India turns to corals and seagrass to shield against rising seas

India is restoring Kariyachalli Island’s natural defenses instead of relying only on hard coastal barriers. The state program uses artificial reefs, transplanted corals, and seagrass meadows to slow erosion, weaken waves, and help the shrinking island withstand storms and rising seas.

Workers are anchoring seagrass shoots to wooden frames on the seabed. Engineers are also placing about 8,500 artificial reef modules around the island. Their locations are chosen using information about seabed conditions, currents, and wave patterns. The structures break incoming waves and give corals new surfaces for growth. Seagrass helps hold sediment in place and supports young marine life.

The $4.79 million restoration program is expected to complete deployment by the end of September. Scientists will monitor the site for at least four years. The effort also provides extra income for local workers, but a severe 2024 bleaching event shows that warming seas could still limit its success.

Coral reefs are built by colonies of tiny marine animals that create hard calcium-based skeletons. Seagrass meadows are underwater habitats formed by flowering plants with roots and leaves. Although they are different ecosystems, both can reduce coastal damage. Reefs provide rough, shallow structures that interrupt waves, while seagrass roots hold seabed sediment together.

Around Kariyachalli, artificial reefs create new surfaces where corals can colonize. Transplanted corals can rebuild reef structure, and seagrass frames help establish meadows on the ocean floor. Together, these habitats soften incoming water, reduce sediment movement, and provide nursery grounds for fish and other species.

Their value extends beyond erosion control. The Gulf of Mannar contains endangered dugongs and hundreds of fish species. Its reefs and seagrass also helped shield Tamil Nadu during the 2004 tsunami. However, pollution, overfishing, and warming seas have weakened these natural defenses.

Kariyachalli Island’s land loss is dramatic. Government estimates show that it covered 20.85 hectares in 1969 but only 3.14 hectares last year. That means it lost more than 80% of its landmass in about half a century. Officials warn that the island could disappear entirely by 2036 if erosion continues unchecked.

The surrounding reef system has also contracted sharply. Around Kariyachalli, reef cover dropped from about 500 hectares in 2005 to 197 hectares in 2021. That is a loss of roughly 303 hectares, or about three-fifths of the earlier coverage. The damaged reefs provide less protection against incoming waves.

These changes are connected to a broader ecological decline. Coral mining in the 1960s and 1970s damaged the reefs. Pollution, overfishing, and climate-driven coral bleaching added further pressure. Restoration now aims to rebuild both the island and the weakened reef barrier.

Artificial reefs, transplanted corals, and seagrass each address a different part of coastal damage. Artificial reefs create bulky, rough structures that interrupt incoming waves. Transplanted corals can grow across those surfaces, gradually rebuilding a living reef. Seagrass roots and shoots hold loose seabed sediment in place, reducing the amount that waves can carry away.

At Kariyachalli, modules weighing several tonnes are being lowered into carefully selected locations. Designers used seabed conditions, currents, and wave patterns to place them where they can best break wave force. Nearby seagrass frames are anchored to the ocean floor, while experts transplant corals. This creates physical barriers and strengthens the surrounding habitat.

The protection is important during cyclones, storm surges, and rising seas. It is not a complete solution, because warming water can kill corals and undermine restoration. Still, results on Vaan Island and Shingle Island suggest that carefully planned restoration can recover land and improve reef survival.

The Gulf of Mannar’s marine ecosystems have been weakened by several pressures acting over many years. Coral mining during the 1960s and 1970s severely damaged reefs around the islands. Removing reef material destroyed important habitat and reduced the natural structures that help soften waves.

Other human activities added more stress. Pollution harmed marine conditions, while overfishing disrupted the ecosystem and reduced its biological richness. Climate change brought warmer seas and coral bleaching. Bleaching can leave corals weakened or dead, making reef recovery more difficult. Around Kariyachalli, reef cover fell from roughly 500 hectares in 2005 to 197 hectares in 2021.

The result is a less effective natural defense for islands and nearby shores. The Gulf remains one of Asia’s richest marine environments, with endangered dugongs and hundreds of fish species. Restoration is therefore meant to protect both coastal communities and biodiversity, even as continuing warming makes recovery harder.

Coral reefs, seagrass beds, and small islands helped reduce the tsunami’s energy before it reached the mainland. Their uneven surfaces and shallow habitats disrupted fast-moving water, spreading and absorbing part of its force. This natural buffering could lower the height and speed of waves arriving at the coast.

The Gulf of Mannar’s ecosystems formed a connected barrier. Reefs provided solid, rough structures offshore. Seagrass meadows helped hold sediments and occupied shallow seabeds. The islands themselves added another layer of protection. K. Diraviya Raj said these tiny islands, seagrass, and coral beds absorbed the impact of the 2004 Indian Ocean tsunami.

The protection did not prevent tragedy. More than 8,000 people were killed in Tamil Nadu. However, marine scientists believe the ecosystems prevented a much larger disaster. Their role explains why restoration matters today: rebuilding natural barriers could reduce risks from future storm surges, cyclones, and rising seas.

Rising ocean temperatures challenge coral restoration because corals are living organisms with narrow environmental limits. When water becomes unusually warm, corals can bleach, meaning they lose essential algae and their color. Severe or prolonged bleaching can kill them. That makes it difficult to establish transplanted colonies or grow new reefs, even when projects are carefully designed.

The Gulf of Mannar experienced a severe bleaching incident in 2024 that killed large numbers of corals. On Shingle Island, transplanted corals survived that event better than older colonies, offering a hopeful sign. Scientists are also searching for naturally heat-resistant coral species that may withstand future marine heat waves.

The larger problem remains climate change. Restoration can preserve biomass and create opportunities for corals to adapt, but it cannot remove the underlying heat stress. Scientists therefore say intervention is more necessary, not less. Kariyachalli’s site will be monitored for at least four years to track whether the restored habitats endure.