Cold corals in hot water?

Cold-water corals face an uncertain future as increasing CO2 in the atmosphere changes the chemistry of our oceans. Laura Wicks set sail on the Changing Oceans Expedition to find out how these amazing animals are likely to fare.

Hidden deep in all the world’s oceans are the vast mounds and reefs of cold-water corals. They grow much more slowly than their tropical counterparts but can still extend over huge areas: reefs of Lophelia pertusa off Norway cover around 2000km2 – more than tropical reefs in the Seychelles, Belize or Mozambique. The corals build their skeletons from calcium carbonate dissolved in the seawater, creating complex 3D structures that persist even after the animal itself has died. These skeletons support thousands of species, including many that we eat.

But the life of these hidden corals is under threat.

The amount of CO2 in the atmosphere has increased exponentially since the Industrial Revolution and much of it is dissolving into the oceans, slowly increasing the acidity of seawater. The pH of the sea is currently about 8.1, but this is predicted to drop by about 0.3 pH units by 2100. It doesn’t sound like much, but this small change can have huge implications for marine organisms that rely on calcium carbonate, like corals, shell-producing animals and calcareous algae. This is because the increasing concentration of dissolved CO2 in the oceans decreases the carbonate saturation of the water, so there is less carbonate available for coral skeletons and shells.

This acidification of the oceans, often referred to as ‘the other CO2 problem’, may be the biggest threat facing marine calcifying organisms today.

In May 2012, I set sail for the North Atlantic on the RRS James Cook, part of an international team of scientists on the Changing Oceans Expedition. Our mission was to examine the potential impact of ocean acidification and warming on cold-water coral reefs and the creatures they support.

In our four weeks at sea we visited a range of cold-water coral sites, from the ‘shallow’ reefs of Mingulay in the Outer Hebrides (around 130m deep), dominated by Lophelia pertusa, to the Logachev Mounds, west of Ireland; nearly 1000m deep and spectacular with both Lophelia pertusa and Madrepora oculata. We don’t know a great deal about either species of coral, or the ecosystems they form, because they are so inaccessible – you can’t just dive in and explore them like you can in the tropics.

Before we could even think about collecting samples we had to find the reefs. To do this we used advanced acoustic techniques, such as multibeam and sidescan sonar, that use sound waves to create an image of the seabed from which we could pick out possible mounds of coral. The next step was to send down our robot, the Remotely Operated Vehicle (ROV) Holland I, to take a closer look.

During each ROV dive the excitement in the lab was palpable. The ROV’s high-definition cameras beamed up spectacular images of the reefs beneath us. Fish darted across the screen and unusual sponges and crabs came into view, causing a buzz as we tried to work out exactly what they were.

But we weren’t just there to watch. As part of ‘Team Coral’, I carried out short-term experiments to see what effect ocean warming and acidification have on the growth and overall health of the corals. Using samples carefully collected by the ROV’s robotic arms, we kept Lophelia pertusa and Madrepora oculata in specially designed tanks for the duration of the cruise. We manipulated the temperature and CO2 levels in these ‘mini-oceans’ to mimic one possible set of future conditions, in this case a 3°C increase in temperature and a near-doubling of atmospheric CO2. We then measured the respiration and growth rates of the corals over three weeks. My team-mates looked at how other aspects of the corals’ biology responded to their changing environment, including microbial communities and protein expression. All of an animal’s biological processes, such as growth and respiration, are controlled by proteins, so changes in the concentration of various proteins give us a clue as to how processes like calcification may respond to increased temperature and ocean acidification in the future.

Along with longer-term experiments under way at Heriot-Watt University, these will help us to work out how the corals will respond to global climate change – whether they can adapt, or whether ultimately it will be impossible for them to survive.

Alongside the ROV campaign, a host of other activities took place out at sea. One was the deployment of the CTD and SAPS. CTD stands for conductivity, temperature and depth, which this particular instrument measures at the bottom of the ocean. Attached to the CTD frame we had a SAPS – Stand Alone Pumping System – which is a big pump with a filter and timer.

We use the SAPS to look at the amount of particulate organic carbon (coral food) that is reaching the reefs. When it reaches the seabed the pump switches on and records how much water flows through its filters, which capture the organic carbon. Back in the lab, we analyse the amount of carbon on the filters and the water flow, to calculate how much food the corals have access to. Combined with surveys of the reef and CTD data, this information can help us understand why the corals live where they do, and how any future changes in climate and currents may affect these ecosystems.

Four weeks at sea passed by in a flash, and everyone on board collected a wealth of information. Now we’re all back on dry land, it’s time to process samples, extract data and try to understand what the future holds for these cold-water creatures as our oceans change.

More information
Dr Laura Wicks is a postdoctoral researcher at the Centre for Marine Biodiversity and Biotechnology, Heriot-Watt University. Email: L.Wicks@hw.ac.uk.
The Changing Oceans Expedition is part of the UK Ocean Acidification Research programme.
http://www.changingoceans2012.blogspot.com

Natural Environment Research Council, 23 November 2012. Article.

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