Most people associate the word “coral” with sunshine, blue skies and Australia’s Great Barrier Reef. In fact, more than half of the 5,100 species on the planet exist as “cold-water corals” in deep and dark parts of the world’s oceans.
Unlike most other animals, corals are immobile and therefore rely heavily on currents to transport small pieces of organic matter to feed on.
Over time, in some cases millions of years, cold water corals can grow into huge skyscraper-sized structures on the sea floor called “coral mounds.”
These structures are common in the Northeast Atlantic Ocean on the edge of the Irish continental shelf. They can be several miles long and reach 100 meters or more in height – higher 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 that these mounds of fossilized coral and sediment are incredibly varied.
Some are completely covered with living coral, while others have a lot of dead coral on the surface, and the mounds themselves have very different shapes and sizes.
A point of interest is the Porcupine Bank Canyon, the largest submarine canyon on the edge of Ireland’s continental shelf. Colleagues and I wanted to understand why the coral there varied so much over short distances.
To do this, we used the Irish Marine Institute’s deepwater research submarine to collect sonar data and deploy monitoring systems.
This equipment is essential to retrieve information from habitats nearly one kilometer (0.62 miles) below the surface. We recently published the results of our work in the Nature log Scientific reports.
Images show that corals thrive on the edge of the canyon on a nearly vertical rock face. Monitoring stations placed nearby showed that the currents here were fast, sometimes in excess of one meter per second, the fastest ever recorded in a cold water coral habitat.
Nonetheless, there was also more coral debris at these locations, possibly due to these faster currents.
We then used video footage from the submarine to generate 3D reconstructions of the coral habitats, which we were able to analyze to understand how deep water currents affected them.
Interestingly, although the corals can survive these extreme conditions, they still prefer it when the current slows down as they find it easier to feed.
Because the 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 will the oceans. Winds above the sea surface are getting stronger, accelerating average ocean currents by about 5 percent per decade since the 1990s.
It is still unclear how these vast mounds of coral deep below the ocean’s surface will respond to these changing conditions, especially since coral lives on such long time scales. After all, these coral mounds grow very slowly, no more than just 12 centimeters (4.7 inches) per thousand years.
Yet despite their slow-growing nature, colleagues and I previously found that these mounds have changed in just four years, with increased amounts of coral debris and significant decreases in the coverage of a particular coral species.
That is why our team recently used the measuring stations for another year. We are looking for things like increased coral debris production or coral growth on the hills.
Ultimately, our goal is to determine how these corals will respond in the long term to these difficult and changing conditions. 
Aaron Lim, Postdoctoral Fellow, Marine Geoscience, University College Cork.
This article has been republished from The Conversation under a Creative Commons license. Read the original article.