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In Dead Water

Coral Reefs

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 diversity of other organisms such as coralline algae and shellfish.
Coral reefs provide a unique habitat able to support a high diversity 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 worldwide (Freiwald et al., 2004), but they are most likely distributed 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 Aleutian 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, feeding and nursery grounds for a vast number of other organisms, including commercial fish and shellfish species.

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.

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 operated vehicles revealed that most of the reefs found on the continental shelf in the North Atlantic show signs of impact by trawling. Lost fishing gear entangled in the corals, and scars from the heavy net doors, rollers and lines, are a common sight. In some places reefs that took over 8.000 years to grow have been completely destroyed, leaving only coral rubble behind.

Warm-water coral reefs are found in circum-tropical shallow waters along the shores of islands and continents. Here, corals feed by ingesting plankton, which the polyps catch with their tentacles, and also through the association with  symbiotic algae called zooxanthellae. Stony corals deposit calcium carbonate, which over time forms the geological reef structure. Many other invertebrates, vertebrates, and plants live in close association to the scleractinian corals, with tight resource coupling and recycling, allowing coral reefs to have extremely high biodiversity in nutrient poor waters, so much so that  they are referred to as the ‘Tropical Rainforests of the Oceans’. Corals have certain ranges of tolerance to water temperature, salinity, UV radiation, opacity, and nutrient quantities. The extreme high diversity of coral reefs have led to the erroneous belief that they prefer nutrient rich environments, but, in fact, corals are extremely sensitive to silt and sewage at far lower concentrations that what is classified as hazardous to humans (Nyström et al. 2000). Hence, even minor pollution in apparently clear waters can severely impact coral reefs and their ability to support thousands of fish species and other marine life. Sea water quality and human impacts are particularly critical to coral reefs when they are exposed to other stressors or when they are recovering from storms or bleaching events (Burke et al., 2002; Wilkinson, 2002; Brown et al., 2006; UNEP, 2006)

Corals are beautiful living animals that are enjoyed by millions of snorkelers and divers world wide, but they are also of vital importance for the whole coral reef ecosystem and for coastal fisheries. One of the largest declines in fishing has, in fact, been recorded in the catches of coral reef fishes, probably as a result of overexploitation of the more vulnerable species (Cheung et al., 2007). If corals die, the characteristic three dimensional structure of reefs that is essential to so many of the services provided, will be lost through natural physical and biological erosion as waves, storms, tsunamis, predators, and other factors affecting corals break it down to rubble. Coral reefs support over a million animal and plant species and their economic value exceeds US$30 billion a year.