Deep water reefs

A few species of azooxanthellate corals form deep water reefs in high current areas associated with the hard substrate and topographic rises such as ridges and pinnacles. The best known is Lophelia pertusa, a colonial coral which forms structures ranging from small patches a few metres in width to reefs many hundreds of metres in size, at depths of 100-3000m and temperatures 4-8 C. The distribution of these reefs is global (1) though the species is best known from the north east Atlantic in part because of concentrated deep sea trawling and oil exploration activities there.

Distribution of known deep water coral reefs

Typically the reefs are surrounded by areas of loose sediment. The complex matrix of living and dead branches of Lophelia therefore increases the spatial heterogeneity of the seabed and maintain higher levels of biodiversity with an invertebrate species list of 800 being a likely underestimate. Nonetheless boring sponges, anemones, bryozoans, gorgonians, polychaetes, barnacles and bivalves occur in large numbers and their diversity is comparable to some shallow water tropical systems. Other corals such as Madrepora oculata and Solenosmilia variabilis frequently occur. Although large aggregations of fish are associated with Lophelia reefs, and reef areas support higher catches than adjacent seabed, the fish and coral species diversity is much lower than in tropical coral reefs: only 23 species of fish have been recorded on Lophelia reefs in the north-east Atlantic.

The reefs are delicate structures easily destroyed but demersal fishing gear which routinely operate to depths of 2000m. The total destruction of some Norwegian reefs have already been documented and 30-50% of others has been estimated. Slow growth, in the region of 4-25 mm per year, severely limits the ability of Lophelia to recover. The trawl fishery for orange roughy (Hoplostethus atlanticus) and oreos (Allocyttus niger) on seamounts south of Tasmania has been responsible for substantial destruction of Solenosmilia reefs with more than 90% bare rock in the most heavily fished areas. If recovery ever occurs it will takes hundreds of years.