The pioneering Arctic coral expedition
Corals are not only found in warm shallow tropical waters, in fact they also live at depths ranging from 40 metres right down to 2,0000 metres in water temperatures as low as 4 degrees Celsius. Unlike tropical corals, cold-water corals don’t have symbiotic algae living in their polyps, as such they do not need sunlight to survive. They feed by capturing food particles from the surrounding water and so their polyps are often larger than tropical corals. Specifically, these corals are dependent on particulate organic matter (POM) and zooplankton for their food and energy. Organic matter (dead algae, faeces, dead animals etc.) produced in the sunlit zone sinks in the water column and transports carbon and other biogenic materials downwards. This POM becomes a food-source for bacteria and more complex organisms living in the water column and on the sea bed. As the particles sink they can aggregate and accumulate to form what has been described as floating blizzards, or marine snow.
Corals are home to many species
Corals, whether in tropical or cold-water, can build hard calcium carbonate skeletons as the grow, forming giant and complex reefs. The reefs take thousands of years to build as coral usually only grows a few millimetres or centimetres per year. Cold-water coral reefs are important biodiversity hotspots and provide a habitat for a variety of marine invertebrates including sponges, worms, molluscs (mussels etc.), echinoderms (starfish etc.) and crustaceans (crabs and lobsters etc.). The coral reefs also provide shelter for organisms to live on, in and around. They also provide important nursery areas for various fish species in the deep sea.
Ideal habitats for cold-water corals
Cold-water corals are typically found in places where the current flow is accelerated as well as on the continental shelf and in deep sea areas with topographic heights that include seamounts, mounds, ridges and pinnacles. The most spectacular reefs are constructed by stony corals, which form colonies that vary in size from small, scattered colonies no more than a few metres in diameter to vast coral reef complexes that are several tens of kilometres across.
The commonly found cold-water coral
The most common habitat-forming, reef-building cold-water coral is the Lophelia pertusa that forms bush-like colonies that measure several metres across and consist of thousands of coral polyps. As the colony develops, adjacent branches typically join together to strengthen the overall framework. The northernmost Lophelia occurs in the southwestern Barents Sea. Scientists have determined that the spread of Lophelia northwards occurred during the climatic warming that followed the last glacial period, with radiocarbon dating indicating the coral made its way in the Arctic circle waters around 8,600 years ago.
Temperature and salinity limits of Lophelia
Knowledge of temperature and salinity limits for cold-water corals is lacking significantly. Historically, few relevant measures have been made on cold-water corals nor has a great deal of research been undertaken. What is known is that the Lophelia tolerates temperatures between 4-13 degrees Celsius with some evidence from studies of the species in the Faroe-Shetland Channel indicating that the species can tolerate even sub-zero values for a short period of time.
The Arctic coral reef
In the Arctic circle, the waters of continental shelf off Lofoten, Vesterålen, and Troms is one of the world’s largest known reef complex. In this area, the marine landscape varies dramatically, from 0 to 3,000 metres with shallow banks and plains alternating with deeper troughs formed during the last glaciations. Along the shelf are banks and plains, with water depths that vary from around 40 metres to 200 metres, along with troughs that have depths of 200-300 metres. In the troughs, around 330 coral reefs, made up of Lophelia occur in the south-eastern part of the trough in water depths of 200-270 metres. The Røst Reef as it’s known can be considered by scientists as both geomorphic features and biogenic structures. The reefs vary in length from 31 metres right up to 334 metres-long, and 27-114 metres wide and between 4-17 metres high. Most of the reefs are smaller than 100 metres in the longest direction whereas only 81 reefs are longer than 200 metres. Meanwhile, chains of coral reefs in the area can up to 500 metres-long. The shape of the reefs changes from circular to elongated with increasing size and age.
The pioneering Arctic coral expedition
While cold-water corals have been known about for a few centuries the observation and study of cold-water coral in their natural habitats has only begun over the past decade or so due to technological advances in manned and robotic submersibles. One of the pioneering expeditions to find out more about the Arctic marine environment and coral reefs was made in 2007 when scientists from various countries went on a month-long expedition aboard RV Polarstern, a research vessel run by the Alfred Wegener Institute for Polar and Marine Research in Germany. The expedition provided scientist the opportunity to investigate cold-water reef ecosystems. The ice-strengthened ship carried a two-person submersible called Jago, on loan from the Leibniz Institute of Marine Sciences at the University of Kiel, enabling researchers to undertake the first manned submersible operations to the Røst Reef, the world’s largest reef found in cold-waters. The submersible had a grabber, sampling container, digital-camera and a high definition video camera enabling scientists to take samples at depths of 300-380 metres.
Taking a range of samples
Scientists analysed the samples collected from the Arctic expedition and found the samples to be rich in diversity and showed an abundance of species living in the habitat; showing scientists that cold-water reefs were hot-spots of biodiversity in the deep, cold waters of the Arctic. In addition to biologists, geologists analysed the area with sonar systems to visualise the sea bed. This provided the ability for scientists to determine the reef structure and the surrounding sea bed. Oceanographers also took water samples to measure temperature, salinity, oxygen and pH with a CTD-sensor (Conductivity-Temperature-Depth).
Corals in the Alaskan Arctic
More recently in 2014, Scientists from the National Oceanic and Atmosphere Administration (NOAA) conducted a first-ever, large-scale survey targeting corals on the Bering Sea Slope. In this sea, the Bering Sea starts sloping down into the abyss. Most of the slope makes a gradual descent however in several places canyons interrupt the regular topography and dig deeply into the slope and shelf. One of these canyons, the Pribilof Canyon, is so broad that it covers around 10 percent of the entire Bering Sea continental slope. Parts of these canyons have coral communities that are an important habitat for a variety of fish species and crustaceans. To understand these corals more, the scientists dropped a video camera along 300 transects, allowing them to identify coral species, estimate their abundance, and because the camera is stereoscopic, measure their dimensions to gauge the importance of deep-coral as a structural habitat for fish and crabs.
A Bering Sea coral hotspot
Scientists recently found evidence that a canyon in the eastern Bering Sea shelf region – the Pribilof Canyon – is truly a biodiversity hotspot with the canyon containing half of the soft corals and sponge habitat found in the eastern part of the sea. Specifically, scientists have found that the one canyon contains over 50 percent of the estimate high-quality deep sea coral habitat and 45 percent of sponge habitat, despite making up only 1.7 percent of the entire study area. To determine these figures the researchers used a computer model to find the best locations for coral and sponge habitat in the Bering sea with data provided from bycatch records, NOAA surveys, Greenpeace submersible expeditions as well as physical oceanographic data including temperature, slope and current speed with showed the Pribilof Canyon was an ideal location for the coral to thrive in.