Most people associate the word “coral” with sunshine, blue sky and the Great Barrier Reef in Australia. In fact, more than half of the 5100 species on the planet exist as ‘cold coral’ in deep and dark parts of the world’s oceans.
Unlike most other animals, coral is immobile and therefore relies heavily on streams to transport small pieces of organic material to feed.
Over time, in some cases millions of years, cold water corals can grow and eventually form large skyscraper-sized structures on the seabed, called ‘coral heaps’.
These structures are common in the northeastern Atlantic Ocean on the edge of the Irish mainland. They can be a few kilometers long and 100 meters (328 feet) or higher – longer than any building in Ireland.
I have been studying the cold water coral habitats off the coast of Ireland for a number of years and have found these piles of fossil coral and sediment to be incredibly diverse.
Some are completely covered with living coral, while others have very dead coral on the surface, and the hills themselves have many different shapes and sizes.
One interesting spot is the Porcupine Bank Canyon, the largest submarine gorge on the edge of mainland Ireland. Colleagues and I wanted to understand why the coral differs so much over short distances.
To do this, we used the Irish Marine Institute’s deepwater research to collect sonar data and deploy monitoring systems.
This equipment is essential to obtain information from habitats that can be found almost a kilometer below the surface. We recently reviewed the results of our work in the Earth journal Scientific reports.
Images show that coral at the edge of the gorge thrives on an almost vertical rock face. Monitoring stations deployed nearby 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.
Nevertheless, there was also more coral debris on these sites, which may be the result of these faster currents.
Next, we use video recordings captured by the submarine to generate 3D reconstructions of the coral habitats that we could analyze to understand how deep water currents affect them.
Interestingly, although the coral can survive these extreme conditions, it seems that they still prefer it when the current slows down, as it is then easier to feed.
Since the cold water corals live in such remote parts of the planet, experiments have been performed in tanks in laboratories in the past that show similar results.
As the world warms, so will the oceans. Winds over the sea surface are getting stronger, causing the average ocean currents to accelerate by about 5 percent per decade since the 1990s.
It is still unclear exactly how these large coral heaps deep under the ocean will respond to these changing conditions, especially since corals live on such long scales. After all, these coral hills grow very slowly, no more than just 12 centimeters (4.7 inches) every thousand years.
Yet, despite their slow-growing nature, colleagues and I have previously found that these hills have undergone changes in just four years, with increased amounts of coral debris and a significant decrease in the coverage of a particular coral species.
That’s why our team recently deployed the monitoring stations for another year. We are looking for things like increased coral litter production or coral growth on the hills.
Ultimately, our goal is to determine how this coral will respond to these difficult and changing conditions in the long run. 
Aaron Lim, Postdoctoral Researcher, Marine Geoscience, University College Cork.
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