The Guardians of the Coast: Mangroves

Today, the 26th of July, is International Mangrove Day. Mangroves, often called the Guardians of the Coast, are vital ecosystems with diverse trees and shrubs that possess intricate root systems and are salt-tolerant. The devastating effects of the tsunami in December 2004 highlighted the importance of these coastal systems. In Sri Lanka, areas with mangrove ecosystems suffered less damage compared to open coastlines, as mangroves helped reduce the destructive energy of the tsunami tides by dissipating their force.

Mangrove belts, several hundred meters wide, have been shown to reduce tsunami height by 5% to 30%.

Mangrove forests are part of unique wetland ecosystems, comprising woody, seed-bearing plants ranging from shrubs to tall trees. These ecosystems inhabit the edge of land and sea, thriving in saltwater along coastlines and tidal rivers. Mangroves have special adaptations to take in extra oxygen and remove salt, allowing them to tolerate conditions that would kill most other trees. They first appeared during the Late Cretaceous period, about 70 million years ago, with the palm genus Nypa being one of the oldest extant genera.

Mangroves are halophytes, plants capable of sustaining in saline conditions. They possess a complex salt filtration system and a root system designed to cope with saltwater immersion and wave action. These plants thrive in low-oxygen, waterlogged mud, and the upper half of the foreshore. Their fine sediments, high in organic content, help protect the coast from high-energy wave action.

In Sri Lanka, mangroves consist of 14 true mangrove species and 12 mangrove associate species. The most extensive mangrove areas are found in the Puttalam-Kalapitiya region, which is associated with estuaries. Other dense, localized mangrove stands are along estuaries on the southern, southwestern, and northeastern coasts, such as in Koggala Lagoon, Kalamatiya Lagoon, and Kokilai Lagoon.

Mangroves have unique adaptations to thrive in harsh saline conditions. Prop roots arising from the branches and stilt roots from the main stem help anchor the plants to the loose substrate. These roots also filter out salt entering the plants. Some species, like Avicennia, have salt-secreting glands to remove excess salt. Their xeromorphic leaves, with a thick cuticle and extensive water storage tissue, reduce water loss due to transpiration.

Given that mangrove soil is poor in oxygen, air-breathing roots, or pneumatophores, grow upwards from the soil surface to facilitate gas exchange for submerged roots. Knee roots, which grow upward in the form of bent knees, also enable air breathing. Mangroves exhibit vivipary, a rare reproductive adaptation where embryos germinate directly on the maternal plants.

Mangroves support diverse animal species. Birds use the trees for nesting and foraging in the rich surrounding waters. Wading birds tiptoe through the shallows, extending their long necks to catch small fish hiding among the roots. Many mangrove crabs are semi-terrestrial, living in burrows extending below the water table and staying on land for extended periods due to their branchial chambers retaining moisture. The mudskipper, a unique mangrove fish, has adaptations for terrestrial life, using its fins to skip and climb. Mangrove oysters attach to roots and close their shells as the tide recedes.

Mangroves trap sediments, contributing to land building, preventing erosion, and stabilizing coastlines. They protect nearby areas from tsunamis and extreme weather events. For generations, coastal communities have relied on mangroves for various needs. These ecosystems provide fish, prawns, crabs, and molluscs. Mangrove timber is valued for its strength and insect resistance and is used for fuel wood, construction, fish traps, boat building materials, tool handles, and furniture. The bark of Rhizophora and Ceriops tagals is used for extracting tannins for dyeing fishing nets, while the bark of Bruguiera is crushed to create poultices for minor cuts. The leaves of Avicennia serve as green fertilizer, enriching the soil and supporting sustainable agriculture.

Mangroves play a crucial role in coastal ecosystems, serving as nursery grounds for fish, prawns, and crabs, supporting fisheries production. Juvenile marine species feed in the mangroves before returning to the sea. Mangroves produce leaf litter and detrital matter, which bacteria, fungi, and other microorganisms break down, providing a valuable food source for marine animals. These ecosystems also protect shorelines from erosion and reduce coastal water pollution by trapping and absorbing pollutants. Additionally, mangroves offer recreational opportunities such as bird watching and serve as excellent field laboratories for biology students due to their high diversity and unique adaptations.

Extensive destruction of these ecosystems for development purposes has occurred. However, it is essential to understand that mangroves are not “wastelands” or “marginal lands” awaiting “development”; they are among Sri Lanka’s most valuable natural resources. Everyone should be aware of their importance to the environment, animals, and humans. Mangrove areas should not be cleared for housing, aquaculture, or other developments, and mangrove trees should not be indiscriminately cut for fuel wood or construction materials. Sri Lanka’s limited mangrove resources are under threat, and it is crucial to prevent further destruction. Let us take the initiative to safeguard these national treasures for future generations.

Written by:

Yoshini Kumarasinghe

2^nd Year

Biological Science

Faculty of Science,

University of Colombo.

References:

• Reid, D., Dyal, P., Lozouet, P., Glaubrecht, M., & Williams, S. (2008). Mudwhelks and mangroves: The evolutionary history of an ecological association (Gastropoda: Potamididae). Molecular Phylogenetics and Evolution, 47(2), 680–699. https://doi.org/10.1016/j.ympev.2008.01.003

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