Seagrass helps Biscayne Bay’s mangroves keep up with rising seas

The mangroves of Biscayne Bay have been getting help from an unlikely neighbor — the seagrass meadows just offshore.

Dead seagrass blades and seaweed drift loose until the tide carries them into the mangroves where they pile up, rot slowly, and then become part of the ground.

In a paper published in Estuaries and Coasts, researchers from FIU and the National Park Service found the natural buildup of soil in Biscayne Bay's mangrove forest isn't building up fast enough to keep pace with rising seas. But seagrass washing in from nearby meadows is helping in two ways — adding material to the forest floor and slowing decomposition, allowing more of that material to accumulate over time.  

The soil beneath the mangroves is rising 2 to 4 millimeters a year while the sea-level is rising by 8.5 millimeters a year. Seagrass alone will not close the gap, but the study shows that it makes up a much larger part of the mangroves defense.

To track this challenge, biology Ph.D. student, Kevin Montenegro loaded a 16-foot canoe onto a truck before sunrise every month for two years. He would paddle an hour into Biscayne National Park, and wade into the mangroves to see what washed in.

“I see mangroves and seagrass ecosystems as part of our city and community, just like streetlights, schools and businesses,” Montenegro said. “They’re puzzle pieces that make our community whole. If we lose mangroves, we lose a huge piece of our community and identity — but also a major part of our infrastructure.”

Wrack is usually studied on beaches, where researchers can walk up to it or measure it from satellite images. But mangrove canopies are difficult to view through satellite imagery, and mangrove forests are difficult to access by foot.

For this reason, he and a field partner worked at two sites in the park, recording where wrack landed and how deep it piled at set points moving inland from the shoreline. Every few months they collected soil samples for chemical analysis and checked mesh bags of mangrove leaves and seagrass left on the swamp floor to see how fast each broke down. They combined this with elevation measurements the National Park Service has taken at the same sites since 2011.

South Florida's mangroves are generally thought to build their soil from their own leaves, wood and roots. Chemical analysis found between 36% and 53% of the carbon near the surface came from seagrass. The highest share was in wetlands around Cutler Bay, where more than three times as much wrack reached the inner forest as the study site.

It was also where the ground was rising fastest, and the mesh bags helped explain why.

Where more wrack piled up, seagrass in the mesh bags broke down more slowly, leaving more material behind to build up the forest floor.

Montenegro had studied both ecosystems before. As an FIU undergraduate, he worked in a seagrass lab studying how poor water quality damages seagrass beds. He later measured mangrove elevation with the National Park Service before returning to FIU for graduate school.

The new findings tie those two worlds together; what harms the seagrass offshore eventually reaches the mangroves onshore.

"We know seagrass ecosystems are very sensitive to water quality," he said. "And so if we lose those seagrasses, now we're losing a huge contributor to mangrove resilience, mangrove blue carbon storage, as my paper shows."

Mangroves also help protect Miami from storm surge and give fish and other marine life a place to live.

"While these mangroves protect us from the water and all the scary things from nature out on the ocean, they also protect nature from us," Montenegro said. "A lot of what washes out from our town to the water has to pass through these ecosystems. And thankfully, these ecosystems can act as important filters for dirty water runoff and fertilizer and sewage and also, unfortunately, plastics."

Montenegro conducted much of his research as part of FIU’s CREST Center for Aquatic Chemistry and Environment in the Institute of Environment. He was also a 2022 FIU Everglades Scholar, a program hosted and supported by the Everglades Foundation. He graduated with his Ph.D. earlier this year. Today, he works in the Department of Environmental Resources Management for Miami-Dade County.

For him, being an access point is about connection. Sometimes that means paddling out so others can see what’s happening under the canopy. But the connection runs both ways. Just as the mangroves get help from the seagrass next door, the bay gets help from neighbors onshore who pay attention to what they send down the drain, one small choice at a time.