Climate Change Threatens to Dissolve the Ocean’s Deep-Water Corals
Courtesy Marine Conservation Biology Institute
“Black corals are one of the oldest continuously living organisms on earth,” J. Murray Roberts, a marine biologist with the Scottish Association for Marine Science, tells me eagerly. “Some are 4,000 years old.” They are but one of a family of corals, some of which, Roberts explains, may be 1.5 to 2 million years old. These ancient, fantastically branched, intricately sculptural sea creatures live not in balmy, sunlit tropical waters but in cold, dark ocean depths of up to 13,000 feet, where temperatures can drop to 39° F. “Far too deep for diving,” Roberts explains. The foundations of the deep-sea corals living today were laid down during the last Ice Age; some live on giant carbonate mounds first established in the Pleistocene Era. They are literally living fossils.
Deep-water corals are found all around the world – off the coasts of Scotland, Norway, the Pacific Northwest and South Atlantic coasts of North America, as well as in the waters off Australia, New Zealand, Africa, and South America. But now, just as they are beginning to be scientifically understood, these corals are at risk of disappearing, falling victim to swift destruction from fishermen’s trawling gear and a slow death brought about by ocean acidification, a process spurred by out-of-control carbon emissions.
These corals have been known since the 18th century. Linnaeus – the Swedish botanist often called “the father of biological taxonomy” – and his contemporaries wrote about them. So did 19th-century fishermen who trolled the seas off Scotland, Norway, and Canada. “Historically, a coral reef was a protrusion you didn’t want to run your boat into,” says Lance Morgan of the Marine Conservation Biology Institute, and not much more about their biology and ecology was known until the past few decades. Today they are being explored with remotely operated vehicles, submersibles, and multi-beam sounders that can map the ocean-floor environment. Yet because of their inaccessibility, even now relatively little is known about these diverse corals.
What is known is that the mound and reef structures of these corals are deep-ocean hubs of what biologists call “productivity” – the cycling and production of nutrients. This activity in turn attracts other sea animals – sponges, crustaceans, sea urchins, and snails among them – so that coral mounds become hot spots of biodiversity. This, Roberts explains, is part of deep-water corals’ importance to ocean life.
There is a great diversity of cold-water corals – about 1,300 different species have been identified in the northeastern Atlantic alone – and new species are just now being discovered. A single expedition, Roberts says, identified 349 deep-water coral species new to science. Adding to the complexity of understanding their role in the marine ecosystem – and the significance of any such species loss – is the fact that many deep-water corals are endemic, meaning that many are found only in one very specific location. This has led scientists to suspect these corals play an important, perhaps unique, part in their ecosystem.
What’s also known is that, given their extreme age and the nature of coral chemistry, which enables them to retain a precise chemical and temperature record, cold-water corals are what Roberts calls “an incredible archive of climate change, one that we’ve only begun to unravel.” As greenhouse gas emissions continue to rise, loading the global atmosphere with excessive amounts of carbon dioxide, the oceans – the world’s primary sink for atmospheric CO2 – are becoming increasingly acidic, a condition that is literally eating away at corals, including those that live thousands of feet beneath the surface.
This threat to the ocean’s deep-water coral is yet another example of the dangerous dynamics of global climate change. In acidifying the oceans, we are at once jeopardizing unique ecosystems and, by altering water chemistry and circulation patterns, running the risk of further altering the climate – creating still more of the “negative feedback loops” climatologists keep warning about. Acidification is also endangering the very biological assets that might hold valuable information about past atmosphere shifts, information that might allow us to better understand our current plight.
What makes deep-water corals particularly vulnerable to changes in ocean conditions, explains Roberts, is that they cannot move. Once they begin to grow, they are fixed in place, so if they’re to survive and thrive, they need to be where food can come to them and where temperature and chemistry favor their growth. The forces behind climate change – temperature, circulation, and chemical balance – are precisely those that determine corals’ growth and their ability to find and process food. Changing temperatures affect how nutrients and tiny organisms move through the deep ocean. Changes in circulation mean alterations in deep-water temperature and chemistry. In turn, such changes at the bottom of the sea influence what happens throughout marine ecosystems. As Roberts wrote in Science, “Shifts in deep-ocean circulation patterns profoundly affect global climate, and there is now evidence that cold-water corals have recorded these oceanic shifts in their skeletons.”
Now those skeletons are starting to break down. They’re suffering from lack of adequate calcium as if, Roberts says, the corals have osteoporosis. Corals rely on calcium carbonate to build their skeletons. Availability of this calcium compound depends on a balanced exchange of carbon dioxide between ocean and atmosphere. When CO2 enters the ocean, it dissolves and combines with water to form carbonic acid. Under conditions of normally balanced ocean chemistry – when the atmosphere is not overloaded with CO2 as it is now – this process results in free-floating carbonate ions becoming the calcium carbonate that corals, mollusks, and other marine organisms rely on to make skeletons and shells.
But now that the atmosphere has such a high concentration of CO2, this system has been thrown out of whack. Oceans are now absorbing so much CO2 from the atmosphere that this chemical reaction – which results in free carbonate ions – is not taking place as expected. As Richard Feely, chemical oceanographer at NOAA’s Pacific Marine Environmental Laboratory explains: If it’s going to absorb any more CO2, “the ocean needs to be less concentrated than the air above it.” This is no longer the case. There are now so many carbonate ions floating around in the seawater that instead of remaining on their own in a form that allows them to become calcium carbonate, these carbonate ions are crowding together and bonding with each other to form strongly acidic bicarbonate. “The precipitous decline in free carbonate ions is increasing bicarbonate levels on a logarithmic scale,” says Roberts.
Consequently, the oceans are now more acidic than they have been at any point in the past century. If present trends continue – as is likely, given atmospheric carbon’s longevity – the oceans could soon be more acidic than they have been in tens of millions of years, if ever, says Scott Doney, a marine biologist at the Woods Hole Oceanographic Institution.
© Juan Carlos/Oceana
Scientists worry that more acidic oceans will dissolve corals and imperil important deep-water coral food sources – plankton, zooplankton, and copepods that also rely on calcium carbonate to form their tiny skeletons. The oceans have absorbed so much CO2 that they’re approaching a saturation point. This makes calcium carbonate scarce and, Feely explains, “changes animals’ ability to form shells dramatically.”
That deep-water corals are suffering the effects of acidification indicates that oceans may rapidly be approaching the point where they cannot continue to absorb CO2 and support what’s considered a healthy diversity of marine life. Doney reports that given current levels of atmospheric carbon, the acidification may now be essentially irreversible.
What deep-water corals also reveal about the process of ocean acidification is where these changes are occurring. This information can then help us understand how climate change is affecting ocean circulation. “There are strong geographical patterns to how the oceans are acidifying,” explains John Guinotte, marine biogeographer with the Marine Conservation Biology Institute, a nonprofit based near Seattle that advocates for ocean protection. “Because colder water holds more CO2, high latitude waters at the poles are acidifying at the fastest rate,” he says. “And the Arctic is acidifying faster than the Southern Ocean.”
Feely points out that the finely tuned polar ecosystems most sensitive to the impacts of excess CO2 are those farthest removed from the sources. Because the majority of the world’s industry, power plants, and other fossil fuel emissions are released in the Northern Hemisphere, atmospheric circulation sweeps these gases toward the Arctic. There the exchange of CO2 between air and sea begins its lengthy journey to the bottom of the sea.
When trying to follow CO2’s journey to deep seas where cold-water corals live, it’s important to remember that atmospheric carbon can persist for years – even decades – and how correspondingly long these effects play out in the ocean. Doney explains that water becomes acidified where it was exposed to the surface. As it cools and evaporates, the surface water becomes saltier and heavier and begins to sink and travel with ocean currents. This is a slow process, Doney says, so deep water can be many years removed from contact with the surface. “As this deep water sloshes up onto the continental shelf, it brings acidic water with it. And just a little extra fossil fuel carbon can put things over the edge.”
This is not good news for deep-water corals. As Guinotte explains, these beautiful and complex creatures evolved to grow in cold, deep, nutrient-rich waters, and they grow very slowly. Northern seas, where a great many of these corals are found, are loaded with CO2. “Stony corals in the North Pacific are now surviving but not flourishing like they are in other areas of the world,” Guinotte says. “And this could be the future if we keep pumping CO2 into the atmosphere at the rate we are.”
What Guinotte and his colleagues are now trying to determine is how the relative current carbonate saturation of different ocean areas will affect different species of deep-water coral. Part of the reason this is a key piece to understanding the cold-water coral puzzle is that different corals use different forms of calcium carbonate to build their bones and branches. There are several different types – aragonite, calcite, and high magnesium calcite – and each has a different solubility under high CO2 conditions.
“We don’t yet know if they’re opportunistic or species specific,” says Guinotte of the different forms of calcium carbonate. “And we don’t yet know if they [the corals that use a specific type] can adapt to changing conditions.” Given the great diversity of deep-water coral species and that so many of these species appear to be site-specific and limited in range, a change in conditions that foster a particular coral’s growth could have significant ramifications – not only for the coral, but also for the other species that depend on it for habitat and nutrient cycling.
The changes we’re now seeing in ocean conditions, says Roberts, are unprecedented, and are likely to have dramatic impact for species like cold-water corals that live on a geologic timescale. Or as Morgan puts it: “Ocean acidification is a geologic time showstopper.” If we lose these corals, we may lose essential deep-sea anchors of biodiversity. In doing so, we will also lose the longest-living records of Earth’s changing climate and chemistry, fossils that may reveal key details to understanding how the planet will respond to the changes now underway.
© Juan Carlos/Oceana
So what do we do? Can we stop the destruction? If so, how?
Morgan and Guinotte remind me that while we’re busy burning coal and gas, loading the atmosphere with CO2 and other greenhouse gasses at a great clip, what destroys deep-sea corals even faster than ocean acidification is bottom trawling. This type of fishing gear is dragged over the sea floor, literally bulldozing whatever it encounters. Unlike atmospheric carbon, which takes years or decades to dissipate, bottom trawling can stop immediately. With the aim of implementing an international ban on deep-sea bottom trawling, the Marine Conservation Biology Institute co-founded the Deep Sea Conservation Coalition and began UN negotiations among fishing nations. No such agreement has yet been reached, but there are now marine reserves and protected areas around the world where bottom trawling is prohibited.
“Resiliency is key,” Morgan says. “Reserves make a lot of sense when it’s hard to nail down which incremental stress is going to be the straw that breaks the camel’s back.” Reserves, he explains, can help preserve a full complement of biodiversity while limiting the introduction of additional elements of stress.
Closing sensitive benthic habitat to destructive bottom trawling will be essential to preserving corals and fish habitat, says Guinotte. “No matter how remote the location, if you talk to scientists who go down in submersibles, there is trawler evidence.” Yet, he says, “the devil is in the details,” explaining that United States National Marine Sanctuaries are “multiple use” areas, with regulations specific to each area. In some, trawling and fishing are restricted, but in others they are not. Some of these regulations are federal, some are state, and some more local.
Meanwhile, in late January, the US Environmental Protection Agency agreed to review how ocean acidification may be addressed under the federal Clean Water Act. At the same time, a panel of over 150 scientists working with UNESCO issued a statement calling for immediate action by international policy-makers to reduce CO2 emissions to avoid severe acidification damage to marine ecosystems.
“There is no magic bullet to turn around ocean chemistry,” cautions Guinotte. “So we have to look at creating protected areas and refugia.”
Listening to these scientists talk about deep-water corals, I find myself thinking about fossils in a race against the clock. The pace at which we are changing ocean chemistry is happening in a time frame utterly alien to these mind-bogglingly intricate deep-sea corals. When I ask Murray Roberts why it is so important to understand what’s happening to these animals hardly anyone will ever see, he asks a question back: “Why does our generation have a right to stop the next generation from having a positive environment? Some of these corals have never been seen before. Does our generation have the right to remove them?”
Elizabeth Grossman is the author of High Tech Trash. Her next book, Redesigning the Future, will be published this year.