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Sea floor erosion causes coral reefs to sink

April 28, 2017, by Tim Radford

Coral reefs Molokini crater near Maui

Antler coral at the Molokini crater, near the Hawaiian island of Maui, where the sea floor is being scoured of sand and sediments. Image: Yury Velikanau via Flickr

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Five US coral reefs are sinking beneath the waves due to the erosion of the sea floor, robbing coastal communities of their natural storm barrier.

LONDON, 28 April, 2017 – The world’s coral reefs are not just in hot water and under threat from acid attack; they may even be getting out of their depth. New research around five US coral reefs shows that even as sea levels rise, the sea floor around the reefs is being eroded.

And coral growth simply may not be fast enough to keep up, which means that coastal communities in Florida, the Caribbean and Hawaii could become increasingly at risk from storms, waves and erosion.

The news comes close after revelations that great tracts of Australia’s Great Barrier Reef, like other coral colonies, have been devastated by bleaching, as ocean temperatures rise above the levels that corals – animals that live in symbiosis with algae – can tolerate, and researchers have warned that this could soon be happening to reefs almost everywhere, every year.

Coral under threat

There is already widespread alarm among marine scientists as the seas become measurably more acidic due to an increase in levels of carbon dioxide in the atmosphere, and this too poses a threat to corals everywhere.

But while researchers in the tropics had monitored the living reefs of the surface waters, hardly anybody had paid attention to the sea floor around the reefs.

Now, scientists of the US Geological Survey report in Biogeosciences that – possibly as a consequence of the degradation of the reefs of the Florida Keys, the US Virgin Islands and the Hawaiian island of Maui – the sea floor is being scoured of sand and sediments, just as sea levels continue to creep to a predicted rise of up to a metre by 2100.

Around Maui, they report, they measured the loss enough sand, rock and shell to fill the Empire State Building in New York 81 times over.

At current rates, by 2100 sea floor erosion could
increase water depths by two to eight times more than
what has been predicted from sea level rise alone”

This means that the seas along those coasts have become unexpectedly deep. Since tropical corals depend for nourishment on light photosynthesised by their algal partners at the surface, this raises yet another hazard: if the sea floor is falling at the same time as the seawater ceiling is going up, can corals grow fast enough to keep up?

Our measurements show that seafloor erosion has already caused water depths to increase to levels not predicted to occur until near the year 2100,” says Kimberly Yates, a biogeochemist at the USGS’s St Petersburg Coastal and Marine Science Centre, who led the research.

At current rates, by 2100 sea floor erosion could increase water depths by two to eight times more than what has been predicted from sea level rise alone.”

Healthy coral reefs are among the richest and most diverse habitats on the planet. They represent an immediate asset to human communities: they underwrite tourism and fisheries, and they deliver protection against storm surge and tsunami for around 200 million people in low-lying coastal communities.

Sea level rise presents a threat to communities along the coasts of all the inhabited continents, and coastal flooding could by 2100 be costing the world $100 trillion a year.

One group has calculated that money spent on protecting and restoring reefs would represent a bargain, at about one-twentieth the cost of artificial breakwaters.

Reefs in decline

The USGS team identify no specific cause for erosion around the reefs, but they point out that reefs worldwide are in decline because of coastal development, pollution, coral bleaching, disease and acidification.

They worked from seafloor measurements taken by the US government’s own National Oceanic and Atmospheric Administration between 1934 and 1982, and more recent surveys by the US Army Corps of Engineers and LIDAR data gathered by remote sensing. Across all five sites they examined, they found that the sea floor had become lower, in some cases by as much as 0.8 metres.

We saw lower rates of erosion – and even some localised increases in seafloor elevation – in areas that were protected, near refuges or distant from human population centres,” Dr Yates says. “But these were not significant enough to offset the ecosystem-wide pattern of erosion at each of our study sites.”
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