Most people associate the word “coral” with the sun, blue sky and the Great Barrier Reef in Australia. In fact, more than half of the 5,100 species on the planet exist as “cold water corals” in the deep, dark parts of the world’s oceans.
Unlike most other animals, corals are immobile and therefore rely heavily on currents to carry small pieces of organic material to feed on.
Over time, in some cases millions of years, cold-water corals can grow to eventually form huge sea-sized skyscraper structures called “coral mounds.”
These structures are common in the Northeast Atlantic, on 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.
We have been studying the habitats of cold-water corals off the coast of Ireland for several years and have found that these mosses of fossilized corals and sediments are incredibly varied.
Some are completely covered with live corals, while others have a lot of dead corals on the surface, and the mounds themselves have very different shapes and sizes.
A place of interest is Porcupine Bank Canyon, the largest underwater canyon on the edge of Ireland’s continental shelf. Colleagues and I wanted to understand why the coral there varies so much over short distances.
To do this, we used the Irish Navy Institute’s submersible research to collect sonar data and implement monitoring systems.
This equipment is essential for retrieving information from habitats located almost one kilometer (0.62 miles) below the surface. We recently published the results of our work in The nature diary Scientific reports.
The images show that the corals thrive right at the edge of the canyon on an almost vertical rock. Nearby monitoring 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.
However, there was also more coral debris in these places, which could be the result of these faster currents.
We then used video images captured by the submarine to generate 3D reconstructions of coral habitats that we were able to analyze to understand how deep water currents influence them.
Interestingly, while corals can survive these extreme conditions, they still seem to prefer it when the current slows down because it is easier for them to feed.
Because cold-water corals live in such remote parts of the planet, experiments have been conducted in laboratory tanks in the past that show similar results.
As the world warms, so do the oceans. Winds on the sea surface are becoming stronger, causing average ocean currents to accelerate by about 5% per decade since the 1990s.
It is still unclear how these huge mounds of deep coral beneath the surface of the ocean will meet these changing conditions, especially since coral lives on such long scales. After all, these mounds of coral grow very slowly, no more than just 12 centimeters (4.7 inches) every thousand years.
However, despite their slow-growing nature, my colleagues and I have previously found that these mounds have changed in just four years, with increased amounts of coral debris and significant declines in coverage of a particular coral species.
This is why our team recently deployed the monitoring stations for another year. We are looking for things like the increased production of coral debris or the growth of coral on mounds.
Ultimately, our goal is to determine how these corals will respond to these harsh and changing conditions in the long run. 
Aaron Lim, Postdoctoral Researcher, Marine Geosciences, University College Cork.
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