Mangrove Forests Calm Coastal Waters

aerial view of a green coastal mangrove forest next to clear water with sand beneath

Biological Strategy

Mangrove forest

AskNature Team

Image: Waranont (Joe) / Unsplash / Free non-commercial use

Capture, Absorb, or Filter Solids

Some living systems must secure solid particles such as sediment, usually to keep the particles from hindering their health or activity. The most common way in which they do this is through filtering. To be effective, a filtering system must be appropriate to the sizes of solid particles to be captured and must capture only what is needed. It must also be effective in the appropriate media–air, water, or sometimes solids like soil. An example is mangroves, which are trees that grow along ocean coasts. Their root system slows down and settles sediment out of the water, building up soil to support the mangrove ecosystem.

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Regulate Hydrological Flows

Water flows into, through, and between ecosystems. Because ecosystems need this water for growth and survival, they play an important role in maintaining and regulating hydrological flows. The Earth’s hydrological cycle depends on ecosystems to serve this role, so that water flows to the sea and is evaporated into the air to provide rain. Water is subject to gravity, which carries it down and away from ecosystems. Therefore, ecosystems must slow water to keep it on-site long enough for plants and other organisms to use it. For example, beavers build dams to make ponds where they can store food and travel underwater to avoid predators. These dams slow water, enabling it to seep into the earth surrounding the ponds. A community of plants grows in this moistened soil, supporting other life and transpiring water into the atmosphere.

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Control Erosion and Sediment

Ecosystems must maintain their soils on site and prevent it from being washed away. Organic matter, minerals, and other nutrients in soil support organisms from microorganisms to the largest mammals. But because of the pull of gravity, water flows unless impeded. Therefore, the presence and actions of many, diverse organisms hold soils in place and slow water to allow sediments to settle out rather than be washed downhill. For example, in forests, downed logs that fall crossways to the slope efficiently slow water flow, trapping soils carried by the water and causing water to soak into the soil. In areas lacking logs and other structures, water can flow so fast that it cuts into the soil, causing erosion that washes the soil away.

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a mountain with snow is in the background of a landscape with trees and shrubbery

Ecosystems

Eco- ("household"), system ("organized whole")

An ecosystem consists of all the biological and nonbiological elements that make up a defined area. That definition might seem vague, but that means it’s flexible: We can apply the word across different scales and times. Ecosystems can be small, like a single tide pool, or large, like coastal wetlands. Ecosystems can include plants, animals, bacteria, fungi, rocks, temperature, light, soil, and climate—all linked through the flow of energy and cycle of nutrients. Each part of an ecosystem interacts with, and is influenced by, other members of the system. These interactions are dynamic, leading to changes to any ecosystem over time.

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Roots of red mangrove forests protect coastal shorelines by absorbing energy from waves.

Climate change is bringing stronger and more intense storms to coastal areas across the world. Powerful storm surges and other types of large waves carry enormous amounts of energy. When they wash up on a shoreline, they can cause rapid erosion, major damage to buildings, and loss of human life. Mangrove forests can protect shorelines from damage by powerful waves. The forests’ layered architecture helps to slow down and scatter wave energy so that it loses power before it reaches the shore.

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Also called mangrove swamps, these dense ecosystems occur in tropical and sub-tropical coastal waters. They are dominated by tree and shrub species that have unusual root shapes. For example, “prop roots,” or “stilt roots,” look like multiple legs branching out from the base of the tree trunk, anchoring the tree in the soil. Other species have “knee roots” emerging like bent knees from the soil. They anchor the tree and also help the main roots, nestled in oxygen poor soils, to get oxygen. A third type of roots are tall finger-shaped growths poking out of the soil near the main plant. These are called pneumatophores, and also aid in gas exchange. A wave rolling into a mangrove forest at high speed is met by a dense tangle of obstacles and it loses more and more energy as it gets sliced up by all the hurdles in its path. Scientists found that a wave can lose up to two-thirds of its energy after moving just 100 meters into a mangrove forest. The further it travels through the mangrove forest, the more energy it loses. The special root structures create so much friction and drag on the water that when the wave finally reaches the shoreline it has often been reduced to a gentle ripple.

Without mangrove forests, waves from coastal storms reach the shore with more intensity.

How much the wave slows down depends on how high it is, of course. Because the obstacles of the mangrove forest are close to the ground, they might not have as much of an effect on higher, more powerful waves. The forest canopy, however, can have the same type of effect as the mangrove roots on an incoming wave. Waves that are high enough to reach the height of the canopy – with its thick trunks, sprawling branches, and numerous leaves – get broken up and dissipated much the same way that shallow waves do. Overall, the dense mangrove ecosystem and forest canopy can be very beneficial to shoreline communities by helping to protect them during storms.

Image: Ian Seckington / CC BY NC - Creative Commons Attribution + Noncommercial

Image: Waranont (Joe) / Unsplash / Free non-commercial use

Aerial view of a coastal mangrove forest in Thailand.

Image: Gary Stolz / BioLib.cz / Public Domain - No restrictions

Image: Hayden Dunsel / CC BY - Creative Commons Attribution alone

Last Updated July 2, 2020

References

“All evidence suggests that mangroves can reduce the height of wind and swell waves over relatively short distances: wave height can be reduced by between 13 and 66% over 100 m of mangroves. The highest rate of wave height reduction per unit distance occurs near the mangrove edge, as waves begin their passage through the mangroves. A number of characteristics of mangroves affect the rate of reduction of wave height with distance, most notably the physical structure of the trees. Waves are most rapidly reduced when they pass through a greater density of obstacles. Mangroves with aerial roots will attenuate [weaken] waves in shallow water more rapidly than those without. At greater water depths, waves may pass above aerial roots, but the lower branches can perform a similar function.” (McIver et al. 2012:3)

Report

Reduction of wind and swell waves by mangroves.

Natural Coastal Protection Series: Report 1. Cambridge Coastal Research Unit Working Paper 40. The Nature Conservancy and Wetlands International. 27 pages. ISSN 2050- 7941. | McIvor AL, Möller I, Spencer T, and Spalding, M.

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