Mangroves are healthier when allowed to grow naturally, research shows

Letting mangroves grow naturally is key, Suncoast Waterkeeper said after conducting new surveys. 

Suncoast Waterkeeper recently completed mapping surveys of several mangrove sites around Manatee County to better understand the impacts of trimming mangroves versus letting the trees grow naturally. 

Executive Director Abbey Tyrna said these surveys aim to advocate for better protection of mangroves, or for better enforcement of existing policies. 

“Mangroves are the only protected tree in the state of Florida, and enforcement of the laws are seriously lacking,” she said. “And so using this information to bring attention to the lack of enforcement and the need for greater protection is definitely what we’re working to do.”

Six sites — one reference site and five studied sites — were a part of the survey in the North Sarasota Bay and Palma Sola Bay areas. The surveys included aerial imaging of the mangrove canopies and spectral analysis. 

In February, the organization started this effort with the help of volunteers who went out to complete the initial surveys. The team also included drone operators, a data manager and a boat captain to survey the sites.

The team went out again in March and April to survey the sites a second time before data analysis began. 

Suncoast Waterkeeper received support from Manatee Fish & Game and the Charles & Margery Barancik Foundation to help fund a data analysis intern, a multispectral drone and other project costs. 

The new drone helped to capture new data that added more insight into mangrove mapping: Color analysis. 

If the team solely used canopy volume as a measurement — that is, how full the canopy appeared from aerial views — then many of the sites would appear to be in good health, Tyrna said. But she indicated that it’s not enough to only go by volume. 

Suncoast Waterkeeper used the multispectral drone to capture Landsat Normalized Difference Vegetation Index (NDVI) data. According to Tyrna, it’s a way for scientists to get information by looking at colors. 

“It definitely classifies or quantifies the greenness,” Tyrna said. “Greener vegetation is typically healthier vegetation.”

Data analysis took until about July, which is when Suncoast Waterkeeper was able to share the results with the public. 

Mangrove health metrics 

The five surveyed sites were compared to the selected reference site, which Tyrna said is a protected area in which mangroves can’t be trimmed. This reference site was the site in the best health of all six. 

Tyrna said some of the five sites were selected because they are currently threatened by development, though she did not give specifics as to where the sites are located. 

Overall, Tyrna’s team found that trimmed mangroves show significant losses to canopy volume and leaf health. Those areas with trimmed mangroves experienced more significant losses than surveyed areas that were previously impacted by sewer pipe blowouts. 

Each site was given a Mangrove Health Index Score, which is created from the volume and NDVI data. The reference site scored a 90. 

Site 1 in North Sarasota Bay scored a 52. It was affected by a sewer pipe blowout in 2022.

Site 2, also in North Sarasota Bay, had the lowest score of 34. This is one site that was permitted for trimming and received annual trimmings for two years prior. The trimming was most likely the reason behind the low score, a graphic from Suncoast Waterkeeper states. 

Courtesy image

Tyrna said that trimming or reduction of a mangrove habitat due to sewage impacts can cause a reduction in the function of the ecosystem. 

“A lot of scientists look at the structure of an ecosystem and relate that to the function of an ecosystem, and higher structure means higher function,” Tyrna said.

According to the Florida Fish and Wildlife Conservation Commission, mangroves are important habitats and can be nurseries for juvenile fish. Mangroves also act as shoreline stabilizers, which can offer more coastal protection. 

Tyrna said reducing structure by any percentage would reduce the function of the mangrove ecosystem by a similar percentage.

This data shows one snapshot of time, and the organization hopes to continue these surveys next year and beyond. 

This year’s study was a pilot for Suncoast Waterkeeper. Tyrna said they learned a lot through the process and figured out what needed to be tweaked for future surveys. She said the organization hopes to continue next year in February and is looking for more funding opportunities. 

“We have a lot of work coming up in the spring, and we look forward to fine-tuning this program,” Tyrna said.