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Mangroves: The first line of defense

By DR. SURI SEHGAL - | Sep 29, 2020

Those of us who live on islands realize the threat of sea level rise (SLR) to our homes. We also know that a healthy mangrove ecosystem is the first line of defense as they protect coasts against erosion and protect coastal inhabitants against storm surges, high winds, and high tides.

For visitors to the islands, mangroves are a striking attraction. We admire their beauty as we peddle through estuaries or visit the Everglades and see how they nurture rich biodiversity.

A mangrove tree has a rich underwater component that harbors a large variety of fish, crab, shrimp, and mollusk species; a surface component and a tangled root system that prevents coastal erosion from storm surges and high tides; and an aerial component that moderates wind flow and serves as a nesting and roosting site for birds. Of the three types of mangroves found in Florida, red mangroves thrive closest to the shore (mostly seaward). Black mangroves grow immediately inland of red mangroves in sites that may periodically be inundated. White mangroves grow inland as well and are found above the high tide line. They range in height from 2 to 10 meters depending upon the species.

Defending against storm surges and high tides, mangroves act as a natural barrier, mitigating flooding by reducing wave energy, slowing down storm surges, and providing stabilization. A healthy deep and wide mangrove forest can help significantly in breaking up wave energy and decreasing storm surges, thus protecting property and people. A mangrove forest can considerably reduce wind velocity of hurricane. It is estimated that a healthy forest can reduce the effects of a Category 5 hurricane to a Category 3 hurricane. Further study is required to determine to what extent wind velocity decreases with the health and depth of the mangrove forest.

Florida has 469,000 acres of mangrove forest, making it a stable ecosystem of coastal zones. Mangroves are adapted to shallow brackish water and have other special adaptations that allow them to live in sediment with high salinity and low oxygen. If the water becomes too deep or too high in salt content, they can’t survive. Mangroves have coexisted for centuries with the sea level rise. Dr. Erica Ashe, a researcher at Rutgers University, states that SLR has been modest historically (1.2 mm per year) as compared to now (3.4 mm per year), and mangroves have kept up with the SLR and are still growing upward. The SLR rate that has accelerated in recent years is likely to go much higher, causing a concern as to whether mangroves will be able to cope if it goes above 5 mm per year. The threshold for their growth is 7 mm or less per year, according to Dr. Neil Saintilan, a professor at the Macquaire University in Sydney, Australia.

For mangroves to keep pace with rising sea levels, they rely on buildup of organic sediments, according to Jeremy Conrad of the J.N. “Ding” Darling National Wildlife Refuge. Sediment buildup is largely attributed to root growth, decomposition of leaf litter, and deposition of mineral sediments from tides and waves. To maximize the buildup of sediments, the mangrove forest must be healthy, protected, and restored wherever possible. Root production and leaf decomposition rates are altered in stressed mangrove forests, reducing the ability to build up sediments and keep pace with rising sea levels. Eventually, these stressed forests can begin to die off and result in a loss of sediment and the conversion of forest to open water.

With projected SLR, the question being asked is if mangroves will move inward if there are no barriers in the way? Or will they simply die off if the water becomes too deep or too acidic? Further study of the entire subject is needed.

Mangroves are an excellent carbon sink. Recent studies have found that the sequestration rate of carbon dioxide is several times that of terrestrial plants — as much as four times more than a tropical rainforest. Their ability to absorb carbon dioxide from the atmosphere and store it in their roots and sediment for centuries is sizable and makes mangroves important for mitigating climate change. However, when mangrove trees die, the carbon trapped in their roots and in the sediments is released back into atmosphere creating a new carbon source which has a negative impact on climate.

Mangrove forests can play an important role in carbon removals because they are among the most carbon-dense ecosystems in the world, and if kept undisturbed, mangrove forest soils act as long-term carbon sinks.

More comparative data on carbon sequestration efficiency of mangroves is essential to make the most of their important role in climate mitigation. We would do well to learn as much as possible about this scientific wonder so that we do not lose this vital coastal relationship.

Captiva resident Dr. Suri Sehgal is an India-born American philanthropist with a long career as a crop scientist, seedsman, entrepreneur and leading expert in the global hybrid seed industry. Along with his wife, Edda, he now operates two nonprofit organizations that focus on water security, food security and social justice. He is a member of the Captiva Sea Level Rise Committee and chairs the Captiva Island Yacht Club’s Environmental Awareness Committee.