How will cold-water corals fare in a Changing Ocean?

17 September 2012

Out of sight in the deep oceans, cold-water corals are facing yet another threat. As the chemistry of the oceans changes due to increasing CO₂ in the atmosphere, these coral ecosystems, home to thousands of other species, may be unable to continue growing or even survive. The Changing Oceans Expedition set out to the North Atlantic during the summer to find out what the future holds for these amazing animals.

Ocean acidification, often referred to as the ‘the other CO₂ problem’, is arguably the biggest threat facing marine calcifying organisms today. The amount of CO₂ in the atmosphere has increased exponentially since the industrial revolution, with much of this CO₂ dissolving into the oceans. The problem with this is that dissolving more CO₂ into the sea decreases its pH, produces more hydrogen ions and hence increases it acidity. Currently, the pH of the sea is about 8.1, but increasing atmospheric CO₂ levels are predicted to potentially drop this by about 0.3 pH units by 2100.Whilst it may not sound dramatic, this small drop in pH can have potentially huge implications for the vast number of marine calcifying organisms, such as calcareous algae, shell producing animals, and coral. This is because increasing concentrations of dissolved CO₂ in the oceans decrease the carbonate saturation of the water, which means there are less carbonate ions available for corals to make their calcium carbonate skeletons.

In May 2012, the RRS James Cook set sail for the North Atlantic, with an international team of scientists onboard as part of the Changing Oceans Expedition. The mission was to examine the potential impact of ocean acidification and warming on cold-water coral reefs and the associated reef-creatures. Hidden in the depths of all the world’s oceans, cold-water corals form vast mounds and reefs, with the complex 3D structures supporting thousands of species, including commercial fish species. Although they grow much slower than their tropical counterparts, the calcium carbonate skeletons that cold-water corals create can form extensive reefs which persist even after the animal itself has died. In fact, the Lophelia pertusa reefs off Norway which cover approximately 2,000km², are larger than tropical reefs in the Seychelles, Belize or Mozambique.

In the 4 weeks at sea, a range of cold-water coral sites were visited, from the ‘shallow’ reefs of Mingulay in the Outer Hebrides, dominated by Lophelia pertusa, to the reefs on the Logachev mounds, spectacular with both Lophelia pertusa and Madrepora oculata at nearly 1000m depth. Little is known about both species of corals and the ecosystems they form, because of their inaccessibility – you can’t just dive down and explore the reefs like you can in the tropics. Before exploring the reefs and collecting samples could begin,the location of the reefs had to be identified. Once at each site, advanced acoustic techniques, such as multibeam and sidescan sonar systems, were used to dynamically image the seabed and to identify possible coral reefs that form mounds growing up from the seafloor. The Remotely Operated Vehicle (ROV) Holland I was then deployed to examine these ecosystems in greater detail.

During each ROV dive, the excitement is the lab was palpable, as high definition images of the spectacular reefs beneath were beamed back from cameras to the ship. Fish darted across the screen as the robot ran along its transect, and unusual sponges and crabs came into view, causing a buzz in the lab. But it wasn’t all about watching for the team of scientists; short-term experiments were undertaken to examine the effect of ocean warming and acidification on the growth and overall health of the corals. Along with longer term experiments underway at Heriot-Watt University, this will help to determine whether corals can adapt to such changes, or whether it will be impossible for them to survive. Using coral samples carefully collected with the robotic arms of the ROV, Lophelia pertusa and Madrepora oculata were maintained in specially designed tanks for the duration of the cruise. The temperature and CO₂ levels in these ‘mini-oceans’ were manipulated to mimic future conditions, so a 3°C increase and a near-doubling of atmospheric CO_{2. R}espiration and growth rates of these corals were then measured over a 3 week period, using an optode system and radioisotopes, respectively. Concurrent experiments looked at changes in other aspects of the coral’s biology in response to their changing environment, including protein expression, microbial communities and DNA:RNA ratios as a proxy for health. Together, these parameters will allow us better prediction of how corals will respond to global climate change.

Alongside the ROV campaign, a host of other activities took place out at sea. One such activity was the CTD and SAPS deployment. CTD stands for conductivity, temperature and depth; the parameters that the instrument measures at the bottom of the ocean. Attached to the CTD frame was also a SAPS (Stand Alone Pumping System), which is a big pump attached to a filter rig with a delayed timer. SAPS was used to look at the amount of particulate organic carbon (coral food) that is reaching the reefs. The pump switches on when it is at the bottom of the ocean, and records how much water flows through the filters. Following laboratory analysis of how much carbon is on the filters, how much carbon the corals have access to can be calculated. This information, combined with surveys of the reef and CTD data, can help improve understanding of why the corals live where they do, and what any future changes in climate and currents may have on these ecosystems.

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