Coral reefs are marine ridges or mounds, which have formed over millennia as a result of the deposition of calcium carbonate by living organisms, predominantly corals, but also a rich diver- sity of other organisms such as coralline algae and shellfish. Coral reefs provide a unique habitat able to support a high di- versity and density of life. They occur globally in two distinct marine environments; deep, cold water (3–14ºC) coral reefs, and shallow, warm water (21–30ºC) coral reefs in tropical latitudes. Cold-water corals have been recorded in 41 countries world- wide (Freiwald et al ., 2004), but they are most likely distrib- uted throughout the World’s oceans. They occur wherever the environmental conditions (cold, clear, nutrient-rich waters) are present, from Norwegian fjords in 39 meters depth to several thousand metres in the deep-sea. Living mostly in perpetual darkness, cold-water corals do not possess symbiotic, single- celled algae, and rely solely on zooplankton and detritus, which
they capture with their tentacles. Some species, such as Lophelia , can form large, complex, 3-dimensional reef structures several metres in height. The largest reef so far was discovered in 2002 is the Rost reef off the Norwegian coast. It spans twice the size of Manhattan, is part of the Lophelia reef belt stretching all along the eastern Atlantic continental shelf and slopes from within the Arctic Circle to the coast of South Africa. Other soft corals living in colder waters such as Gorgonia species do not form reefs but large ‘gardens’, covering vast areas for example around the Aleu- tian island chain in the North Pacific. The ecological functions of such reefs and gardens in the deeper waters are very similar to tropical reefs: they are biodiversity hotspots and home, feed- ing and nursery grounds for a vast number of other organisms, including commercial fish and shellfish species. Living in highly productive areas, cold-water coral reefs and gardens are threatened by bottom fishing, especially with trawls and dredges. Observations with submersibles and remotely
Figure 7. Distribution of coldwater and tropical coral reefs. The coldwater reefs are highly susceptible to deep-sea trawling and ocean acidification from climate change, which has its greatest impacts at high latitudes, while tropical reefs will become severely damaged by rising sea temperatures.