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Under the sea

Coral reefs losing ability to keep pace with rise in sea levels

Research

By Kelly Foss

Many coral reefs will be unable to keep growing fast enough to keep up with rising sea levels, leaving tropical coastlines and low-lying islands exposed to increased erosion and flooding risk, new research suggests.

An international team, which includes Memorial University’s Dr. Evan Edinger, a professor of geography, biology and earth sciences, Faculty of Science, compared the maximum upward growth rates of coral reefs in the Caribbean and Indian Ocean regions with predicted rates of sea-level rise, and found many will be unable to keep pace.

Coral reef
Many Caribbean coral reefs have been largely overgrown by fleshy algae, as shown in this photo from the Bahamas. The small yellow coral at centre-left, Agaricia, grows much more slowly than the Acropora corals that used to dominate these environments.
Photo: Submitted

The growth of coral reefs is strongly influenced by the amount and types of coral living on the reef surface.

This growth is now being hampered by combinations of coral disease, deteriorating water quality and fishing pressure, along with severe impacts from coral bleaching caused by global warming.

Coral bleaching can occur when water is too warm. Corals expel the algae living in their tissues, causing the coral to turn white. After bleaching, some corals survive and some die.

Coral cover — the proportion of reef surface covered by live stony coral — is a strong predictor of the extent to which rates of reef growth will lag behind sea-level rise, and thus how much additional submergence will occur.

Dead coral colony
Acropora palmata was the dominant shallow-water reef forming coral species on Caribbean reefs. This dead coral colony on the Belize barrier reef has been overgrown by sponges that break down the coral skeleton.
Photo: Submitted

“If we look at the long-term definition of reef health, it comes back to the ability to produce more calcium carbonate than will get eroded away,” he said. “Healthy reefs are very easily able to keep up with sea-level rise, even during the rapid sea-level rise after the last glaciation.

“But, what we’re seeing now in the Caribbean and Indian Oceans is that we’re throwing so many different threats at coral reefs, the amount of live coral cover is actually low enough that they’re not able to keep pace with the natural things that take away calcium carbonate.”

Dr. Edinger helped to design the protocol used for measuring how fast the reefs can produce limestone and helped collect and analyze the data from the Caribbean sites.

“We’ve seen a lot of talk and not much action.” — Dr. Evan Edinger

He says one depressing aspect of environmental science is that, all too often, researchers end up documenting the destruction of something in excruciating detail.

“We’ve known about a lot of these threats for quite a long time. We also know what should be done about those threats, but actually doing it is hard. We’ve seen a lot of talk and not much action.”

Despite that, Dr. Edinger says coral reefs are very resilient systems, and that researchers know from a number of examples around the world that recovery is possible.

“However, they are only resilient if we do something about the threats to them,” he said. “When we look ahead to the next 80 years, the future of reefs is quite grim if we don’t do something about climate change impacts.

“With sea levels continuing to rise faster and as reefs fall more behind,” he continued, “they also become susceptible to storm waves. These waves can remove even more limestone from the reefs, making them fall even further behind.”

Financial implications

One of the benefits to coral reefs is the ecosystem services they provide, including coastline protection.

Dr. Edinger says healthy reefs break the waves and, if you allow them to become degraded, they can’t function effectively as a breakwater. An offshore reef can take the impact of storms instead of the land and there will be less flooding.

He says the Maldives have a long history with this because their capital city, Malé, is an atoll — a ring shaped island formed of coral surrounding a lagoon. Back in the ’80s and ’90s they degraded their coral reef by mining it for building stone. The country had no choice but to construct a seawall to protect the island — at a cost of $10,000 per linear meter.

“If you want to put it in terms of dollar value, which politicians might relate to, the cost of not doing something is astronomical,” said Dr. Edinger.

“We should take this as a call to action and start getting serious about doing what we need to do to protect our global coral reefs.”

The paper, published in the June 21 issue of Nature, is titled Loss of Coral Reef Growth Capacity to Track Sea-level Rise under Climate Change.


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