Cold-water coral reefs

Coral reefs can not only be found in the warm, sun-drenched waters of tropical regions, they also grow in cold, dark waters within a huge geographical range between 55°S to 70°N. Off the coast of Britain, cold-water coral reefs form mainly off the west of Scotland and Ireland. Near Barra and Mingulay in the western isles of Scotland, complex reef covers a large area of approximately 100km2.

Cold-water corals are different from tropical/warm-water corals in the way that they do not rely on symbiotic algae to provide them with nutrients. Algae require sunlight to produce these nutrients, so warm-water corals usually only exist in shallow water. Without algae restricting them to sunny shallows, cold-water corals tend to grow deeper, at around 200 to 400m, and sometimes over 3000m deep. They particularly like the temperature range of 4 to 8 °C.

So what is a coral reef? A coral reef can be defined as a large underwater structure made of dead and living corals. They are formed by hard coral species, otherwise called reef-forming corals. An individual coral animal (called a polyp) is in fact tiny and soft-bodied, but hard coral species secrete sufficient quantities of calcium carbonate required to form the substantial mass that is a reef. Hundreds to hundreds of thousands of individuals join together through their hard, branching, calcium carbonate skeletons.

Within the chilly, deep, dark waters they reside, cold-water corals feed solely by catching food particles from the surrounding water, using tentacles that protrude from the small polyps on the end of their hard, skeletal branches. You can find cold-water corals growing on silt and rock slops, seamounts, mounds, ridges and pinnacles, where there is enough current to deliver a sufficient amount of food to the corals and where there is, or was, a hard surface for coral larvae to settle on.

The largest cold-water coral reef yet discovered is off the coast of Norway’s Røst Island and measures 40km in length and 2 to 3km in width. Another Norwegian cold-water coral reef has grown to a height of 165m above the surrounding seabed. Despite the vast sizes of some of these reefs, deep-sea/cold-water corals grow very slowly, at about 5 to 25mm a year. Therefore it can take hundreds or thousands of years for these reefs to reach a substantial size. Some reefs around Britain are over 4,000 years old and can be more than 8,500 years old elsewhere

Lophelia pertusa, a reef-forming coral.

Lophelia pertusa, a reef-forming coral.

Lophelia pertusa is the only reef-forming coral in British waters. This coral is highly branched, forming a beautiful, delicate structure. The living tissue of this coral tends to be white, but can be pink or orange. As colonies of this coral grow bigger and bigger they begin to break up under their own weight. The fragments that break off are then colonised by a huge variety of organisms, such as sea fans, sponges, large bivavles, prawns, squat lobsters, basket stars and worms. In fact, 614 species have been recorded from cold-water reefs. These reefs provide breeding grounds and refuges for many fish species, including commercial species of fish such as redfish and pollack.

Because cold-water coral reefs are very slow growing and fragile, they are easily damaged. The fact they are very species-rich also makes them a prime target for fishermen, who often use destructive trawling methods. Other threats include oil and mineral exploitation, which causes smothering and marine pollution, and the laying of cables and pipelines. Since these reefs are threatened and are biodiversity hotspots, they are a priority habitat under the UK Biodiversity Action Plan.

Cold-water coral reefs are fascinating, beautiful and highly important. They host a large variety of marine species, but large areas around Britain have been heavily damaged or totally destroyed. Forming vast and ancient communities, their levels of biological diversity can be up to three times as high as the surrounding soft sediment. These reefs may grow in dark, cold seas, but they are just as fantastic as their tropical counterparts.

Arkive, (2014), coral reef conservation, [online], available at: http://www.arkive.org/coral-reef-conservation/ accessed 6 September 2014.

Natural England, (2014), cold water coral reefs, [online], available at: http://www.naturalengland.org.uk/ourwork/marine/mpa/mcz/features/habitats/cold-watercoralreefs.aspx accessed 6 September 2014.

Scottish Natural Heritage, (2014), cold water coral, [online], available at: http://www.snh.gov.uk/about-scotlands-nature/species/invertebrates/marine-invertebrates/cold-water-coral/ accessed 6 September 2014.

WWF, (2014), cold water corals, [online], available at: http://wwf.panda.org/about_our_earth/blue_planet/coasts/coral_reefs/coldwater_corals/ accessed 6 September 2014.

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Kate Dey

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