Independent Scuba

Coral reefs in Scandinavia

General

Little is known about the coral reefs that can be found in Scandinavian waters. They normally live at great depths but can at some places be found as shallow as 40 meters (the Trondheim fiord). Although several types of coral exists in cold waters, only three are able to build reefs: Lophelia pertusa, Madrepora oculata and Solenosmila variabilis where the most common is Lophelia pertusa.

History and overview

Scandinavian coral reefs are mentioned as early as 1752 but discovery was slow until the oil-age came along with its subsequent mapping of the sea floor of the North Sea which resulted in the discovery of several hundreds of reefs, although most of them are far from land at between 200-400 m depth. The structures can be very impressive with reef areas covering between 1300 and 12000 m2. The largest reefs are up to 140 m long, 120 m wide and 25 m high. Most reef explorations have been carried out with ROV’s (remote operated vehicles). 

Lophelia pertusa and sponges at 200 m, Nordleksa, West Norway

The diversity of life is impressive at Lophelia reefs and can in fact be compared to the diversity of tropical coral reefs.

Biology

There are several differences between tropical and cold water corals. The cold-water corals do not have zooxanthellae in their tissues which means that they need to rely upon their tentacles in order to feed themselves. This is not surprising, considering the depths where most reefs are located, which means that no light is present for zooxanthellae to utilise. Reproduction is similar to tropical corals. The larvae are free swimming, but it is unclear how far they can swim. The corals grow very slowly, only about 6 mm per year, which means that it takes thousands of years to build the massive reef structures.

Threats

The major threat to these reefs is deep-sea trawlers. Previously fishermen tried to avoid the reefs in order to save the trawls, which were damaged by hitting the structures. This, however, is not the case with modern trawlers and the impact can be severe.  The reefs in Sweden are badly damaged but agreements have been made to reduce the risk for further damage.
 

The arrow shows a trench from towed fishing gear. Smashed coral is littered around.