The ocean has been our savior. Besides generating about two thirds of the oxygen we breathe, oceangoing phytoplankton — those floating microscopic plants that form the base of the aquatic food chain — absorb about a third of all the carbon dioxide we pump into the atmosphere.
In this way, the oceans have managed to slow the buildup of heat-trapping greenhouse gases and stave off even more dramatic warming of the planet.
But John Guinotte and colleagues are discovering that the critical role of “carbon sink” comes at a potentially devastating cost for the world’s oceans: acidification.
Guinotte is a coral specialist at the Marine Conservation Biology Institute in Bellevue, Wash. The changes he sees in ocean chemistry spell trouble for the coral that he studies closely.
If the acidification process continues on its current trajectory, it poses a dire threat to the whole marine ecosystem.
“What I’m really concerned about with ocean acidification is that we are facing the prospect of a crash in marine food webs.” says Guinotte.
“There is no question that many of my colleagues in marine science are scared about what is happening. We know we need a more precise understanding of the changes and biological responses now under way — and we need it as quickly as possible, before it is too late to turn things around.”
Guinotte has dedicated his life to the study of coral, especially the less well understood deep-sea varieties. Growing up in rural Kansas, his only exposure to corals was through the pages of National Geographic.
But that changed when he learned to scuba dive at his grandfather’s winter home in the Florida Keys. The experience, plus his interest in biology and geography, led him to Australia, where he earned his Ph.D.
Guinotte still remembers the thrill of exploring Australia’s Great Barrier Reef for the first time.
“I was absolutely blown away by the abundance and diversity of coral,” he recalls. At that time, back in the late-1990s, scientists were increasingly concerned about coral bleaching caused by environmental stresses such as warming ocean temperatures.
Those threats remain, Guinotte says, but ocean acidification may be an even more serious and intractable problem.
On the macro scale, Guinotte explains, the chemistry of ocean acidification is relatively clear. Based on some 25 years’ worth of measurements scientists know that oceans absorb about 22 million tons of carbon dioxide every day. The oceans are vast.
But even so, the absorption of CO2 is now occurring at such an unprecedented rate that ocean chemistry is approaching a state not seen in many millions of years. Guinotte fears that many marine species might be unable to adapt quickly enough to survive these dramatic changes.
As carbon dioxide is absorbed by seawater, hydrogen ions are released. This lower the pH, making the water more acidic. Measurements indicate that Earth’s oceans are already about 30 percent more acidic than they were before the industrial revolution.
As the number of hydrogen ions has risen, the number of carbonate ions available in seawater has gone down. This carbonate deficit makes life more difficult for the “marine calcifiers,” species such as coral and shellfish that use carbonate to build their skeletons and protective shells.
“Ocean water becomes increasingly corrosive to calcium carbonate,” says Guinotte. “A reduction in carbonate ions not only impedes corals’ ability to build their skeletons, but once the calcium carbonate drops below critical levels, the ocean erodes the framework they have built up previously — the reefs upon which corals live.”
Even if select coral species can survive ocean acidification, Guinotte says, when the coral reefs begin to dissolve, the effects on the entire marine ecosystem are likely to be devastating.