Carbon Dioxide, Absorbed by the Seas, Changes the Chemistry of Water and the Growth of Marine Life
In the living laboratory of a submerged volcano, marine biologist Verena Tunnicliffe glimpsed sea creatures trying to survive in acidic oceans.
Carbon dioxide that bubbles up in the sulfur chimneys of the undersea Eifuku volcano near the Pacific’s Mariana Islands has turned the water into an acidic broth, with striking effects on sea life. Scientists say the corrosive conditions there offer clues to how rising levels of man-made CO2 in the air could unbalance oceans world-wide.
To her surprise, Dr. Tunnicliffe found that mussel shells she collected at Eifuku were so thin that she and her colleagues could see right through them. The water chemistry made it impossible for the mussels to extract enough calcium carbonate to form a proper covering. Compared with shells of the same species collected in more normal waters, “they were half the thickness and half the weight,” she said.
To live in these inhospitable conditions, the mollusks cannibalized their own shells, leaching from them the carbonate needed to maintain their internal muscle chemistry. “They are dissolving whatever shell they do have,” says Dr. Tunnicliffe at the University of Victoria in British Columbia. They manage to survive despite their weakened shells, the scientists speculated, because the water’s harsh chemistry is too much for the hard-shell crabs that prey on these mussels elsewhere.
When the mussels die, their wafer-thin shells disintegrate even faster than their soft tissues can decay.
To be sure, the sea chemistry of Eifuku is unusual by any measure. Located on the volcanic rim of the Mariana Trench near the island of Guam, the mollusks live in water pressure 44 times that at the surface, at one of only two spots in the world where CO2 rises from the seabed as a liquid. Dr. Tunnicliffe and her colleagues explored the beds of exotic vent mussels during a 2006 expedition via the sensors of a sturdy deep-sea robotic explorer called Jason-II. They reported their findings last week in the journal Nature Geoscience.
Conditions on the volcanic slopes of Eifuku have been this acidic for millennia, giving these creatures more than enough time to acclimate. But many oceanographers worry that increased CO2 — likely created by burning fossil fuel — is changing sea chemistry world-wide more quickly than most marine life can adapt. A host of experiments are underway to assess just how the increased CO2 levels are changing ocean life.
From the rocky inlets of Tatoosh Island in the Pacific Northwest to Australia’s Great Barrier Reef, seawater is turning acidic. Mounting evidence suggests plankton, sea urchins, squid, coral and other marine life already find it harder to grow, reproduce and survive. If acidification intensifies, it could ultimately threaten the marine food chain, including commercial fisheries.
All told, the oceans have absorbed 118 billion tons of carbon in the 200 years since the beginning of the industrial revolution, an international research team led by oceanographer Christopher Sabine at the Pacific Marine Environmental Laboratory in Seattle has calculated. Every second of the day, the oceans absorb an additional 300 tons of CO2 emissions.
In seawater, CO2 forms carbonic acid, steadily lowering the ocean’s pH value on a scale used to gauge a liquid’s acidity or alkalinity. The number gets lower as a liquid gets more acidic. Fresh milk has a pH of about 6.7; lemon juice has a pH of 2.4 or so. The concern is that quickly falling pH levels could overwhelm a species’ chemical stability.
“If CO2 levels in the atmosphere rise, then the oceans become more acidic,” says marine ecologist Jon Havenhand of Sweden’s University of Gothenburg. “The chemistry is unavoidable.”
For at least 600,000 years, the oceans maintained a steady pH of about 8.2, according to levels measured in ancient ice cores that preserve an annual chemical record of times past in the same way that tree rings do. Since 1800, however, the pH of seawater has dropped to 8.1. “The number is small but the change is substantial,” says marine biologist Donald Potts at the University of California, Santa Cruz. By the end of this century, the pH of seawater is expected to drop to 7.8 or so.
The change in sea chemistry affects how easily marine creatures can form the calcium carbonate materials for shells and skeletons. “As it gets more acid, they lay down skeletons more slowly and they make a softer skeleton, with less strength,” he says.
Last month, President Barack Obama signed a new wilderness law that calls for federal agencies to assess the impact of rising ocean acidity. The measure also authorizes an ocean acidification research program led by the National Oceanic and Atmospheric Administration. Last week, the U.S. Environmental Protection Agency for the first time began weighing the possibility of revising pH standards under the federal Clean Water Act to prevent ocean acidification.
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Robert Lee Hotz, The Wall Street Journal, 24 April 2004. Full article.
