What Are Mangrove Communities?

In tropical environments all over the world, mangrove communities consist of about 70 different species of trees, palms, shrubs, and ferns that live along the Earth’s coastlines in the intertidal zone, which is where the ocean meets the land [1]. Often, they are found in estuaries—places where freshwater rivers flow into the ocean. Freshwater that arrives in estuaries often carries soil sediments, nutrients, and pesticides. Mangrove communities, also called mangrove forests, slow down the flow of this water and filter it, helping to capture sediments and decrease impacts from harmful substances like pollutants. Mangrove communities also shelter and support many creatures, including humans. In addition, they help prevent erosion by slowing down waves as they crash into the shoreline. At the foundation of mangrove communities are the mighty mangrove trees. Over thousands of years, mangrove trees have evolved unique traits that allow them to survive in a salty world.

How Do Mangrove Trees Live In Saltwater?

It is not normal for trees to grow in water, much less saltwater—but mangrove trees do it. So how do they do this? First, mangrove trees must deal with living in a lot of water, and then they need to figure out what to do with all the salt. Over time, mangroves have developed unique root systems that allow them to live in flooded habitats. Their roots are different from those of ordinary plants and have names like pneumatophores (pronounced new-mat-uh-fours), knees, aerial roots, and prop roots (Figures 1A,B). These special roots stick out of the water, which helps the trees breathe through special pores, called lenticles, that let in oxygen (Figure 1A). Other structures move the oxygen to the parts of the trees that are underwater. This unique root system prevents the trees from drowning.

“Water, water, everywhere, Nor any drop to drink.” Just like the sailor observed in “The Rime of the Ancient Mariner” poem by Coleridge (1798), ocean water is salty and unsafe for both humans and plants. Mangrove trees can handle salt in three ways: by blocking the salt from entering the roots, by letting the salt in and then sending it to older leaves that eventually die and fall off, or by letting the salt in and pushing it out through special salt glands on the surface of leaves, where it is washed away by rain [2]. Salt that the mangrove roots block out slowly builds up in the surrounding soil over time, making it saltier. This makes it harder for the mangroves to keep the salt out. Luckily, rainfall and ocean tides help move the extra salt back into the ocean. If this flushing process did not happen, mangrove communities would get so salty that even the mighty mangrove tree could not survive.

It Is A Give-And-Take Mangrove World

Healthy mangrove forests are not always flooded—water flows in and out (Figure 2). The amount of water and the length of time that water is present are constantly changing. Water levels can change one or two times a day, due to the tide cycle, and sometimes over several weeks to months due, to seasonal rain events (monsoons), water pushed by the wind, or floods. As water comes into the mangrove communities, it brings in beneficial nutrients (like phosphorous) from the ocean. When the water flows out, it gets rid of materials that can be bad for the trees, like the extra salt. Water movement also brings oxygen to the tree roots and soil. In addition, leaves, nutrients, sediments, and other things that can be useful to the nearby ocean are carried away.

Shifting water levels also help mangrove communities to support many ocean creatures (Figure 2C). When the mangrove forest is flooded, the roots provide places for animals to eat and hide. Fish, crabs, shrimps, and other marine species come into the mangrove forests to find food. Even larger animals like the smalltooth sawfish, rays, crocodiles, and manatees use mangrove forests when they are flooded (Figures 1, 3). Once the water moves out, a whole different group of creatures arrives, searching for food on newly uncovered surfaces. It is common to see wading birds, raccoons, snails, lizards, and crabs like the mangrove tree crab and the fiddler crab, feeding in the mangrove forest mud (Figure 3). There is even a fish called the mangrove rivulus that can live in mangrove forests when there is no water at all. The rivulus fish has been found living inside dead mangrove logs for up to 60 days with no water [3]!

Mangrove Forests Are A Fish’s (And Other Species) Best Friend

Without a doubt, mangrove forests support abundant wildlife. Many animals spend part of their lives, or their entire lifetimes, in mangrove communities. This is especially true for fish. In South Florida, U.S.A., the home of Biscayne National Park and other nearby national parks, an estimated 90% of commercially caught fish (fish that are caught and sold for a profit) and 75% of game fish (fish caught for fun) need mangroves for some part of their lifespan. In Malaysia, scientists have found over 119 different fish species in one single mangrove creek!

Clearly, mangrove forests ARE a fish’s best friend, but other life forms benefit from mangrove forests as well. Mangrove roots provide a solid structure that organisms like sponges, sea fans, anemones, clams, oysters, and corals can attach to and live on (Figures 1, 2). At US Virgin Islands National Park, scientists recorded the same number of coral species in mangrove forests as they did on nearby coral reefs. That is amazing! In addition, the scientists found over 60 species of sponges attached to mangrove tree roots.

Mangrove Superheroes: Rising Sea Levels And Capturing Carbon Dioxide

Even the mighty mangrove will eventually drown if water levels do not vary enough. If the water is too deep for too long, mangrove seedlings drown. Without mangrove seedlings, there are no young trees to replace those that die. To adapt to rising sea levels within mangrove forests, mangrove trees can bio-generate materials to make peat (a soil made of partially decomposed leaves and roots) and/or capture natural materials to build up soil levels. During bio-generation, mangroves help capture and trap carbon dioxide (CO₂), making these trees important players in the fight against climate change.

Through photosynthesis, mangroves collect CO₂ from the atmosphere and lock it up for long-term storage in their peat soil. Mangrove forests are one of the most carbon-dense forests in the world, containing on average 1,023 metric tons of carbon per hectare [4]. What does this mean? Well, if an average passenger car uses a 75-L tank of gas, then it produces about 178 kg of carbon. So, each hectare of a mangrove forest stores about 5,750 tanks of gas in carbon. Now that is a lot to honk about!

What Else Can Peat Tell Us?

Not only can peat sequester CO₂, but it can also help scientists understand how mangrove communities have survived over time. Mangrove communities have existed at the boundary between the ocean and the land, in places like Belize, for thousands of years! How do we know this? Well, scientists can gather a lot of useful information from studying the peat beneath the mangrove roots. Scientists took 12-meter-long soil cores out of the ground from a Belize mangrove forest. Back in the laboratory, they used microscopes and other machines and techniques to identify the types of material in the soil and how long they had been there. They found that the mangroves in Belize today are in the same spot as mangroves 8,000 years ago [5]! Who would have thought that dirt—oh sorry, peat—could tell us so much?

To understand how mangrove forests are managing sea-level changes today, National Park Service scientists use information collected from soil-monitoring stations in mangrove forests in numerous national parks. These stations are sampled twice a year to see if the mangroves are generating peat or capturing enough soil to keep pace with the rising sea levels. And so far, mangrove forests seem to be doing their part!

Protecting The Earth’s Coastlines

By now, you have learned about the numerous ecological services mangrove forests provide. But did you know that mangrove communities are also extremely important in protecting Earth’s coastlines? Their presence in the intertidal zone helps buffer shores from all sizes of waves—from small, rippling waves that can result in minor erosion, to massive waves from storm surges that can wipe out entire coastal ecosystems and human developments. As our Earth experiences more frequent and extreme storm events with climate change, mangrove communities are increasingly vital to preserving these fragile areas where the land meets the sea.

Conclusion

Next time you visit a beach or go fishing, remember the valuable role of mangrove forests in keeping our coasts healthy. From sheltering and supporting ocean and land animals to keeping the Earth’s shorelines intact, this important job is accomplished by the mighty mangrove!

Glossary

Intertidal Zone: ↑ The area of the marine coastline that is either flooded by water at high tide or exposed to air at low tide.

Estuary: ↑ The place where freshwater flows into the ocean.

Erosion: ↑ The process of wearing away of the soil or rock by water, wind, or other natural agents over time.

Lenticle: ↑ Special pores in woody plant stems or roots that allow gas exchange.

Bio-Generate: ↑ The building blocks of a substance or process come from biologically made components.

Peat: ↑ Soil formed from partially decomposed plant materials (leaves, roots, etc.) in wet, low-oxygen conditions.

Soil Core: ↑ A vertical soil collection that samples down the soil column (profile). It is typically collected in a tube or cylinder.

Ecological Services: ↑ Benefits that an ecosystem provides for humankind, such as oxygen, habitat for animals, place for recreation.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

The authors acknowledge the South Florida/Caribbean Inventory and Monitoring Network of the National Park Service for making the time available to work on the product. The authors also acknowledge the 2040 Science Based Solutions goals program of the Inventory and Monitoring Division of the National Park Service, for supporting a science publication for youth, concentrating on Research Topics that are currently monitored under this program.