Finding Meaning in the (Mangrove) Mush
We were tracking a lemon shark with telemetry gear, and things weren’t going well. The tide was going out; we were losing water fast. And we already lost the shark. Our wily foe won the chase. It was either start the engine and pull out now or curse ourselves while walking the skiff out later.
Before we left, however, we needed to return and get the small net covering a channel entrance in the mangrove trees. The shark used the channel to escape perceived dangers — like nosy researchers with telemetry gear. If left behind, the net would become a deadly trap for the shark if we weren’t there to pull him out. There wasn’t enough water for the boat to get across the marsh. One of us had to take a hundred-yard walk to the mangrove edge. I went because I knew no one else wanted to go into tree-covered mush that reeked like rotten eggs and provided a well-used breeding ground for the loathsome mosquito.
Funny thing about mangroves. The closer I got to the mangrove roots, the more the mushy bottom sank beneath my feet. By the time I neared the trees, I was 24 inches deep in the mud below the water. Forward progress was virtually impossible.
I flopped onto my stomach, figuring that its larger — much larger — surface area wouldn’t sink so much. True, but now forward progress consisted of a crawling motion.
And then came the mosquitos. Dozens swarmed my head, sticking out above the waterline. I could hear the irritating high-pitched buzzing in my ears and choked as they flew in droves up my nose. With my arms buried in the mush, I had no choice but to mash my head down into the thick brown soup of saltwater and sediment. The ooze crept down my suit, boldly going where ooze had never been and should never be.
At this moment, I was sure of two things. One, I wasn’t leaving until I got that net. Two, it would be a long ride home while trying not to scratch some of the places where I had hydroid stings.
I slithered back out, net in hand. I had a brief moment of kinship with those first amphibians emerging from the primordial swamp. Then I reached terre firmer and stood up. Bipedal again.
Sounds awful, right? So, it may not seem surprising that others destroy these “unsightly” mangroves in the name of land reclamation, farming and aquaculture, resort hotels, and condominiums. Since my days of shark research, the global mangrove area decreased by 1.04 million hectares, about 2.3 million acres.
But, mangroves are not areas to be “reclaimed.” They not only belong where they are, but they are actively reclaiming and preserving land. And that function is essential for the mangrove ecosystem, its inhabitants, the juvenile marine species that grow up there and move to other ecosystems, those ecosystems, and humans.
The whole process is cyclical, beginning with the mangroves’ role as recyclers and ending with protecting the more tourist-appealing coral reefs and protecting humans, including tourists.
The reclaiming process is as simple as it is vital. The mangrove tree colonizes an area via floating seedlings. The seedlings take root in the sandy bottom. The roots act as natural filters, trapping sediment and runoff from the land, preventing or slowing erosion.
Mangroves not only reclaim, but they also recycle. Leaves from the trees fall into the water, sink, rot, and become a rich source of nutrients, that deep muck I waded through. One acre of red mangroves sheds an estimated 3 tons of leaves annually. This humble, albeit slimy, substance is the supporting strand of an extensive food web and carbon cycle.
Many organisms feed on the rotting leaf litter, called detritus. In turn, hundreds of species feed on the detritus-eaters and small marine species associated with mangroves. Juvenile fish grow up in the protected environment of dense mangrove roots. Species that rely on mangroves for part of their life cycle grow into the reef inhabitants we love to photograph or eat with a wedge of lemon and some garlic butter. Other species, such as the juvenile lemon sharks we were studying, become the predators in reef ecosystems.
Though mangroves provide the base for an extensive and commercially valuable ecosystem, humans clear the misunderstood marine plants for more “useful” purposes — beachfront development, aquaculture, and agriculture. Losing the mangrove ecosystem places two other marine ecosystems, coral reefs, and seagrass beds, in peril.
Mangroves filter and trap much of the runoff of pollution and sediment from the land. With the mangroves cleared, sediment and pollutants, often sewage and fertilizer, run unchecked into the seagrass beds. The nutrient-rich effluent promotes the growth of phytoplankton and filamentous algae, which, combined with the sediment, clouds the shallow water and blocks out light necessary for seagrass beds to flourish and feed sea turtles and manatees. The environment becomes anoxic, and the seagrass ecosystem dies. Such eutrophication from nutrient runoff in Florida killed the seagrass beds and triggered a mass die-off of manatees.
The combined filtration of soil runoff and pollutants by healthy mangroves and seagrass beds helps keep the water clear of sediment. Sedimentation is a major cause of coral reef destruction. Because mangroves are often removed for shoreline development, that development increases the amount of sediment runoff from shore and increases stress on coral reefs.
One example of this human-caused cycle of destruction is the Florida Keys reef system, which is now in a state of collapse with a loss of up to 90 percent of coral cover. A reef ecosystem requires at least 25 percent coral cover to sustain ecosystem functions. The Florida Keys reef system is the only barrier reef in the continental U.S. and one of the world’s largest reef systems. Historical sedimentation, overuse, nutrient runoff, and devastating disease called Stony Coral Tissue Loss combined have placed the reef system in a state of collapse. Climate change has accelerated the decline of coral reefs, including the Keys, where water temperatures rose to record highs in 2023, triggering mass bleaching and threatening one of the world’s largest reef restoration projects.
In turn, healthy, intact coral reefs protect mangroves. Coral reefs buffer waves and currents, which could uproot vulnerable marine plants. Combined, reef and mangrove systems protect shorelines during storms. The shoreline protection provided by mangroves totals an estimated $17 billion in flood protection in Mexico, Florida, and the Bahamas. The ecosystem protects half a million people from flood risk throughout the Caribbean. The ecosystem services are so valuable that the protection of mangroves may be the basis for new insurance products.
In addition to flood prevention, mangroves sequester an estimated 6.23 billion metric tons of carbon annually. For comparison, rainforests globally sequester 7.6 billion metric tons of carbon annually. Climate threats to mangrove forests are significant, including extreme storms, sea level rise, and changes in temperature and precipitation patterns.
Yet, clearing mangroves in the name of land reclamation is a worldwide activity. Clearing doesn’t always pay off for the humans (and never pays off for the mangroves). High, long-term costs of battling increased erosion can result. The acidic, organic soil is often unfit for development and farming — two primary reasons humans clear mangroves. Thirty-five percent of global mangrove loss is due to aquaculture and palm oil production.
A few places have come to appreciate the true beauty of the mangrove and then go on permitting (literally) their killing. In Florida, it is illegal to clear mangroves without a permit. But, in that sunny land of prime real estate, the property values rise exponentially with a waterfront view. After the land owner applies for a permit — or does not apply for a permit and trims illegally — they trim down “unsightly” mangroves to expose a high-dollar glimpse of a jeopardized ecosystem.
Trimming is better than total removal, but it cuts the productivity of the mangrove ecosystem by reducing the amount of leaf litter, or detritus, produced. Reducing leaf litter means reducing the critical base of the food chain in the mangrove ecosystem.
The impact of mangrove trimming was the subject of a study by Jim Beever of the Florida Department of Natural Resources. Trimming two-thirds of the mangrove height in the study area initially cut productivity by as much as 84 percent. Sightings of the visible animal life decreased by 79 percent after trimming. Worse still, the decrease in productivity for the area studied may eventually become as much as 99.8 percent as the mangrove continues to decline.
Destruction of the mangrove habitat by clearing or trimming impacts several marine species. Simply put, fewer offspring equals fewer adults to carry on the population.
Outside of human-caused destruction, mangroves are specialists at survival. The plant leaves have a waxy texture like those of desert plants. This leaf design allows mangrove trees to retain moisture and excrete salt but at a high “energy cost” to the tree. Tannin in the mangrove bark makes the leaves inedible for most would-be predators. The hostile saline environment, accompanied by rotten smell and hordes of mosquito bodyguards, eliminates the rest — except humans with development plans.
Because of the degree to which mangroves are specialized, they are highly vulnerable to change and cannot adapt well. The introduction of unnatural stressors, including defoliants and pollutants, sewage, sedimentation, and oil spills, destroy the fragile balance on which mangroves depend. Unable to cope, adapt, or recolonize quickly under such new stresses, the delicate mangroves are lost, taking the base of the ecosystem with them.
A mangrove forest may look like a wasteland to some. There are mosquitos. Parts of the forest can smell like an Easter egg left unfound for ten years. There is a definite slime factor. But life at the base of the trees is fascinating. Mangrove habitats possess a unique beauty. To fully appreciate them, you should view mangroves from below.
Don’t think of it as a bunch of roots. Think of it as an upside-down forest wrapped in golden light and filled with animals. It is. On prop roots, space, not food, is the limited resource. Barnacles, clams, snails, and oysters live in a competition for housing that rivals any New York apartment search.
Some of the most prevalent prop root inhabitants are species of tunicates (about 30 species in the Caribbean alone). Some species look like sponges; others look like lumps of two-month-old pudding with openings but no fuzzy mold. Some species are crust-like. All species look like stuff I wouldn’t want to step on if I saw it on the sidewalk.
Yet, underneath this slightly disgusting exterior beats a genuine heart next to intestines and a stomach. This lump is no spineless sac of animal matter. It is a full-fledged Chordate. Its notochord, similar to our spinal cord, is a feature found only in the larvae. The chord and the larvae’s muscular tail disappear as the adult forms. One theory even suggests that vertebrates evolved from the tadpole-like larvae of these attached sea squirts. If so, that would make them very distant cousins. The roots are also home to shellfish like oysters that clean and filter water.
Above the peaceful underwater habitat, it’s anything but quiet. Many species of marine birds, including egrets and the magnificent blue heron, roost in the mangrove trees. Lizards and land crabs thrive. Mangroves are the home of the biggest land crab in North American and Caribbean waters. A large male can wield one 12” pincher. Not an easy catch.
At night, sharks move in to hunt the fish and the smaller juvenile sharks that inhabit the relatively protected mangroves. Other predators like stingrays and barracuda cruise the flats on the edge of the mangroves in search of midnight sushi.
Mangroves are rich in life, a far cry from “wasteland” awaiting bulldozers and clippers, aquaculture ponds, and palm oil trees. The ecosystem is a primary link in the marine environmental chain, a vulnerable link that needs protecting just as much as the “prettier” coral reefs. Globally, mangroves provide critical habitat for 341 threatened and endangered species.
The good news is that mangrove protection and restoration efforts globally are beginning to slow the rate of decline. Blue carbon credits, insurance products, ecotourism projects, or public funding can support mangrove restoration. However, early projects with replanted mangroves have a low survival rate of 20 to 40 percent. The most successful projects lead with removing human-caused barriers, building community support and financial benefit, and letting the forests regenerate. Planting seedlings is a riskier strategy that is necessary if the mangroves cannot regenerate. The most crucial action is to prevent existing mangroves from further destruction.
You can learn more about mangrove protection through organizations like The Mangrove Alliance or view data on mangrove forest status at the Global Mangrove Watch website. Awareness and advocacy for mangrove protection are two critical steps. When you travel to coastal areas, support destinations that care for mangrove habitats or consider a sustainable ecotour or volunteer effort to help care for a mangrove ecosystem.
Beever, L., J. Beever, R. Lewis, L. Flynn, T. Tattar, E. Donley, E. Neafsey. 2016. Identifying and Diagnosing Locations of Ongoing and Future Saltwater Wetland Loss: Mangrove Heart Attack. Charlotte Harbor National Estuary Program. Punta Gorda, Florida.
http://chnep.wateratlas.usf.edu/upload/documents/Mangrove-Heart-Attack-Draft-30Sept2016.pdf
Carlson, R. R., Evans, L. J., Foo, S. A., Grady, B. W., Li, J., Seeley, M., Xu, Y., & Asner, G. P. (2021). Synergistic benefits of conserving land-sea ecosystems. Global Ecology and Conservation, 28, e01684. https://doi.org/10.1016/j.gecco.2021.e01684
FAO. 2023. The world’s mangroves 2000–2020. Rome. https://doi.org/10.4060/cc7044en
NOAA. (2024, February 12). Extreme ocean temperatures are affecting Florida’s coral reef. National Environmental Satellite, Data, and Information Service. https://www.nesdis.noaa.gov/news/extreme-ocean-temperatures-are-affecting-floridas-coral-reef
NOAA. With “Mission: Iconic Reefs”, NOAA aims to restore Florida Keys with climate-resilient corals. (2023, March 6). NOAA Climate.gov. https://www.climate.gov/news-features/features/mission-iconic-reefs-noaa-aims-restore-florida-keys-climate-resilient-corals
The Nature Conservancy. (2020, October 22). Could insuring mangroves save them — and protect coastal communities? The Nature Conservancy. https://www.nature.org/en-us/what-we-do/our-insights/perspectives/three-things-insuring-mangrove-forests/
