Most associate the word “coral” with sun, blue skies and Australia’s Great Barrier Reef. In fact, more than half of the planet’s 5,100 species exist as “cold-water corals” in the deep and dark parts of the world’s oceans.
Unlike most other animals, corals are immobile and therefore rely heavily on currents to carry tiny pieces of organic matter.
Over time, in some cases millions of years, cold-water corals can grow to form large skyscraper-sized structures on the seabed known as “coral mounds”.
These structures are common in the north-east Atlantic at the edge of the Irish continental shelf. They can be several kilometers long and reach 100 meters (328 feet) or more in height – taller than any building in Ireland.
I have been studying the cold water coral habitats off the coast of Ireland for several years, and have found that these heaps of fossil coral and sediment are very different.
Some are completely covered with live coral and others with a lot of dead coral on the surface, and the heaps themselves have very different shapes and sizes.
One place of interest is the Porcupine Bank Canyon, the largest submarine canyon at the edge of the Irish continental shelf. Colleagues and I wanted to understand why the coral there was changing so much over short distances.
To do this, we used the Irish Marine Institute deep water survey to collect sonar data and use monitoring systems.
This equipment is essential for retrieving information from habitats found nearly a kilometer (0.62 miles) below the surface. We published the results of our work in the Nature iris Scientific Reports.
Images show that corals thrive at the very edge of the canyon on a nearly vertical cliff face. Nearby-used survey stations showed that the currents here were fast, sometimes more than a meter per second, the highest speed ever recorded in a cold-water coral habitat.
Nonetheless, these sites had more coral debris, which may have been the result of these faster currents.
We used video film captured by the submarine to create a 3D reconstruction of the coral habitats that we could study to understand how they were affected by deep water currents.
Interestingly, although the corals may survive in these awful conditions, they still seem to prefer when the current slows down as it is easier for them to then feed.
Because the cold-water corals live in such remote parts of the planet, experiments have previously been run in tanks in laboratories showing similar results.
As the world warms, so will the oceans. Winds over the sea surface are becoming stronger, causing average ocean currents to accelerate by about 5 percent every decade since the 1990s.
It is not yet clear how these large piles of coral deep beneath the ocean surface will cope with these changing conditions, especially since corals live on such long levels. After all, these coral heaps grow slowly, to no more than just 12 centimeters (4.7 inches) per thousand years.
But despite their slow-growing nature, colleagues and I have found that these mounds have shown changes over just four years, with increased coral debris and a significant reduction. in coral-specific species coverage.
That’s why our team recently used the monitoring stations for another year. We are looking out for things like increased coral debris production, or coral growth on the mounds.
Ultimately, our goal is to determine how these corals cope with these difficult and changing conditions in the long run. 
Aaron Lim, Postdoctoral Researcher, Marine Geology, University College Cork.
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