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

For years, the mangroves in Biscayne Bay have relied on an unexpected ally—seagrass meadows located just offshore. Dead seagrass blades and seaweed gradually drift into the mangrove forests through the tide, piling up and slowly decomposing to become part of the ground. However, a recent study published in Estuaries and Coasts has revealed that the natural soil accumulation within Biscayne Bay's mangrove forest is not sufficient to keep up with the rising sea levels.

Researchers from Florida International University (FIU) and the National Park Service found that seagrass, which drifts into the mangroves, has been aiding the forests by adding material to the forest floor and slowing down the decomposition process, allowing more of that material to accumulate over time.

The soil beneath the mangroves is rising at a rate of 2 to 4 millimeters (0.08 to 0.16 inches) per year, while the sea level is rising by an alarming 8.5 millimeters (0.33 inches) annually. While seagrass alone cannot close the gap, the study demonstrates that it plays a more significant role in the mangroves' resilience and defense.

The study's lead author, Kevin Montenegro, a biology Ph.D. student, spent two years paddling a canoe to and from the mangroves every month, meticulously documenting the material washing into the mangrove forest. Montenegro emphasized the importance of these ecosystems, stating, "Mangroves and seagrass ecosystems are part of our city and community, just like streetlights, schools, and businesses.

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."

Tracking this challenge, Montenegro and his field partner studied two sites within Biscayne National Park. They recorded where wrack (seagrass and seaweed) landed and how deep it piled up along the shoreline. Over time, they collected soil samples for chemical analysis and monitored how fast different materials broke down. By comparing this data with elevation measurements taken by the National Park Service since 2011, the researchers discovered that seagrass contributed between 36% and 53% of the near-surface soil carbon in Biscayne Bay's mangroves.

The highest percentage of carbon from seagrass was found in wetlands around Cutler Bay, where more than three times the amount of wrack reached the inner forest compared to the other study site. The mesh bags containing mangrove leaves and seagrass also revealed that where more wrack accumulated, seagrass decomposed more slowly, leaving more material behind to build up the forest floor.

Montenegro's research highlights the interconnectedness of these ecosystems and the crucial role they play in protecting Miami from storm surge and providing a habitat for marine life. He emphasized that seagrass ecosystems are highly sensitive to water quality, and their loss would undermine mangrove resilience and blue carbon storage—a natural mechanism for carbon sequestration.

Mangroves also act as a filter, trapping pollutants like dirty water runoff, fertilizer, sewage, and plastics, which ultimately benefit both the bay and the surrounding communities.

Montenegro's work was conducted as part of FIU's CREST Center for Aquatic Chemistry and Environment, supported by the Everglades Foundation, and he graduated with his Ph.D. earlier this year. He now works in the Department of Environmental Resources Management for Miami-Dade County, advocating for the connection between the mangroves, seagrass, and the community, emphasizing that small individual choices can collectively make a significant difference.