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Climate Surprise: Warming Planet Helps Florida Mangroves Conquer Oysters

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Kevin Spear
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Mangroves are overtaking oysters in Mosquito Lagoon with the help of the planet's warming temperatures.

Within a coastal refuge of tiny islands and sea water saltier than the ocean is a climate battle among natural Florida’s titans.

It’s happening in Central Florida’s remote and alluring Mosquito Lagoon, which nearly abuts Kennedy Space Center launch pads at the Atlantic Ocean.

Reigning guardians of the lagoon are oysters. They assemble in fortresses of low mounds, or reefs that appear above water at low tide. Their closely clustered shells suggest an invincible bristle of daggers and shields.

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Kevin Spear
Mangroves are overtaking oysters in Mosquito Lagoon with the help of the planet's warming temperatures. Held by UCF professor Linda Walters, this is a mangrove propagule, which floats and is sort of like a seed.

Invaders of Mosquito Lagoon are mangroves. Leafy and leggy, they are the only tree species fond of saltwater. Mangroves expand their turf by launching hordes of floating invaders that are sort of like elongated seeds and are called propagules.

While far different in lifestyle -- one as a critter and the other a bush – oysters and mangroves are both good guys that are critical for the wellness of coastal ecosystems. As with salt marsh and seagrass, they serve as nursery, sanctuary and water-conditioner for a wide range of marine life.

But within the slender, 30-mile long Mosquito Lagoon in Brevard and Volusia counties, mangroves are asserting dominance. Scientists think that mangroves have an ally in the rising level of carbon dioxide in the atmosphere that is trapping more of the sun’s heat and rapidly warming the planet.

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Kevin Spear
Mangroves are overtaking oysters in Mosquito Lagoon with the help of the planet's warming temperatures. Seen here is Paul Walters, who helps with research by his wife, Linda Walters, a UCF professor.

Mangroves are cold-weather wimps. But freezes – those well into the 20s – have vanished in the past couple of decades in the Mosquito Lagoon. The shift mirrors Florida’s climbing temperatures, with winters and nights measurably less frigid.

While oyster reefs appear to be impregnable, they can’t thwart all of the mangrove propagules that resemble thick green beans. Of thousands of propagules that may settle on an oyster reef not much bigger than a living room, it takes only one to squiggle through the armor of shells to root in sediment – and become a living stake in new territory.

Evidence of the rise of mangroves over oysters in the Mosquito Lagoon is presented by researchers from the University of Central Florida in the March edition of the scientific journal Global Change Biology.

The title – “Replacement of oyster reefs by mangroves: Unexpected climate-driven ecosystem shifts” – tells of the authors’ surprise at their discovery.

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Kevin Spear
Mangroves are overtaking oysters reefs in Central Florida's Mosquito Lagoon with the help of the planet's warming temperatures. Oysters reefs had seem invulnerable to mangroves taking root.

“All of sudden there were a lot more mangroves here,” said Linda Walters, a UCF biology professor who focuses on marine conservation and has intensively studied the Mosquito Lagoon for more than two decades. “It did sneak up on us.”

“One of the takeaways,” said Giovanna McClenachan, another author and now an assistant professor of biology at Nicholls State University in Thibodaux, Louisiana, “is that this shift is probably happening in a lot of other areas.”

“It’s something that was right in front of peoples’ eyes and nobody saw it happening,” McClenachan said. “It’s kind of interesting to think about how many more of those there are with climate change. How many more shifts are we not noticing?”

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Kevin Spear
In Central Florida's Mosquito Lagoon, mangroves are overtaking oysters with the help of the planet's warming temperatures. A mangrove seen here rises from an oyster reef.

Is the rising conquest of mangroves over oysters a good or bad thing? Walters and McClenachan hear that question often. Their response: it depends.

Oysters provide food, while mangroves slap down storm waves, McClenachan said.

Mangroves do well in locking up carbon, keeping it from warming the atmosphere and entombing it in soggy, underground layers. They can even hold the line on rising sea levels by building up coastal soils.

“But where the oysters win, and some people may disagree with me, they are the ones that clean the water,” Walters said. “An oyster can filter 10 gallons a day and that’s probably a conservative amount. If there are a thousand live oysters per square meter, that’s a lot of water being filtered.”

The UCF researchers suspect mangroves are encroaching on oysters worldwide but it so happens that Mosquito Lagoon is an ideal environment for measuring the transformation.

The range of oysters in Florida spans from the Stuart area to the northern Atlantic coast of North America. The range of mangroves extends from South Florida to the St. Augustine and Jacksonville area.

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Kevin Spear
A mangrove rises from an oyster reef in the Mosquito Lagoon.

That makes Mosquito Lagoon – at the south end of oyster habitat and at the north end of mangrove habitat – the center arena for a contest between the two.

The lagoon is full of seawater but largely landlocked. With evaporation, its saltiness regularly exceeds Atlantic Ocean salinity. Wrapped by protective boundaries of the Canaveral National Seashore, Mosquito Lagoon is as much of a marine wilderness as there is along coastal Florida.

During an outing with Walters, and her longtime research helper and husband, Paul, it was apparent they know in fine detail every island and surrounding waters. She has guided hundreds of students investigating seabirds, shorelines, microplastic pollution and restoration of oysters reefs assaulted by boat wakes, harvesting, algae, infections and predators.

A fierce cold snap in 1989 inflicted a mass die off of mangroves in the lagoon and today there are a few sun-bleached skeletons still standing from that freeze.

Since then, mangroves have been staging a comeback – not, it seemed, on the backs of oyster reefs, but at the expense of another victim. “There used to be a lot more marsh grass,” Walters said.

Already well understood is that mangroves readily overpower Florida’s coastal salt marshes, which has occurred noticeably in the lagoon since that 1989 freeze of mangroves.

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Kevin Spear
Mangroves are overtaking oysters in Mosquito Lagoon with the help of the planet's warming temperatures.

From the 1700s, mangroves have expanded extensively along the Atlantic coasts of Central and North Florida at least five times. Spreading mangroves obliterated salt marsh before being beaten back by freezing weather, which allowed the grasses and sedges of salt marsh to return.

That’s the finding of the 2019 study “Climate-driven regime shifts in a mangrove–salt marsh ecotone over the past 250 years” in the journal Proceedings of the National Academy of Sciences.

Kyle Cavanaugh, an ecologist and geographer at the University of California, Los Angeles, worked with seven other researchers, assembling historical maps, photographs and satellite images, explorers’ journals, scientific articles and climate models.

“It’s very dynamic, with shifts back and forth between mangroves and salt marsh,” Cavanaugh said. Mangroves are now in their sixth period of dominance, with no end in sight as the planet warms. “The probability of really cold, extreme freeze events is going down,” he said.

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Kevin Spear
Mangroves are overtaking oysters in Mosquito Lagoon with the help of the planet's warming temperatures. This small mangrove rises from an oyster bar.

Much harder, however, is for mangroves to advance on fortified oyster reefs – a feat so seeming to defy the odds that it took a heads-up from Tampa Bay to begin to understand that mangroves were able to do just that in Mosquito Lagoon.

In 2018, a University of South Florida professor of coastal ecology, Susan Bell, spoke to Walters’ UCF students. Bell mentioned that a graduate student, Stephen Hesterberg, had observed mangroves expanding across oyster reefs in Tampa Bay.

Now pursuing a doctoral degree, Hesterberg said during his master’s research on the bay’s oysters “I found it really difficult to find just oysters.”

“Over time, it became clear that these mangroves were colonizing reefs and taking over,” Hesterberg said.

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Kevin Spear
UCF professor Linda Walters inspects mangroves rising from an oyster reef in Mosquito Lagoon.

Walters and McClenachan knew that Mosquito Lagoon is farther north and not as warm as Tampa Bay, and probably not as favorable for mangrove expansion.

Still, thought McClenachan, “let’s go out and check it out.”

Launching a study of the lagoon, McClenachan and Walters assembled previous documentation of oyster reefs and matched that with aerial images held by the St. Johns River Water Management District from as early as the 1940s.

“Once we ran through all of the data, it was strikingly obvious what had happened,” McClenachan said.

The mangrove expansion from the 1940 to the 1990s wasn’t remarkable, but since then there has been a “very distinct change in growth,” McClenachan said.

While some reefs became dominated by mangroves, the most notable expansion was on reefs where there had been no mangroves previously.

Next up for research by Walters is to understand whether mangroves after they firmly latch onto an oyster reef will be able to withstand erratic cold snaps brought by the climate convulsions of a warming planet.

Mangroves may be affecting their own survival the longer they stay in place, potentially dissolving oysters and creating a more suitable soil.

“It could be a positive, feedback loop,” McClenachan said. “The longer they are on there, the more sediment they can trap, the better the environment they are making for themselves to survive.”

This story was produced in partnership with the Florida Climate Reporting Network, a multi-newsroom initiative founded by the Miami Herald, the South Florida Sun Sentinel, The Palm Beach Post, the Orlando Sentinel, WLRN Public Media and the Tampa Bay Times.