How Did February's Freeze Impact the Mangroves in Canaveral National Seashore?

By Giavanna Harrington and Dr. Linda Walters

University of Central Florida

 Adult red mangroves on an open oyster reef showing freeze damage.

Over the past few decades, mangroves have steadily expanded northward throughout the Indian River Lagoon and up into Georgia and South Carolina. Why? Mangroves are tropical trees, historically limited to central and south Florida. With warming winters and no subfreezing temperatures, however, mangroves are successfully establishing themselves farther up the east coast, transforming habitats previously dominated by oyster reefs and salt marshes into mangrove stands. This process is called tropicalization.

Since 2023, students in Dr. Walters’ Advanced Marine Biology undergraduate course at the University of Central Florida have been monitoring red mangrove expansion on intertidal oyster reefs in Mosquito Lagoon each year in March/April. No freezing temperatures were experienced during the first three years of monitoring. Then in February 2026, the lagoon experienced a historic freeze which made this year’s study

particularly interesting. Red mangroves (Rhizophora mangle) have a minimum survival temperature threshold of -4° C (24.8° F). So how cold did it get during this freeze? And, how did the mangroves fare?

What happened to the mangroves?

Throughout the course of the study, UCF students recorded mangrove numbers and sizes across reefs with different levels of mangrove coverage (reefs with mangrove stands vs. open oyster reefs without mangrove stands). On 1 February 2026, we recorded temperatures of -5.5° C (22.1° F) on open reefs. On reefs with adult mangrove stands, however, the temperatures within the stand were never below freezing. Open

reef temperatures dropped below the red mangrove survival threshold temperature for three hours after sunset. Overall, 23 percent of adult red mangroves died across our 10 study reefs, and all were on open reefs. No adult trees perished within large mangrove stands. And most surprising of all - no seedlings and propagules perished on either reef type.

Why did the mangroves survive?

Previous laboratory studies suggest that mangroves died due to prolonged exposure to freezing temperatures (12 to 24-hour exposures). The February 2026 freeze only dipped below the -4°C threshold for three hours, suggesting that short-term freezes are less harmful than we anticipated. We also determined that temperatures within mangrove stands were 3.5°-4.5° C warmer than open oyster reefs. These warmer

conditions appear to create a “nursery effect,” helping protect younger mangroves from the cold. On open reefs, larger mangroves died, but the small mangrove propagules and seedlings were somehow protected by the lagoon – possibly the water was warmer than the air, especially during high tide.

The future of mangroves

Despite the historic freeze, mangroves have replaced most saltmarsh vegetation along shorelines and are on track to expand farther onto oyster reefs in Canaveral National Seashore. While a previous Mosquito Lagoon study published in 2019 reported a 6 percent increase in mangrove coverage per year on intertidal oyster reefs, our study projects a higher rate with an 18.4 percent annual increase in mangrove coverage. We are currently experiencing an ongoing regime change from oyster reefs to mangrove habitat, and the freeze of 2026 does not appear to have slowed down that transition.

As climate conditions continue to change, long-term monitoring of oyster reefs and mangrove expansion will be critical for understanding ecosystem impacts and informing conservation efforts throughout the Indian River Lagoon. Moving forward, UCF hopes to continue to investigate this form of tropicalization.