Lagoon's 'incredible refuge' may hold secret to coral survival

The secret to coral survival in a warming world could lie in Aldabra's vast inland 'sea'.

Coral reefs in the huge lagoon, which is big enough to swallow Manhattan twice over, survived the mass bleaching event in 2016 in much better health than those that ring the outside of the atoll.

Preliminary results from new research show that tiles placed on the bottom of the lagoon attract many more coral larvae than identical tiles placed on the fringing reef.

The work is being conducted by scientists at the Seychelles Islands Foundation (SIF), working with teams from Oxford University and Germany.

April Burt, from The Queen's College, Oxford, told Sky News: "There are pockets of resilience around Aldabra.

"The coral in the lagoon is doing much better than the coral outside."

Coral bleaches when water temperatures reach 30-31C (86-87.8F). It ejects the symbiotic algae inside its tissues that provide it with much of its food.

If the warm water persists for more than a few weeks the coral dies.

Yet the coral inside the lagoon experiences temperatures far above this daily, when the tide rushes out leaving only shallow water that rapidly heats up in the tropical sunshine.

Ms Burt said its survival gives hope that coral may adapt to rising sea temperatures caused by climate change.

"The corals may just be used to it," she said.

"There are huge daily fluctuations in temperatures as the tide goes in and out.

"We want to know whether they have adapted to it and express certain genes when they need to.

"We need to understand what makes coral more resistant to temperature rises and Aldabra is key to finding out."

Conservationists from the island research station, which is run by SIF, took us into the lagoon through a narrow channel of water almost hidden from the open ocean.

We drift-snorkelled with the rushing tide, as millions of tonnes of water flowed out of the lagoon.

We saw shoals of fish using the channel as a highway, dodging the prowling sharks, groupers and other apex predators.

The lagoon is three metres deep at high tide. But just six hours later, at low tide, more than two-thirds is dry land.

Lindsay Turnbull, a professor at Oxford University and a trustee of SIF, described the lagoon as the "world's biggest infinity pool".

"There are very few ways in," she said.

"That's why when people came here they were not aware it was even there.

"Navigating the channels is extremely dangerous.

"But once in the lagoon you have this incredible refuge. It's always calm because the fringing reef absorbs the power of the waves."

The lagoon is unusual in having coral, seagrass and mangrove habitats all together in pristine condition.

The seagrass attracts scores of green turtles and rare dugong, or sea cows, the only marine mammal to exclusively eat vegetation.

The forests of mangrove provide shelter for a range of wildlife.

Below the water, eels, fish and juvenile sharks are protected from larger predators by the lattice of roots.

In the leafy canopy the Indian Ocean's largest colony of both greater and lesser frigate birds jostle with red-footed boobies.

Cheryl Sanchez, Aldabra's science coordinator, described it as "paradise".

She said: "It is just teeming with life everywhere you look, which is what makes it so special.

"You have every ecosystem that you would want.

"Life in the water and above water - all over."

Aldabra, an outer island of the Seychelles, is strictly protected by the government and is a UNESCO World Heritage Site.

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