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How will climate change impact cold-water corals? Mostly through food loss, study says

Cold water corals in the deep sea.

Cold water corals in the deep sea. Image by Submarine Ring of Fire 2002, NOAA/OER via Flickr (CC BY-SA 2.0).

  • A new study warns that cold-water corals, also known as deep-water corals, could be most impacted by a decrease in food supply as climate change shifts the dynamics of the planet’s oceans.
  • The authors came to this conclusion by examining how cold-water corals survived the last major period of global warming that occurred at the end of the last glacial period and the start of the current interglacial period, which is somewhat analogous to how the Earth is projected to warm by the end of this century.
  • However, experts point out that cold-water corals today are subjected to a number of additional stressors, including ocean acidification, destructive fishing practices, and pollution, and that the climate is changing far more rapidly than it did in the past.
  • Cold-water corals are considered to be equally important — or perhaps even more important — than tropical corals, which makes understanding their chances of survival of the utmost importance, researchers say.

Between 1869 and 1870, the H.M.S. Porcupine sailed into the North Atlantic Ocean and scraped a dredge along the seafloor. When the sailors pulled the dredge back to the surface, it held pieces of stony coral that lived in the cold, sunless depths of the sea.

Examining the coral back on land, English paleontologist Peter Martin Duncan noted in a report, with some wonder, that these corals could live as easily in the deep, cold ocean as well as other organisms could in the warmer, shallower parts of the sea. “It suggests that a great number of the Invertebrata are not much affected by temperature,” he said, “and that the supply of food is the most important matter in their economy.”

Today, the ocean is a very different place than what it was when the H.M.S. Porcupine set sail. Climate change is causing global ocean temperatures to rapidly rise, while lowering oxygen levels and acidifying the water. The seas are also being damaged by overfishing, pollution, and other human activities like transportation. But according to a new study published in PLOS Biology, the biggest threat to cold-water corals, like the ones scooped up by the H.M.S. Porcupine all those years ago, is a lack of food. As it turns out, Duncan’s observations appeared to be correct.

Large colony of the cold-water coral Lophelia pertusa colonised by crinoids and soft corals at 700 m water depth (Porcupine Seabight, Irish continental margin). Credit ROV image: MARUM ROV Cherokee, Bremen, Germany.

How will climate change impact cold-water corals?

Cold-water corals, also called deep-water corals, don’t garner the same amount of attention as tropical corals living in warmer, shallower water. This isn’t surprising when you consider that cold-water corals tend to be found hundreds or even thousands of meters below the surface, beyond the reach of the sun. Tropical corals, on the other hand, are situated in the photic zone of the ocean, easily accessible by snorkelers and scuba divers.

Besides being visually accessible, tropical corals are known to support an abundance and diversity of marine life. In fact, it’s estimated that tropical corals support about 25% of fish and other marine species living in the global ocean. Yet researchers argue that cold-water corals are equally important to the marine ecosystem, providing food and habitat for a diversity of species. Murray Roberts, a marine biologist and cold-water coral expert at the University of Edinburgh, even argues that cold-water corals could be more important than their tropical counterparts.

“When you add up all of the types of animals we call coral — not just the reef-building corals, but the black corals, the bamboo corals, the whip corals — more than 50% of the species are found deeper than 50 meters [164 feet],” Roberts, who was not directly involved in the recent study, told Mongabay. “So in fact, corals in general are a phenomenon of deeper waters, but we tend to think of them as a phenomenon in shallow waters.”

The new study probes a central question: what will happen to cold-water corals as the planet warms and shifts due to climate change? Since it’s difficult to track the demise of cold-water corals in the hard-to-access deep sea, the authors looked for answers in the past, starting at about 20,000 years ago, when the world was just beginning to warm up at the end of the last glacial period. The warming that occurred during the start of the current interglacial period generated similar conditions to the warming that is expected to happen by the end of this century, the authors say.

Using sediment samples collected from six cold-water coral sites in the North Atlantic Ocean and the Mediterranean Sea, the researchers tried to reconstruct ocean conditions and record the abundance of Lophelia pertusa, a reef-building coral found in these parts of the ocean. Their analysis suggested that it wasn’t temperature change that determined the survival of cold-water corals, but food supply. This appears to be in contrast to tropical corals that are heavily impacted by rising sea temperatures, which can force them to expel their life-sustaining zooxanthellae, weakening them and turning them that characteristic white that’s become associated with climate change.

Deep-water sea fans (Plumarella pellucida) with a basket star at 150 meters (492 feet). Image by NOAA Exploring Deep Sea Coral Expedition 2010 via Flickr (CC BY 2.0).

Lead author Rodrigo da Costa Portilho-Ramos, a scientist at the University of Bremen, Germany, explained that during the change that occurred at the start of this interglacial period, the temperatures at the cold-water coral sites didn’t actually appear to fluctuate that much. But the changing climate would have altered ocean circulation and stratification, which likely prevented food from reaching some of the cold-water corals. And the same thing could happen in the future, he and his co-authors say.

“Shallow corals have symbionts that produce food for them and they also get food from the environment,” Portilho-Ramos told Mongabay. “But cold-water corals don’t have symbionts — they are really dependent on the transport of food to them. If [food dynamics] change because of climate change, they can be damaged or they can die.”

The study also found that deoxygenation was a stressor for L. pertusa during the last warming period, causing the coral to die when oxygen dropped below a certain level. But as the authors point out, this finding is in conflict with the fact that researchers have recently discovered cold-water coral colonies thriving in low-oxygen, or hypoxic, conditions in some parts of the ocean.

Portilho-Ramos said the study’s findings are particularly important for modeling purposes, especially since a lot of previous laboratory-based research on cold-water corals have mainly examined the impacts of temperature and pH, but have largely overlooked food. However, he added that a growing amount of research is looking at the role of food in cold-water coral survival, including a study that found that cold-water corals could adapt to climate change stressors like temperature change and acidification if they had enough food to nourish them.

“Our paper suggests that food has to be also manipulated in the laboratory, not only to feed the organism, but to manipulate the quality of food, the amount of foods and the effect of foods [when] corals compensate for [changes in] temperature or low oxygen water,” he said.

Squat lobster climbing a colony of the cold-water coral Lophelia pertusa at ~500 m water depth (Gulf of Mexico). Credit ROV image: MARUM ROV Cherokee, Bremen, Germany.

‘Unprecedented rates of warming’

Roberts said the new paper is helpful in broadening our understanding of why cold-water corals occur in certain parts of the ocean. However, he added that the climate is changing faster than it did at the start of the interglacial period, which makes the future for cold-water corals “enormously uncertain.”

“We’re seeing unprecedented rates of warming, and then critically, acidification of huge areas of the global ocean, so the rates of change are very, very much faster,” he said. “Fish can move very quickly, and we’re seeing fish populations migrate with changing water temperatures. Corals, one could argue, could do the same, but that will involve a process of larval release, dispersal, settlement growth, and so forth, into areas that are suitable, so we’re in very uncertain times.”

Besides needing to adapt to a rapidly changing climate, cold-water corals have to deal with other pressures such as trawling and other fishing practices that destroy deep-water habitats, Roberts said. Additionally, there is the looming threat of deep-sea mining operations that are looking to target the metal-rich crusts found on seamounts and ridge systems, which are ecologically important structures that support a variety of deep-water corals and other species, he added.

“It is a tremendous concern that direct damage will be enormously destructive, but then the tailings produced from this mining operation will be released and they will disperse through the water column,” Roberts said. “Corals [reproduce] by spawning — they produce larvae that then spread through the water column — but if you load that water column with mining spoil, those larvae have never seen that in their evolutionary history, they’ve got no reason to, there’s no way they can be adapted to it. So one of the big worries is that [deep-sea mining] will wipe out vast areas of the water column, and anything that migrates through that water column from the largest whale to the tiniest coral lava. So I think we’re really dicing with fire here.”

Deep-sea mining in international waters could begin as early as next year, as mining regulators work to approve a set of rules to govern future operations.

Live polyps of the cold-water coral Lophelia pertusa at 700 m water depth (Porcupine Seabight, Irish continental margin). Credit ROV image: MARUM ROV Cherokee, Bremen, Germany.

The authors of the new study also point out that they haven’t taken ocean acidification into account, since the last major warming event considered in the paper “does not provide any equivalent change in ocean pH conditions as it is expected until the end of this century.” Yet, there is a sufficient amount of research that suggests ocean acidification will have a devastating impact on cold-water corals, since acidic waters decrease the availability of the carbonate ions that corals need to produce their skeletons.

“We do consider ocean acidification to be a threat to all calcifying organisms,” Andrew Wheeler, a marine geologist and cold-water coral expert at the University College Cork, Ireland, who was not involved in this study, told Mongabay. “We know that on the European side of the Atlantic, the acidification of cold-water corals is not so bad, actually. But on the American side of the Atlantic, and in the Pacific, acidification is a threat and these corals are already in waters that are too acidic.”

Ocean acidification is considered to be such a prominent threat that it’s included as one of the nine planetary boundaries that, if breached, can destabilize Earth’s life support systems, depending on whether a buildup of environmental pressures leads to a crossing of the critical threshold. While the theory suggests that a global boundary for ocean acidification has not yet been fully breached, ocean acidification is getting progressively worse across the world’s oceans as climate change — another planetary boundary — accelerates.

If we do lose cold-water corals due to the dynamics of climate change, Wheeler said, there will be a marked decrease in the abundance and diversity of marine organisms, leading to large-scale changes in the ocean. He added that the study in PLOS Biology provides important insights into the possible impacts of climate change on these vulnerable ecosystems.

“The study clearly shows that this food supply and oxygen are key factors that determine whether these cold-water coral decline or thrive,” he said. “And that’s really important because it means that we have an understanding of what’s driving them, so we can look at how to manage them in areas that are threatened.”

Banner image: Cold water corals in the deep sea. Image by Submarine Ring of Fire 2002, NOAA/OER via Flickr (CC BY-SA 2.0).

Citations:

Büscher, J. V., Form, A. U., & Riebesell, U. (2017). Interactive effects of ocean acidification and warming on growth, fitness and survival of the cold-water coral Lophelia pertusa under different food availabilities. Frontiers in Marine Science, 4. doi:10.3389/fmars.2017.00101

Duncan, P. M. (1873). A description of the Madreporaria dredged up during the expeditions of H.M.S. ‘Porcupine’ in 1869 and 1870. The Transactions of the Zoological Society of London, 8(5), 303-344. doi:10.1111/j.1096-3642.1873.tb00560.x

Portilho-Ramos, R. D., Titschack, J., Wienberg, C., Siccha Rojas, M. G., Yokoyama, Y., & Hebbeln, D. (2022). Major environmental drivers determining life and death of cold-water corals through time. PLOS Biology, 20(5), e3001628. doi:10.1371/journal.pbio.3001628

Steffen, W., Richardson, K., Rockström, J., Cornell, S. E., Fetzer, I., Bennett, E. M., … Sörlin, S. (2015). Planetary boundaries: Guiding human development on a changing planet. Science, 347(6223), 1259855. doi:10.1126/science.1259855

Elizabeth Claire Alberts is a staff writer for Mongabay. Follow her on Twitter @ECAlberts.

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