Natural coastal protection of mangrove fringe getting squeezed by sea level rise, development

Florida’s first defense against an encroaching and eroding sea are the protected coastal mangrove forests. But laws can’t protect these plants from sea-level rise, a new threat to Lee County’s many miles of coastline.

Originally published by Karl Schneider in the Naples Daily News on Feb. 14, 2020


Sea-level rise and onshore development are killing Florida’s mangroves. Here’s why that matters

https://www.naplesnews.com/story/news/environment/2020/02/14/sea-level-rise-development-threatens-florida-mangroves/4666293002/   Karl Schneider, NaplesPublished 6:02 a.m. ET Feb. 14, 2020

The mangrove forests lining the Florida peninsula provide vital habitats, protect against severe storms and prevent coastal erosion, but they are caught between two impending threats: sea-level rise and development.

Experts call this “coastal squeeze.”

As sea levels rise, mangroves naturally move inland, but in many areas of Florida they would hit a wall: seawalls, lawns, docks and waterfront homes. It’s hard enough getting a handle on growth as Florida adds nearly a thousand new residents a day. But protecting ecosystems from rising water is a harder task.

“The easiest way to kill a wetland is to raise the water level,” said Bill Mitsch, the director of the Everglades Wetland Research Park under Florida Gulf Coast University. “Mangroves are adapted to shallow water, but if it gets too deep they can’t survive.”

And then there’s the cutting. Mangroves have lost some of the protections they once had. Sometimes a condo association, for example, can trim them just to get a better water view. Many scientists say trimming mangroves can hurt their health and effectiveness.

“Mangroves are one of the most important coastal habitats and safeguards,” Weesam Khoury, a Florida Department of Environmental Protection spokeswoman, wrote in an email. “Their root network provides habitat for oysters and barnacles. Mangroves are also an important habitat for many species of birds, especially wading birds such as the federally endangered Reddish Egret.”

These unique ecosystems, sometimes called “nature’s nurseries” are the incubator of the $7.6 billion seafood industry in the state, which provides 109,000 jobs, according to the DEP.

Under the canopies of the state’s mangroves, 80% of commercial and recreational fisheries depend on the habitat, according to a report by Jim Beever, who recently retired from the Southwest Florida Regional Planning Council.

If the mangrove habitat has nowhere to go under the threat of sea-level rise and coastal squeeze, these fisheries could be in trouble.

Sea-level rise threatens ‘nature’s nursery’

Sea-level rise has been observed and recorded “over the past century, and the rate has increased in recent decades,” according to the federal government’s National Oceanic and Atmospheric Administration and the Environmental Protection Agency.

A recent study from the Virginia Institute of Marine Science shows an increase in sea-level rise along the coast of Southwest Florida.

Molly Mitchell, a marine scientist at VIMS and co-author of the report, said there are a lot of causes for the rising sea levels including increasing temperatures that cause water to expand and ice sheet melt in Greenland and the Antarctic.

“What we are seeing is an acceleration in the rate of (sea-level) rise,” Mitchell said. “In Naples, it is rising 3.1 millimeters per year, but that is accelerating.”

The level of rise and acceleration can vary along Florida’s Gulf coast, though measurements in Key West and St. Petersburg are similar to Naples. Naples is the closest data station to Lee County, which has many coastal and island communities at risk, such as Sanibel.

That leaves a question in Mitchell’s mind: Can mangroves cope with that level of sea rise?

Mangroves rely on sediment built up from the movement of tides to provide the nutrients needed for healthy growth. As sea levels rise, the sediment becomes too deep and mangroves can no longer survive.

While fish and other sea life depend on mangrove nurseries, the forests provide flood and storm protection for people living near the shore.

Oysters form on a mangrove in a tunnel just south of Bunche Beach on Wednesday June, 26, 2019. Mangroves provide shelter and refuge for young fish. Filter feeders like oysters also grow on the trees.  (Photo: Andrew West/The USA Today Network-Florida)

“The amount of protection afforded by mangroves depends upon the width of the forest,” according to the DEP’s website. “A very narrow fringe of mangroves offers limited protection, while a wide fringe can considerably reduce wave and flood damage to landward areas by enabling overflowing water to be absorbed into the expanse of forest.”

A large mangrove forest will absorb the energy from the waves and save beaches from erosion. A 2015 study predicts an increase in storm frequencies and severity and says there is a “potential doubling in the frequency of Atlantic hurricanes as a direct result of anthropogenic (human-induced) climate change … “

Mitchell said her findings on sea-level rise show an increase will affect storm surges in the future.

“Storms will be coming in on higher base level,” she said. “A wave’s energy depends on the shoreline. On steeper shorelines, it will affect it more than a flatter shoreline, but deeper water can make for higher wave energy.”

Essentially, sea-level rise will bring the waters closer to developed areas, and waves will have more energy behind them since the waters are deeper. Mangroves act as a buffer between these storm surges and developed areas in Florida.

Mangroves are adaptable and can migrate inland as sea level rises. The forests run into a problem, however, when seawalls and development block migration and the habitat could shrink and eventually disappear.

 “A basic solution is for all the engineers to get out of the mangrove’s way,” Mitsch said. “Just design with nature and don’t try to come up with some contraption. Contraptions are more favorable because engineers can make a lot of money.”

Seawalls are meant to prevent coastal erosion, but a vertical build will prevent mangroves from migrating inland. Beever said this type of design is old technology that has been promoted as the proper way to build.

“One solution is to change the sea wall design to include a slope and use soil and riprap in pockets along the seawall,” Beever said. “There are designs for this and these would allow the mangroves to migrate.”

If impediments to mangrove migration are removed, the forests will move inland until they run out of subtropical climate, Mitsch said.

Sea walls are not the only impediment to mangrove migration. Large highways such as Tamiami Trail (a southerly portion of U.S. 41) act like a dam, further squeezing mangroves out.

There has been evidence that red mangrove seedlings can move past these roads.

“Red mangrove seedlings float on water,” Mitsch said. “They can flow north through culverts. They will get past the road, but it’s still an impediment. Mangroves south of Tamiami could still get squeezed.”

Many people think it’s illegal to cut mangroves in Florida, but the answer is complicated.

Before the state’s 1996 Mangrove Trimming and Preservation Act, Beever said no one could cut mangroves found in preserves unless it was in the public interest. He said Lee County had an ordinance in place, but the state invalidated those regulations with the act.

Trimming back the forests

The Southwest Florida environment and its economies are dependent on mangroves, and while protections exist, there are exceptions that allow for some mangroves to be cut and removed.

Florida’s mangrove trimming act says, “mangroves are amenable to standard horticultural treatments.” In most circumstances, permits are needed to trim or remove mangroves, but “narrow riparian mangrove fringes” do not fall under the same protections.

The Florida Department of Environmental Protection estimates there are 469,000 acres of mangroves in Florida. In most instances, the DEP oversees the permitting process to trim or cut mangroves.

A 50-foot swath has been cut through mangroves on Hutchinson Island in St. Lucie County to connect State Road A1A to a proposed housing development. Treasure Coast Newspapers

“Just as you have seen on national scale, there has been a weakening under the state to protect wetlands, and it’s easier to get permits for wetland loss,” Beever said. “One main solution to this problem is mitigation.”

In Florida, a professional mangrove trimmer is required under certain criteria. There are more than 140 of these trimmers in Lee and Collier counties.

“Florida laws allow property owners to trim or alter mangroves to improve waterfront views or for access to navigable waterways,” Khoury of the DEP wrote in an email.  “Homeowners are exempt to trim mangroves when the mangrove height exceeds 6 feet but is not taller than 10 feet. Mangroves cannot be trimmed below 6 feet and trimming mangroves over 24 feet tall requires DEP authorization.”  

The department penalizes those cutting and trimming mangroves without permits or prior approval. Since 2015, the DEP has issued about $120,000 in penalties, each fine ranging from $420 to $10,000, according to data the DEP provided.

“While it is true that mangroves can be naturally altered, damaged and destroyed, there is no doubt that there is a significant impact by human activity as well,” Khoury wrote. “That is why state and local regulations have been enacted to protect Florida’s mangrove forests.”

There are some exceptions to the state-run mangrove permits.

The City of Sanibel is one of those six local governments that decided to regulate mangrove trimming shortly after the 1996 state trimming act was passed.

“Within that act, it gave local municipalities the authority to regulate mangrove trimming,” said Dana Dettmar, a biologist with Sanibel’s department of natural resources. “So as a result, Sanibel decided to pursue that designation. It allows us to regulate all mangrove trimming.”

The city is not allowed to be more restrictive regarding the trimming of mangroves, but  can be more selective on who receives the professional mangrove trimmer designation.

Dettmar said a permit costs $72 and contractors need to take a class before trimming. If a mangrove is going to be completely removed, a two-to-one mitigation plan must be followed — the same requirement the DEP requires.

Mitigation is required onsite and the new trees, purchased from a nursery, must be of the same species that was destroyed.

Even with these protections in place, Mitsch said it used to be better. The U.S. Army Corps of Engineers used to be in charge of protecting the wetlands where mangroves are found.

“I loved it,” he said. “They would scare the living daylights out of developers.”

Mitsch describes the reduction in federal attention to wetlands some of the most tragic news he’s heard about the ecosystem since he’s been in the business.

“We’re losing the federal government protecting our wetlands,” he said, “and we need the big boys.”

Karl Schneider is an environment reporter. Follow him on Facebook and Twitter: @karlstartswithk, email him at kschneider@gannett.com

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