Image courtesy of NOAA
The range of creatures in imminent danger from this hazard includes mollusks and crustaceans such as clams, oysters, lobsters, and crabs; large sea creatures for which shellfish is a dietary staple, notably seals, otters, and walruses; and most worrisome of all, plankton and other microscopic organisms that sustain mighty whales and fish big and small.
To make matters worse, German and Japanese researchers recently increased CO2 levels in seawater and found that the greenhouse gas can damage some marine organisms directly: Squid slowly asphyxiated as the excess CO2 crowded out oxygen in their blood, and fish embryos and larvae were abnormally small and less likely to survive.
Dissolving the Coral Reefs
Also endangered by rising acidity are coral reefs, home to an astonishingly diverse range of aquatic life. Though reef resembles rock, it is actually made up of a teeming city of anemone-like creatures known as polyps. These tiny organisms wave their tentacles in the currents to snatch tidbits of food, all the while secreting shells to anchor their trunks. After the animals die, layer upon layer of their skeletons create the exotic structures we call coral reefs, but according to scientists, they will begin to crumble as corrosive waters undo the work of countless generations of polyps.
“Today’s reefs are as much as 5,000 years old, and they will start to fall apart within a decade or so if we don’t radically change how we do business,” contends Christopher Langdon, a biological oceanographer at the University of Miami’s Rosenstiel School of Marine and Atmospheric Science.
The first hint that this might happen emerged more than a decade ago, when Langdon, working in Biosphere 2, grew corals in a swimming pool–size tank. The corals thrived when calcium carbonate was added to the water but did poorly without it. Ocean acidification wasn’t a recognized threat at the time, so Langdon’s findings just sat there. But today, pulled up from the void, they are sounding alarms. Working from his Biosphere data, Langdon calculates that the rise in CO2 pollution since 1850 is stunting the growth of today’s tropical corals by 10 to 15 percent.
Meanwhile, warming seas, human poaching, agricultural runoff, and other forms of pollution have also been taking a toll on coral, as documented by just-published measurements of Australia’s Great Barrier Reef between 1988 and 2003. In that time frame—a mere 15 years—the world’s oldest and largest reef showed an alarming 21 percent decline in growth. This steep downward trend is far greater than even Langdon expected and makes him wonder whether ocean acidification may be acting synergistically with the other destructive forces to greatly compound the damage.
With so many environmental stresses clouding the future of our fragile reefs, the emergence of yet another threat has marine biologists badly shaken. “When I first realized that ocean acidification was happening and the scale of the problem, I was sick about it,” admits Joan Kleypas, a coral expert at the National Center for Atmospheric Research (NCAR) in Boulder, Colorado. The insidious, creeping nature of the threat has her particularly concerned. “Bleaching, caused when rising temperatures lead corals to expel the algae that give them their color, often kills corals outright,” she says. “It’s shocking. But ocean acidification is an invisible, chronic stress that’s hard for people to believe. It’s like hypertension in a person, slowly getting worse and worse without any visible symptoms.”
Lest there be any doubt about the fate that awaits coral in a corrosive world, a recent paper published in the journal Science provides a stark warning. The authors of the report, marine biologists Maoz Fine and Dan Tchernov, raised coral specimens in tanks of water with a pH of 7.3, roughly as acidic as the oceans are expected to become sometime in the next century. In response, the hard coral did a vanishing act, and the polyps that once resided in it reverted to a naked existence. “If seeing is believing,” Kleypas observes, “that picture says it all.”
Should the reefs vanish, the vast populations of aquatic life they support will not be the only casualties. Islands that are atolls, with foundations of coral sediment, could crumble into more acidic seas, experts say. Reefs also form a barrier between land and ocean, preventing beach erosion and creating sheltered sanctuaries for mangroves, birds, and other wildlife. And coral may have still other important functions, as yet unrecognized.
Just two decades ago, scientists discovered that colorful tropical reefs have ghostly counterparts in deep, cold waters throughout the world’s oceans. White as bone, they live as much as three miles down where no light penetrates, feeding off dead marine matter that sinks from above. These corals grow in dense thickets, some of them 30 feet tall, off the coasts of Scotland, Norway, Alaska’s Aleutian Islands, and many other places. Indeed, cold-water reefs turn out to be 10 times as abundant as their much better-known tropical cousins. Yet for all their prevalence, these cold-water varieties have barely been explored because of their inaccessibility. Should corrosive waters soon claim them, we may realize their value only in hindsight.
Peril at the Poles
Coral may be the poster child in the effort to rouse public concern about ocean acidification, yet many scientists worry even more about how the sea’s smallest and least familiar denizens will adapt to the change. Biological oceanographer Victoria Fabry of California State University at San Marcos has spent years studying pteropods, thumbnail-size creatures that flutter through frigid polar and subpolar waters using flaplike wings. When startled they retract into shells that are normally smooth and translucent. But Fabry found that in water as corrosive as their aquatic habitat may be in 2100, the shell of at least one pteropod species turns opaque and begins to dissolve. To Fabry this suggests that pteropods may become vulnerable to predation in a more acidic world and dwindle in number or, in some regions, even die out. Indeed, she says, they may already be suffering adverse consequences, a possibility she is currently investigating.
