Coral reefs, both cold-water (such as the Mingulay reef complex off the coast of Scotland, amongst others), and tropical reefs (for example, the Great Barrier Reef in the Pacific Ocean off of Australia), are a huge part of the marine ecosystem. As of a 2012 study, it is estimated that between one-quarter and one-third of all marine species reside within coral reef complexes. (Alasdair McIntyre: Life in the World’s Oceans, Chapter 4 Coral Reef Biodiversity)
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Whilst being a huge portion of the marine habitat, similar in importance to terrestrial rainforests, reefs are fragile and in a constant need for perfect balance, where a simple tip in one direction or the other can spell doom for the reef system, the corals and all the organisms that live within the habitat.
Such effects, such as coral bleaching, algal carpeting (which can lead to eutrophication of the reef) and human pollution can destroy reefs in a few short years, undoing often hundreds of years of growth.
Cold water reefs tend to face slightly different threats, such as damage from submarines laying communications cables, and bottom trawling, than the shallow tropical reefs. Cold-water corals often do not actually form ‘reefs’, instead forming mounds or other aggregations.
Tropical reefs, on the other hand, can form large atolls (reefs created when volcanic/islands fall back into the sea after tectonic events), barrier reefs and other reef systems.