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Hurricanes benefit mangroves in Florida's Everglades, study finds

Florida's coastal mangrove forests cover 700 square miles. Photo courtesy of U.S. Fish and Wildlife Service
Florida's coastal mangrove forests cover 700 square miles. Photo courtesy of U.S. Fish and Wildlife Service

ORLANDO, Fla., Feb. 18 (UPI) -- Mangrove forests in the Florida Everglades benefit from nutrients and sediment washed up during some major hurricanes, and that helps coastal areas adapt to rising seas, a study concludes.

The study found the effect of hurricanes is so dramatic that two major storms in Florida raised the coastal elevation in parts of Everglades-area mangroves by more than 2 inches each time, said lead author Edward Castañeda-Moya, a research assistant professor at Florida International University in Miami.

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The research measured soil levels and phosphorus, a major nutrient for mangroves following hurricanes that raked Southwest Florida -- Wilma in 2005 and Irma in 2017, Castañeda-Moya said.

"Hurricanes are depositing mud and sediment in mangroves, and we found out this provides natural fertilization that helps the mangroves," he said.

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The findings were published Tuesday online by the Proceedings of the National Academy of Sciences.

Other institutions that contributed to the study were Louisiana State University in Baton Rouge and the College of William & Mary in Williamsburg, Va.

Mangroves are swamp trees that thrive in salty or brackish water. They cover over 53,190 square miles of Earth's surface in tropical coastal regions and over 700 square miles of Florida's coasts, according to NASA.

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Mangroves have been valued for centuries because they prevent erosion, trap runoff from the land and provide a home to aquatic life such as oysters, mussels and small fish, according to the Florida Fish and Wildlife Conservation Commission.

The study noted that wind damage to mangroves in Florida was severe during Wilma and Irma, snapping branches or trunks and uprooting trees. But the outcome had favorable aspects.

"Despite the destructive effect of hurricanes on mangrove forests in tropical and subtropical latitudes, hurricanes are major drivers controlling soil fertility gradients in the Florida Everglades mangroves, and therefore represent a positive influence," the study said.

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It added: "Hurricane-induced mineral inputs to near-coast mangroves in the Everglades enhance phosphorus concentrations in soils, increase plant phosphorus uptake, promote soil elevation gains relative to sea level, and facilitate rapid forest recovery following disturbance."

Castañeda-Moya said hurricane damage from Irma shocked him.

"You see the trees completely defoliated, knocked down, uprooted. We also saw a lot of debris and sediment buildup," he said. "It turns out, it's not all negative, and the trees can use that sediment."

According to the study, Everglades-area mangrove forests were able to recover almost completely five years after Hurricane Wilma.

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Castañeda-Moya noted that South Florida sea levels have increased dramatically in the past decade, and the mangrove trees need that sediment buildup from hurricanes "to keep pace with sea-level rise."

But not all hurricanes may have the same effect, said Betty Staugler, a research biologist with Florida Sea Grant, who was not involved in study. Florida Sea Grant is university-based program that supports research and public education to conserve the state's coastal resources.

"We've seen storms in other areas where mangroves take much longer to recover if they do at all," Staugler said. She is based in Charlotte Harbor, an area that has mangroves but is some 70 miles north of the Everglades mangrove forests.

Staugler noted that hurricane-roiled waters also are viewed as a source of phosphorus that feeds toxic red tide algae blooms.

"The benefit to mangroves is something I had not thought of before. I think this demonstrates in this case that such a natural phenomenon isn't all bad," Staugler said.

But Staugler cautioned that too much sediment from a hurricane could bury mangroves. She said Hurricane Charley damaged them near Charlotte Harbor, and they didn't recover for 10 years -- even with extra planting and restoration work.

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"Some parts of South Florida along the Everglades and Florida Bay are lacking in phosphorus, so a hurricane delivering phosphorus is a pretty cool thing," Staugler said. "But each hurricane and each region is a little different."

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