Cold-water coral ecosystems

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corals_ugot1Spectacular deep-sea cold-water coral reefs several kilometres long are a recent discovery on Europe’s margin. Coral reefs are not restricted to warm, tropical seas but also thrive in the deeper, nutrient-rich high latitude waters where ocean currents prevent sedimentation. The colonial stone corals Lophelia pertusa and Madrepora oculata occur along much of the NW European continental margin and in some Scandinavian fjords. These corals have also recently been discovered in the Mediterranean Sea. Despite intense study in recent years, it is still not known how many coral occurrences there are.

Left: The crab Lithodes maja on a colony of Lophelia with sponge Mycale lingua.

spongemixThe interplay between coral framework, sedimentation and hydrography has enabled the growth of cold-water coral carbonate mounds along major parts of Europe’s continental margins and deep shelves, some measuring more than 300 m in thickness and several kilometres across. The mounds provide distinct habitats for creatures such as sponges, clams, soft corals and crustaceans.

Right: Sponges Stryphnus ponderosus, Aplysilla sulphureus, Polymastia sp. and Geodia baretti.

Deep-sea coral ecosystems represent true biodiversity hotspots where species richness and diversity rivals that of their tropical cousins. The reefs provide shelter and food to a wide variety of animals, act as nurseries for fishes, represent a ‘cabinet’ for natural medicines and play an important role in absorbing carbon dioxide from the atmosphere. Many of the associated species are of commercial interest and are locally heavily exploited leaving physically damaged and ecologically altered seabed behind. Since the mid-1980s the socio-economic value of cold-water coral ecosystems has risen tremendously. In many areas of the European EEZ, major trawling areas overlap with occurrences of coral ecosystems. This has raised the cold-water coral issue onto the political agenda. OSPAR identified cold-water coral ecosystems as one of the most vulnerable ecosystems where action is required now to mitigate further loss of biodiversity. A proposal to ban bottom-trawl fishing to protect cold-water coral reefs around the Azores, Madeira and Canaries was released by the European Commission in February 2004. Another important step was the EU emergency measure to stop trawling in the Darwin Mound coral province in the Rockall Trough, off NW Scotland.

blackbelly_rosefishCold-water coral ecosystems are important archives of climate oscillations but so far have been poorly exploited for this purpose. Unlike shallow-water tropical reefs, the cold-water corals lived deep enough in the ocean to be unaffected by the massive sea level falls which occurred during glacial periods in the past. Moreover, this type of coral ecosystem thrives in the flow of the intermediate water mass, which provides a unique opportunity to measure mixing processes between the productive surface waters and the deeper ocean environment.

Left: The fish Helicolenus dactylopterus among Lophelia corals.

HERMIONE addressed questions relating to the dimensions, distribution and interconnection of deep-sea coral ecosystems along Europe’s Arctic, Atlantic and Mediterranean deep-sea frontier. Emphasis was placed on climate-driven changes, the extent and consequences of human impacts through exploitation of mineral and biological resources at or near coral ecosystems and on understanding ecosystem function. Knowledge was provided to support stakeholders and policy makers in creating deep-sea governance for sustainable management of resources and conservation of cold-water coral ecosystems.

 

Temperature shock in corals

A short slideshow explaining the effects of temperature shock in corals

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1/9: Tisler Reef

The Tisler Reef is a Norwegian cold water coral reef in the northeastern Skagerrak. The reef sits on the sill of the Kosterfjord at an average depth of 120 m. The reef structure is 2 km long.

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2/9: A dynamic ecosystem

The reef is a dynamic ecosystem, home to many species of marine animals, such as crabs, shrimp, sponges, tunicates and fish. The primary reef builder is the stony coral species Lophelia pertusa. (Picture: T. Lundalv, UGOT)

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3/9: Temperature shock

The reef is warmest in October/November and coolest in April/May. In 2006 and 2008 the temperature at the reef increased rapidly above the previously observed maximum of 9° to over 12°, in both cases over 24hrs; a real shock to the reef ecosystem.

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4/9: Deja-vu

The temperature shock events unfolded in very similar patterns. The maximum temperature reached on both occasions was within a fraction of a degree. The temperature event in 2008 happened in October, while the 2006 event occurred in November.

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5/9: Current-flow reversals

The warmer water was brought to the reef following a reversal in the current flow. Water from northern Skagerrak washed over the reef and caused the temperature to increase. Subsequent flow reversals similarly increased and decreased the reef temperature.

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6/9: An ill wind...?

In the weeks before the temperature shocks, the wind blew mostly north across the Skagerrak. The temperature shocks occurred just after the wind direction changed. (Data courtesy of the Swedish Meteorological and Hydrological institute).

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7/9: Sea-level changes

Before the temperature shock, sea-level was highest in the northern Skagerrak as strong winds caused water to pile up there. When the wind changed, water flooded south bringing warm water over the reef. (Data courtesy of the Swedish MHI).

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Further reading

Bibliography of Lophelia coral habitats covering biological and geological aspects