Ecological Consultancy Services Limited (EcoServe)

Coral reefs in Irish Waters

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Yes, it is true, Ireland has its own coral reefs. They are not as noticeable as tropical coral reefs because they are in deeper water, and perhaps not as extensive, but they have a similar structure and richness of marine life.

The discovery of a "major deep-water coral reef province" in the Porcupine Basin in May 1997 (appropriately the Year of the Coral Reef) has highlighted our ignorance of what lives in the sea around Ireland.

It is not widely realised that reef forming coral is present off the west coast of Ireland, reflecting the lack of offshore ecological research. Indeed, the reefs are formed by two coral species, Lophelia pertusa and Madrepora oculata, which interconnect with tubes of the worm Eunice norvegicus.

Tropical coral reefs are a major tourist attraction and fishery resource, and deep water corals also have this potential. Tours in deep water submersibles are already planned to the Titanic shipwreck, and would also be feasible to deep water corals and other marine habitats. The value of the reefs to fisheries has yet to be assessed but is likely to be significant for some species.

The coral was first discovered off Ireland in 1869 by a Royal Irish Academy and Royal Dublin Society sponsored cruise. Until recently little to nothing more was known about this coral in Irish waters, and most research in our offshore waters has been by foreign ships. In the late 1960’s and early 1970’s, fishing trawlers from Dingle caught coral in their nets off the south-west coast, some of it from as shallow as 80-100 m depth.

Recently, geologists from the Dublin Institute of Advance Studies, National University of Ireland college’s Dublin, Cork and Galway, Geological Survey of Ireland, and Petroleum Affairs Division of the Department of the Marine and Natural Resources, have become very active in offshore research. In 1997, they and researchers from Belgium, Germany, Russia, and elsewhere in Europe, noticed hundreds of circular ‘mounds’ on sonar images. Underwater video showed at least some were covered by an abundance of coral. They occurred in a 1200 km 2 area, at depths of 600 to 700 m, and 200 - 400 km from the west coast of Ireland.

The mounds are at least partly composed of fragments of corals, shells and other sediments. There may also be a significant contribution of carbonate material derived from bacteria using methane and sulphates as an energy source.

The distribution of coral has been linked with oil and gas reservoirs.

Information on the coral has largely been obtained from surveys by geologists whose research is driven by the search for oil and gas reservoirs under the seabed. Extraction of oil and gas from such depths is already conducted in other seas.

It may be coincidence, but the occurrence of deep water coral has been linked with seepages of methane and other gases from oil and/or gas reservoirs beneath the seabed. Thus the distribution of coral may be an indicator of the presence of oil and gas. Indeed, there was an unsuccessful court challenge by Greenpeace who tried to prevent oil and gas exploration near coral reefs in UK waters on the basis of the need to protect the coral under the EU Habitats Directive.

What the relationship may be between the coral and methane seeps is a subject of current scientific debate. Opinions range from there being no relationship, to that the coral feeds on bacteria which feed on the methane, and/or that the coral grows on compacted carbonate sediments.

The exploration of the oil and gas industry is stimulating research about the reefs, and it seems that deep-sea oil and gas extraction will become inextricably linked with the study and conservation of their new deep-sea neighbours, the Lophelia reefs.

Deep water reefs have a biodiversity as rich as tropical coral reefs

Over 860 species of animals have been recorded on Lophelia reefs in the north-east Atlantic, and about 300 on single reefs off Norway, Shetland and Bay of Biscay. Such a richness of species is similar to that on shallow water tropical reefs, although Lophelia reefs lack plants and plant eating animals. These numbers are remarkable for a single habitat when one considers that about 6,000 species occur in all of Ireland’s coastal marine environment.

On a seabed largely of sand, mud, gravel and shell, the coral reefs form the most structurally complex physical habitat for species in the deep sea. A rich diversity of animals also occurs around the reefs, some burrowing 2 m down into the sediments.

Amongst the coral branches occur fish (redfish, saithe, cod, ling, and tusk), squat lobsters and other crustaceans, molluscs, starfish, brittlestars, sea pens, and sea urchins. A wide variety of animals grow plant-like attached to the coral, including sponges, bryozoans, hydroids, and other coral species. As is the case for most species living on hard surfaces, most feed by catching plankton and particulate matter from the water.

Some species appear unique to the coral reefs, and different studies have found 5 to 20 species that only occurred on living coral. For example, the worm Eunice norvegicus typically builds its calcareous tube amongst the coral, and large numbers of the scallop-like mollusc Acesta excavata attach to the coral. A snail, Alvania jeffreysi, predates a foraminferan parasite of the coral, and other unique biological interactions probably remain to be discovered.

Species which are not unique to reefs may still benefit from them. For example, reefs provide nursery and resting areas where smaller fish can hide until large enough to avoid predators in open water and larger fish can sleep undisturbed. Animals which collect waterborne food, so called suspension feeders, congregate on the reefs. They pass on this food to other species in the reef food web. The physical banks of reefs may also form barriers to sediment slides which can disturb large areas of sea bed. However, the importance of the reefs to other species and the function of the deep sea ecosystem can only be speculated upon at present.

Comparison of the number of species in major groups found on the coral to those occurring in coastal waters suggest that

  • large immobile or slow moving species have been best recorded, such as sponges, starfish, sea urchins, bryozoans, hydroids and other corals and anemones;
  • fast moving and secretive fish have been overlooked;
  • smaller species, and those more difficult to observe and identify, have not been collected or identified. These include copepod and amphipod crustaceans, and nematode worms;
  • groups with species of varying size and visibility, such as polychaete worms, gastropod and bivalve molluscs, have been only partly sampled and identified.

The numbers of species of the various animal groups associated with Lophelia reefs are 18 - 30 % of their number in coastal seas around Britain and Ireland. If the above factors are taken into account, and we expect about 20 % of the number of coastal species for each taxonomic group to occur in the deep sea, then it is likely that the number of species on Lophelia reefs will be doubled with further research (still excluding protozoans and microbial species).

The coral has been damaged by deep water trawling for about 30 years.

The most heavily trawled parts of the deep sea appear to be (there is no exact location information available) the edge of the continental shelf, where Lophelia reefs are most recorded. Depths down to 1500 m are now trawled, ploughing the seabed and removing large numbers of fish.

Damage by bottom trawling for fish has turned parts of reefs to rubble off the coasts of Norway and Shetland. Despite its deep water habitats, the coral can grow at similar rates to tropical shallow water corals. Norwegian studies indicate it grows at 6 mm per year, so reefs of 1 to 2 m high are hundreds of years old. Trawler damage can thus destroy this unique habitat which would take centuries to recover (and only if not damaged again).

The impacts of trawling on deep sea coral ecosystems will be significant because the fauna of such areas would not be tolerant to physical disturbance. Coral reefs in the tropics are known to be very vulnerable to physical damage. Fisheries may also affect deep sea ecosystems through removal of the fish predators from the food web. The Convention of Biological Diversity obliges countries to protect and research both the economic (fish, shellfish, seaweed) and ecosystem (sea bed species of indirect economic importance) aspects of biodiversity, but it has yet to be implemented in Ireland.

Other countries cannot be relied upon to adequately study and protect Ireland’s natural resources.

The absence of Irish based ecological research on the deep sea is of concern because future opportunities and benefits to society may be lost by uncontrolled and wasteful exploitation of deep sea resources. Mineral resources are eventually used up, but biological resources (i.e. biodiversity) can last forever if managed sustainably. The importance of the reefs as a habitat for other species and consequent economic importance for fisheries, and in the nutrient (carbon, nitrogen, etc.) cycles of marine ecosystems, can not be assessed with the current state of knowledge.

The author thanks Jan Helge Fosså, Pål B. Mortensen, and Torleiv Brattegard (Institute Marine Research, Bergen), Alex Rogers (University of Southampton), Andre Freiwald (University of Bremen), Martin Hovland (Statoil Norway), Ben de Mol and J. P. Henriet (University of Gent), John B. Wilson (Royal Holloway University of London), Geoffrey O’Sullivan and Dan Minchin (Marine Institute, Dublin), Kevin Flannery (Dingle), Declan Quigley (Dublin), Keith Hiscock (English Nature, Peterborough), Murray Roberts (Dunstaffnage Marine Laboratory), Andrew Wheeler (National University of Ireland, Cork), Chris Emblow (Ecological Consultancy Services Ltd, Dublin), and Pat Shannon and Viram Unnithan (NUI, Dublin), for helpful information and unpublished observations used in preparing this article.

This article is dedicated to the government marine engineer who believed there was nothing much living in the deep sea off Ireland.

An article was also printed in the Irish Times

The research to produce this article is a contribution by Ecological Consultancy Services Ltd (EcoServe) to the Year of the Ocean 1998.

[Index][Biology of Lophelia][more information on Lophelia][Distribution]