Hurricanes fertilize mangrove forests, shape coastal landscape of the Florida Everglades

The destructive power of a hurricane appears to do plenty of good for mangroves in the Florida Everglades.

When Category 3 Hurricanes Wilma (2005) and Irma (2017) struck Florida, mangroves took a beating, particularly in the west coast. Trees lost their canopies. Taller ones snapped and several were uprooted. Seedlings were scattered to the winds. Storm surge partially submerged trees on the front line.

Then the storm surges deposited gray, phosphorus-rich mineral sediment from the ocean floor on top of mangrove soils. That sediment increased phosphorus concentrations in the soil, fueling mangrove regeneration and recovery, according to a new study conducted by researchers at FIU, Louisiana State University, and William & Mary.

“When you hear about hurricanes, they create negative impacts to the coastal landscape because of infrastructure damage and ecosystem damage for mangrove forests,” said Edward Castañeda, an FIU research assistant professor in the Institute of Environment and the study’s lead author. “When you do these studies and you dig into information for 20 years, you realize there’s a positive side to these storms.”

Using data from the Florida Coastal Everglades Long Term Ecological Research program funded by the National Science Foundation, Castañeda and fellow researchers realized these storms have a positive effect and were able to help mangroves increase their soil elevation, facilitate their rapid recovery and help their young branch out and find new homes. 

It’s a welcome impact from an otherwise destructive force of nature as mangrove forests do plenty of good for people. They blunt the impact of hurricanes. They absorb damaging winds and prevent floodwaters from moving farther inland. They bear the brunt of the wind and storm surge, so people living in developed areas don’t have to.

They store carbon that otherwise would be released into the atmosphere and provide nursery for fish, crab, shrimp and mollusk species that are a critical component of Florida’s commercial and recreational fishing industries.

How much hurricanes impact mangrove forests depends on how close mangroves were to where the hurricanes made landfall. Mangrove forests closer to the coast were more likely to suffer wind damage, but they were also on the receiving end of most of the nutrients deposited by storm surge.

Data showed phosphorus-rich sediment was pushed up to 6 miles away from the coast, benefitting even trees that weren’t as badly damaged. Wilma deposited twice the average amount of phosphorus found naturally in mangrove soils. That nutrient became the fertilizer that fueled the mangrove forest to recover in less than five years.

Irma had a similar impact on the mangroves. Researchers estimate the storm added up to 14.4 times the regular amount of soil elevation mangroves generate in one year. That’s good news for mangroves who are waging a battle against soil collapse and sea level rise. Higher elevation might give them a fighting chance.

“This natural phosphorus fertilization mechanism from hurricanes may be an adaptation of mangroves to withstand the impacts of both phosphorus limitation and rising sea levels in south Florida,” Castañeda said.

The collaboration among 10 interdisciplinary researchers from three institutions made the study possible.

“Studying the problem from so many perspectives made this long-term project unique to the understanding of natural processes that occur over large spatial and temporal scales,” said Victor Rivera-Monroy, co-author of the study and associate professor in the Department of Oceanography & Coastal Sciences at LSU.

Whether mangroves recover from Irma at the same speed as they did with Wilma is too soon to tell. The researchers are relying on data from the long-term ecological research program to hopefully answer that question in a few years.

The study was published in the journal Proceedings of the National Academy of Sciences.