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Restoring coastal forests can protect coral reefs against sediment runoff: Study

Great barrier reef. Image by Ayanadak123 via Wikimedia Commons (CC BY-SA 4.0).

Great barrier reef. Image by Ayanadak123 via Wikimedia Commons (CC BY-SA 4.0).

  • Corals have declined by 50% over the last 30 years, with losses of 70-90% expected by mid-century.
  • This mass decline is largely attributed to human activity.
  • One of the major threats to coral is sediment runoff from deforested areas, with research estimating 41% of the world’s coral reefs are affected by sediment export.
  • A recent study published in Global Change Biology finds that restoring forests could help reduce sediment runoff to 630,000 square kilometers (243,244 square miles) of coral reefs.

A new study shows that two-fifths of coral reefs globally are seriously threatened by sediment runoff — the transportation of accumulated material from land to sea, often with harmful pollutants tagging along. But it also offers a potential solution: reforestation.

“Sediment excess can attenuate light and reduce the rates of algal photosynthesis, which means algae cannot provide essential nutrients to the corals and this translates in negative impacts on coral growth,” said Andrés Felipe Suárez-Castro, co-author of the study published in Global Change Biology and ecologist at the University of Queensland in Australia.

Suárez-Castro and his colleagues used remote sensing, geographic information system (GIS) modeling and agricultural census data to investigate if forest restoration could reduce the amount of sediment reaching coral reefs, as well as ocean current data to model how and where sediment eroded from land is transported to reef ecosystems. When the team used these methods to assess 5,500 coastal areas around the world, they discovered that nearly 85% of the sediment load leaching from the land eventually settled on coral reefs.

Coral reefs are some of the most biodiverse ecosystems on Earth. Image courtesy of Andrés Felipe Suárez-Castro.

In the last 30 years, corals have declined by 50%, with expected losses of 70-90% by mid-century. Sediment runoff interferes with the ability of corals to survive, feed, grow and reproduce, and also contributes to the exacerbation of other issues, such as ocean warming and acidification as cloudy, darker water captures more heat, and minerals in the sediment change the acidity of the water.

Coral rely on microscopic algae, zooxanthellae, for nutrients, oxygen and waste removal, and the algae rely on sunlight for photosynthesis. But sediment can cloud the water, reducing the amount of sunlight that reaches the reef, which indirectly limits the ability of corals to survive and grow.

If stressed too much, corals will eject their algal partners, causing the corals to turn completely white; if conditions don’t improve, this “bleached” coral will eventually die. Because of this, reducing sediment runoff from degraded land is vital for the protection of coral reefs, the authors write in their study.

“When sediment loads are high, impacts of global warming, light attenuation and smothering become synergistic,” Suárez-Castro said. “This means that negative impacts are greater than what would occur independently.”

Researchers and conservationists consider sediment runoff from disturbed coastal catchments to be a major threat to marine ecosystems. Image courtesy of Andrés Felipe Suárez-Castro.

Suárez-Castro and his colleagues found that Southeast Asia’s coral reefs are faring the worst when it comes to sedimentation, particularly in the Philippines, Indonesia and Timor-Leste.

Of the 82 countries assessed in the study, Indonesia and the Philippines account for a combined 52% of the global sediment export in coastal areas near coral reefs. Tries Razak, a scientist researching coral reef restoration at the Bogor Institute of Agriculture (IPB) near Jakarta, told Mongabay, “everything just goes immediately out into the sea.”

“Our main issue in Indonesia now are these crazy coastal developments. It’s all about projects,” Razak said.

Suárez-Castro and his colleagues say controlling runoff is a promising strategy for improving the health of coral reef ecosystems around the world. They write that by protecting and restoring native, soil-binding vegetation, countries can substantially reduce the amount of sediment that reaches oceans. More specifically, the researchers found that restoring forests could help reduce sediment runoff to 630,000 square kilometers (243,000 square miles) of coral reefs.

“Bringing forest restoration to the forefront of global marine ecosystem conservation discussions is imperative,” Suárez-Castro said. “Countries need to commit to land and forest restoration in coastal regions, which will help reduce the amount of sediment runoff from coastal land areas.”

Mangroves are coastal forests that are particularly adept at binding soil with their roots and reducing sediment erosion into the ocean. Image by Anton Bielousov via Wikimedia Commons (CC BY-SA 3.0).

Protecting and restoring coastal forests isn’t the only way to reduce runoff. The team also found riparian buffers — the assemblages of trees and plants that surround rivers and streams — can stop up to 90% of sediment from entering waterways. In other words, restoring forests across multiple watersheds can help reduce erosion and simultaneously improve coral reef health.

By considering solutions that focus not only on carbon emissions but also the connections between land and sea — in this case, mitigation activities like halting and reversing deforestation near water sources — countries can also tackle climate change, improve human livelihoods, and reduce biodiversity loss, Suárez-Castro said.

This approach, however, may come with difficulties of its own. The authors caution that the financial and time costs of reforestation, as well as political and social barriers, raise challenges that require clear regional and international efforts over a sustained period.

“It may take years before the benefits of forest restoration translate to a clear reduction in sediments reaching marine areas,” Suárez-Castro told Mongabay. “In this sense, reforestation actions must ensure that management in the intervening period is sufficient to ensure the persistence of these systems.”

Land ownership, for example, is another issue that affects restoration commitments all across the world. “Sediment excess,” Suárez-Castro said, “come from a set of jurisdictions with a diverse range of socioeconomic and geopolitical contexts.”

For example, although popular with Indonesia’s current government, “forest restoration is not always successful,” Razak said.

“The most problematic issue with reforestation in Indonesia is the ownership of the coastal land, especially in areas where restoration is suggested to be most needed or beneficial,” Razak said. “Most of these areas, in most cases, are legally owned by someone. Even governments find it difficult to find land for restoration because of land ownership issues.”

Reducing coral-reef stressors such as poor water quality can improve reef health and mitigate the effects of global climate change. Image courtesy of Andrés Felipe Suárez-Castro.

Nonetheless, the researchers behind the study say identifying opportunities to improve land management and marine ecosystem health should be a key priority for all countries, particularly as poor coastal water quality is widely regarded as the most serious problem after climate change for tropical coastal ecosystems such as coral reefs. They say that doing so will help sustain not only the future of the reefs, but the coastal communities that rely on them for their local economies.

“The decline of coral reefs can have extended impacts not only on biodiversity but also on human communities that rely on commercial and subsistence fisheries as well as tourism,” Suárez-Castro said.

The researchers point to a need to identify key coastal zones that are most likely to benefit from reforestation, as well as help land managers improve water quality and coral reef health and resilience. However, they caution that this must be done without harming people who depend on natural resources.

“Reforestation projects will only be effective if there is enough stakeholder engagement to ensure that reforestation actions do not disproportionately impact one group more than another,” for instance by “taking land away from farmers to benefit fishers,” Suárez-Castro said. “This means that reforestation must also be coupled with policy to prevent the displacement of unsustainable land practices to other regions.”


Banner image: a portion of Australia’s Great Barrier Reef, which, like many other coral reefs around the world, is contending with sedimentation and other stressors. Image by Ayanadak123 via Wikimedia Commons (CC BY-SA 4.0).


Suárez-Castro, A. F., Beyer, H. L., Kuempel, C. D., Linke, S., Borrelli, P., & Hoegh-Guldberg, O. (2021). Global forest restoration opportunities to foster coral reef conservation. Global Change Biology, 27, 5238-5252. doi:10.1038/s41598-021-01193-7

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