the state's southeastern edge, left researchers puzzled. All of the sound's wetlands suffered an equal pounding, yet the storms destroyed certain areas and left others relatively intact.

"It was as if the hurricane had never touched it," said Duncan FitzGerald, a professor at Boston University who has spent more than two decades studying the region. "It was night and day in the resilience of these two plant communities."

Researchers wanted to find out why some areas withstood the onslaught while others crumbled, and a new study has provided some answers. It turns out that high salt levels can be an advantage for marshes when it comes to withstanding a beating.

"The saline portions of the marsh lost about a half percent of land area - that's in contrast to the freshwater marshes that lost about 10 percent, so it's quite a difference and it really shows up in a map," said Boston University's Nick Howes, lead author of the new study.

Howes and a team of scientists ran extensive soil tests both in the field and in the laboratory, comparing multiple sites across Breton Sound in the two years following the storms.

It turns out that salt marshes are tough because they support tough plants. In the more saline areas, the plants produce dense webs of thick roots that bore deep into the ground, forming a kind of anchor that keeps the soil from being eroded.

FitzGerald said this has important implications. Louisiana wetlands are at-risk ecosystems, and understanding the relative resilience of salt marshes can help inform policy decisions on how to treat and maintain these coastal areas to help them survive.

In addition, the marshes act as a buffer, protecting inland areas from the ravages of powerful waves. "They're one of the speed bumps along the way for the storm surges," FitzGerald told OurAmazingPlanet.

And in light of the National Oceanic and Atmospheric Administration's prediction that 2010 could be one of the most active hurricane seasons on record, knowing which wetlands best survive storms could be an issue of increasing interest.

Although Louisiana's marshlands are facing complex problems, FitzGerald said, "There are actions that can be taken to prolong the longevity of these wetlands."

Freshwater wetlands 'vulnerable in hurricanes'
BBC News 27 Jul 10;

Freshwater coastal wetlands are more vulnerable to erosion during hurricanes than habitats with higher levels of salinity, a study has suggested.

US researchers say freshwater marshes have shallower root systems, leaving them at risk from wave erosion during storm surges.

They added that the results could have implications for wetland restoration projects in hurricane-prone areas.

The findings appear in the Proceedings of the National Academy of Sciences.

"During the 2005 hurricane season, the storm surges and waves associated with Hurricanes Katrina and Rita eroded 527 square kilometres of wetlands within the Louisiana coastal plain," the researchers wrote in their PNAS paper.

"Low salinity wetlands were preferentially eroded, while higher salinity wetlands remained robust and largely unchanged."

The team said that both freshwater and salt marshes within their study area were exposed to similar conditions during Hurricane Katrina, which struck the US Gulf coastline in August 2005.

"We hypothesise that wave shear stresses generated during the hurricane exceeded the shear strength of the low salinity wetland soils, resulting in failure, whereas greater soil shear strength in the saline wetlands largely precluded erosion," they suggested.

"Soil shear strength and the resistance of the soil to erosion are determined by the properties of the vegetation.

"We propose that resistance to erosion is primarily a function of rooting characteristics, which depend on the dominant species of vegetation - as controlled by salinity."

The scientists identified a "weak zone" about 30cm below the surface in freshwater wetlands, which coincided with the base of the root system of the plants growing in the habitat.

However, in the salt marshes, plants' roots were found to penetrate the soil to depths of about one metre.

As a result of the deeper root system, the vegetation was better suited to withstand the pressure exerted by storm surges and wave action during a hurricane.

The team concluded that the findings could play a part in shaping restoration programmes and land management schemes in regions that are prone to tropical storms.

They wrote: "The dramatic difference in resiliency of fresh(water) verses more saline marshes suggest that the introduction of freshwater to marshes as part of restoration efforts may therefore weaken existing wetlands rendering them vulnerable to hurricanes."