Sebastian Hennige of Heriot-Watt University Releases Research About World’s Cold-Water Corals

The very mention of coral reefs instantly conjures images of tropical seas, white sandy beaches and palm trees; you may have even heard of tropical coral reefs being referred to as the rainforests of the sea. Whilst all of this may be true, emerging evidence has highlighted that cold-water corals also support a vast array of life, and are found in many of the World’s oceans such as the Atlantic, Mediterranean, Indian and Pacific. Yet, as Sebastian Hennige of Heriot-Watt University explains, despite being less well known, these cold-water corals have a much larger distribution range than tropical corals, and have been found to be much more diverse.

Hidden in the depths

One of the reasons why their importance was not appreciated until recently is that cold-water corals and the mounds they form are at great depths and made even more inaccessible by fast currents. Cold-water corals do not contain symbiotic algae (like many of their tropical counterparts), and thus need no light to thrive. Consequently, they have been recorded at depths down to 3000 m which obviously presents logistical problems in fully characterising these habitats.

The reason why these coral mounds and the structures they form support thousands of species, including commercial fish species, is that just like their tropical counterparts, they create calcium carbonate skeletons which persist even after the animal itself has died. Skeletons of these live corals bind together and form complex, and sometimes very large structures. In fact, the Lophelia pertusa reefs off Norway which cover approximately 2,000 km2, are larger than tropical reefs in the Seychelles, Belize or Mozambique. These cold-water reefs provide a complex habitat in an environment where other three-dimensional structures are scarce and as such provide a home for many creatures from sponges to polychaete worms, and fish species such as ling and cod.

Unique reef discovered

At Heriot-Watt University in Edinburgh, the research team headed by Dr. Murray Roberts and comprising of Dr. Sebastian Hennige, Dr. Laura Wicks, Dr. Juan Moreno Navas, Dr. Lea-Anne Henry and three PhD students, is dedicated to studying the biology and ecology of these fascinating reefs, and in particular, those formed by Lophelia pertusa. In 2003, Dr. Roberts discovered and mapped a Lophelia reef complex 130 m deep off the isle of Mingulay in the Outer Hebrides (Figure 3 here), which is the only known coral reef on the coastal shelf around the UK. As such, it provides us with a unique opportunity to study the ecology and importance of UK coastal L. pertusa reefs in comparison to deeper reefs. Over the years since it was discovered the Mingulay Reef Complex has been one of the most intensively researched cold-water coral reefs in the world. In 2006 a series of seabed moorings and landers were used to record the complex water flows across the reefs. This work revealed that the area was swept by tidal currents and that every 6 hours on the turn of the tide the reefs were washed by warmer, food-rich water brought down from the surface. By tying these field observations together with the work on the physiology of the corals themselves a much clearer picture of how cold-water coral reefs function has been developed.

However, just as we begin to understand their basic biology, we are realising the damage that cold-water corals have been subjected to in the past decades by human activities such as bottom trawling. There is even a paper published in 1915 by the French biologist Joubin, called “Deep-water corals, a nuisance for trawlers”, as these deep reefs often fouled and shredded trawler nets. But, since then, trawl gear has increased in size and strength and in many cases bottom-trawling nets have been dragged directly through coral mounds, destroying whole tracts of the reef and the habitats for a diverse array of fauna. Recovery (if any) of these reefs can then take centuries, as cold-water corals grow extremely slowly (about 4 – 25 mm per year). This is why it is crucial to map the occurrence of reefs to prevent the loss of these habitats before we even realise they are there. Norwegian scientists have estimated that up to half of the cold-water coral reefs in their waters have already been damaged, thus, it is crucial that we increase our knowledge of these reefs before we unknowingly damage any more.

Technology aids mapping

The techniques for mapping and sampling these often deep, cold-water coral reefs are becoming more advanced every year both in terms of mapping where the reefs are and identifying individual colonies for sampling. The team at Heriot-Watt uses advanced acoustic techniques such as multibeam and sidescan sonar systems to image, dynamically, the seabed and to identify possible coral reefs that form mounds growing up from the seafloor. Once large areas of the seafloor have been mapped and possible reefs identified, Remotely Operated Vehicles (ROVs) or manned submersibles are sent down to examine these ecosystems in greater detail. Images are then beamed back from cameras to the ship to select specific areas for sampling. Landers can then be deployed on sites of interest where they will carry out a pre-determined work programme to monitor and record various seawater parameters, such as temperature, flow speed, oxygen flux and seawater chemistry. When they have completed their programme, they alter their ballast and return to the surface to be collected. These landers can be deployed for up to a year and give an unrivalled picture of long-term ocean dynamics of these deep-sea habitats over this timescale.

By Sebastian Hennige

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Source: imarest  ,May 12, 2011; Image: coralscience