91av

CORAL reefs will be crippled if carbon dioxide levels increase as projected
this century, say researchers in the US who have studied the impact of changing
ocean chemistry on corals in the Biosphere 2 facility in Arizona.

“I think this is fairly urgent,” says Robert Buddemeier, a coral researcher
with the Kansas Geological Survey in Lawrence. “It’s the first really documented
negative biochemical impact of rising CO₂,” he says.

Coral polyps use calcium and carbonate from surrounding waters to build their
skeletons, which become the framework of coral reefs. But CO₂, a weak
acid, reacts with water and carbonate to produce bicarbonate, which corals can’t
use. Previous research has shown that the predicted increases of atmospheric
CO₂ this century will lead to dramatic decreases in carbonate available
to corals.

But the effects of lower carbonate concentrations on corals have only been
studied in a few small-scale experiments in aquariums. To determine what the
effect would be on an entire coral ecosystem, a team of researchers from
Columbia University’s Lamont-Doherty Earth Observatory in Palisades, New York,
and the Hawaii Institute of Marine Biology in Kaneohe Bay turned to the massive
greenhouse in the Arizona desert near Tucson known as Biosphere 2
(91av, 18 May 1996, p 31).
Originally built as a prototype for a
Martian colony, Biosphere 2 contains an artificial ocean with a volume of 2650
cubic metres, complete with a coral reef and associated organisms, which the
researchers used to study the effects of various carbonate concentrations. “It
was really a clever way to make use of a novel if somewhat strange facility,”
says Buddemeier.

The team manipulated carbonate concentrations by regularly adding acid to the
water to recreate the effect of increased CO₂. For the first two years
of the project, the team held the carbonate concentration near the low level
predicted for 2050, which is 30 per cent lower than today’s ocean
concentrations. The following year they increased it to the current level, and
the next year raised it beyond pre-industrial levels.

To determine calcification rates, they measured how much carbonate and
calcium were removed from the water between acid additions. The reef showed no
sign of acclimatising to lower carbonate concentrations, says team leader Chris
Langdon of Columbia University, and their results suggest that the decrease in
coral growth between 1880 and 2065 will be about 40 per cent. “That’s an
enormous hit on shallow marine ecosystems,” says Buddemeier.

“It was quite surprising to see that it was so strong,” says Langdon.
Previous projections were lower. But the idea that rising levels of CO₂
would have such a heavy impact on corals is well supported in the geological
record, he says. At least twice in Earth’s history, levels are thought to have
risen even higher than those predicted for the end of this century. During those
times large reefs were nonexistent and whole coral families became extinct.