The oceans cover more than 70 per cent of the world’s surface, yet the deep-sea remains largely unexplored. Cold-water corals are one of the deep sea’s treasures. These corals may grow as single polyps, as individual tree-like colonies or form complex reefs providing a range of habitats for many other species. Cold-water coral habitats are recognized by the United Nations as ‘Vulnerable Marine Ecosystems’, and many have been declared ‘Ecologically and Biologically Significant Areas’ under the Convention on Biological Diversity, indicating their importance and need for protection. Mapping marine habitats and species distributions is essential in conservation and resource management. The generation of such maps, however, is particularly challenging within poorly-sampled deep-sea ecosystems.
In 2013 Heriot-Watt University and the National Oceanography Centre (NOC) began a new study to map and characterize cold-water coral sites in the North Atlantic Ocean. This three-year project will integrate a series of datasets including one of the largest archives of high definition video data, acquired by remotely operated vehicles (ROVs) from the Logachev cold-water coral carbonate mound province and other sites on Rockall Bank. See Figure 1.
It was the pioneering work of 19th century biologists that provided the only information on cold-water coral distribution in British waters until the 1970s when distributional data were first collated and manned submersibles were first used in seafloor surveys. Cold-water coral reefs are hard to find as they grow in waters too deep to allow the use of satellite-based remote sensing techniques. In addition, the areas of the continental shelf edge and slope suitable for reef development are too large for extensive visual survey using submersibles or towed cameras. Luckily our ability to chart the seafloor has improved dramatically following the development of multibeam echosounders. These can be used not only to measure water depth, but also to generate a backscatter record giving information about the physical attributes of the seabed. The depth and texture of the seabed can indicate the occurrence of cold-water corals and, alongside ground-truthing with visual surveys and sampling, can be used to build up wide area habitat maps. In deep-sea surveys, sampling and species identification remain vital components of habitat mapping.
Few recent studies have focused on the associated fauna of deep-water coral habitats but cold-water corals, and especially scleractinians such as Lophelia pertusa, are today known to provide biodiversity hotspots thanks to their ecosystem engineering capacities. Their generally tree-like architecture provides biomass, structural complexity, modifies the seascape and produces a great diversity of microhabitats. The Norwegian L. pertusa reefs, for example, are known to support four major types of habitat: the surface of living coral; the surface of dead corals; cavities within the coral skeleton; and spaces between the branches. The corals provide shelter, food sources and spawning habitats to deep-sea fish and shark communities and to a wide range of other organisms, ranging from micro- to megafauna. A recent study recorded more than 1300 species associated with the stony coral L. pertusa on the European continental slope or shelf. Filter feeding organisms like crinoids tend to use the corals as feeding platforms to position themselves higher up in the currents increasing their ability to capture food. Furthermore, as the skeleton of the coral can remain long after the corals’ extensive lifespan, cold-water corals are very likely to maintain benthic communities in different succession states at the same time.
Images: left: Finding shark eggs in cold-water coral habitats demonstrates how a key ecosystem function of coral reefs (habitat provision) co-benefits both sharks and humans. Centre: Cold-water corals are known as ecosystem engineers because, like trees, they change the environment by their own physical structure. Crabs (Gastroptychus sp.) using the black coral Leiopathes to position themselves higher up in the currents to optimize access to food. Right: Munida sarsi can benefit from the protection of gorgonians, scleractinians but also from piles of coral rubble. They use them as a home base that provides shelter against predators.
Unfortunately, the significant loss of live coral, owing to climate change and other anthropogenic disturbances, can cause severe losses to reef-inhabiting macroinvertebrates, especially mobile taxa, including crustaceans and crinoids that rely on the reefs as refuges from predation. A large number of video observations have not only documented the rich biodiversity of deep-sea ecosystems such as cold-water coral reefs, but also gathered evidence that many of these biological communities had been impacted or destroyed by human activities, especially by bottom trawl fishing. The reef-framework forming scleractinian corals build their skeletons from aragonite, the more soluble mineral form of calcium carbonate. Because of this, ocean acidification could cause these complex structures to collapse, resulting in the loss of macroinvertebrate habitat. Ocean acidification can also directly affect the physiology, reproduction, behaviour, neuronal functions and survival of many groups of marine organisms.
Global declines in biodiversity have ignited responses from local to international scales, aiming to establish management methods for the effective protection of species and ecosystems, and the limitation of human impact on the environment.
To date, decisions on area closures for the protection of ‘listed’ deep-sea habitats have been based on maps of recorded presence of species that are taken as being indicative of that habitat. However, large parts of the (deep) ocean have never been sampled or investigated, leaving large blank areas on the management maps. Conservationists, researchers, resource managers, and governmental bodies are increasingly turning to predictive species distribution models to identify the potential presence of species in areas that have not been sampled. Predictive habitat modelling may provide a useful method, but can only give an indication of the probability that a certain habitat will occur. Given the coarse resolution of the models, percentages should be taken as maximal figures, with habitat occurrence likely to be less prevalent in reality. Despite improvements in model algorithms, environmental data and species presences, there are still limitations to the reliability of these techniques, especially in poorly studied areas such as the deep sea.
The Coral Ecosystems Research Group at Heriot-Watt University (Edinburgh, Scotland) takes an integrated, interdisciplinary approach to understanding the biology and ecology of cold-water corals. This approach will be used in our new project to map the habitats associated with cold-water coral reefs and mounds at a series of north-east Atlantic sites, in collaboration with the Seafloor and Habitat Mapping Team of the NOC. Combining ecological, geophysical and hydrographical survey data, our study aims to achieve a more detailed quantification of the roles of the environmental factors that control the biodiversity patterns of macrofaunal species in the North-Atlantic. Data from ROV transects recorded during the 2012 Changing Oceans Expedition (RRS James Cook cruise 073) will be used in combination with data from acoustic sources gathered by Heriot-Watt University and NOC. A large spatial extent was covered, as this is a central requirement for the correct estimation of general patterns of species distribution.
Research on cold-water corals and their associated fauna is necessary to assess their ecological importance, while efforts to map and date these long-lived habitats are vital to develop sound scientific advice on sustainable habitat and fisheries management of deep-water.
1. Centre for Marine Biodiversity and Biotechnology, Heriot-Watt University
2. National Oceanography Center
3. School of Life Sciences, Heriot-Watt University
Vierod A.D.T., Guinotte J.M. and Davies A.J. (2013) Predicting the distribution of vulnerable marine ecosystems in the deep sea using presence–background models. Deep-Sea Research II, http:// dx.doi.org/10.1016/j.dsr2.2013.06.010.
Henry L.A., Navas J.M., Hennige S.J., Wicks L.C., Vad J. and Roberts J.M. (2013) Cold-water coral reef habitats benefit recreationally valuable sharks. Biological Conservation 161, 67–70.
Huvenne V.A.I., Beyer A., de Haas H., Dekindt K., Henriet J.P., Kozachenko M., Olu-Le Roy K., Wheeler A.J. and the TOBI/Pelagia 197 and CARACOLE cruise participants (2005) The seabed appearance of different coral bank provinces in the Porcupine Seabight, NE Atlantic: results from sidescan sonar and ROV seabed mapping. In Freiwald A. and Roberts J.M. (eds) Cold-water corals and ecosystems. Heidelberg: Springer-Verlag, pp. 535–569. The full reading list for this article is available at www.mba.ac.uk/marinebiologist