91av

CLEAR blue sea, brightly coloured fish—and amazing underwater
structures built up over generations from the calcified remains of the tiny
animals called coral polyps. It’s a spectacular sight. No one who has ever
visited one of the world’s great coral reefs is ever likely to forget it. But it
looks like we are one of the last generations who will have the opportunity to
experience this spectacle. Reefs as we know them are on their way out.

Robert Buddemeier, a coral researcher with the Kansas Geological Survey, is
one of a growing band who are bearers of bad tidings about the future of the
world’s coral reefs. He believes that we may have inadvertently switched off
coral’s reef-building ability. If so, we can expect existing reefs to dwindle
away over the next few centuries, and eventually vanish completely in many
places. We would lose one of Earth’s most spectacular ecosystems—the
aquatic equivalent of logging away all but a tiny shred of rainforest.

But this sad story has a bittersweet bright side. Corals, it seems, don’t
have to build reefs. Even if reefs disappear completely, corals—and the
plants and animals that live with them—may settle for a modest life in
exile until conditions are right once again for reef-building.

“Reefs as we know and value them will likely disappear, from most areas if
not the entire world,” says Buddemeier. “But I think they will rise phoenix-like
again in a few thousands or tens of thousands of years.”

The threats to coral reefs are well known. They are being dynamited,
poisoned, overfished, over-fertilised and showered with pollutants and
sediments. Rising temperatures threaten to trigger widespread coral bleaching
(91av, 4 November 2000, p 24). But two years ago, Buddemeier and Joan
Kleypas, a coral researcher at the US National Center for Atmospheric Research
in Boulder, Colorado, were among the first to recognise an even more ominous
threat: as CO₂ builds up in the atmosphere it will gradually turn
seawater more acid, making it harder for corals to extract the carbonate ions
they need to lay down their calcium carbonate skeletons (Science, vol 284, p
118). The following year a team led by Chris Langdon of Columbia University
found that this process—called calcification—did indeed slow down
when they artificially bumped up CO₂ levels in the Biosphere 2 research
facility in the Arizona desert (91av, 27 May 2000, p 8).

By 2065, calcification rates will be 14 to 30 per cent lower than before the
Industrial Revolution, the researchers calculated. And it won’t just slow reef
growth. Reef-building will stop altogether in many cases. Reefs will start to
shrink as erosion eats away at the reef and ocean currents carry eroded
sediments out of the system. Reef growth depends on the net balance of calcium
carbonate gains and losses. Often the margin is a slim one, especially at higher
latitudes where carbonate concentrations in seawater are lower.

There’s some small comfort knowing that even if reef-building stops, coral
ecosystems are unlikely to vanish altogether. But they will not be the rich,
biodiverse places we know now. A century on, the reefs of South-East Asia, for
example, may be transformed into something more like the coral flats of
present-day Hawaii. Kleypas warns that although Hawaiian reefs support many
different fish species, they don’t come close to the biodiversity of those in
the Philippines and Indonesia, where more than 2500 fish species, 3200 molluscs
and 500 species of coral form some of the most elaborate ecosystems on Earth.
Think of the difference between a tropical rainforest and a northern alpine
forest, Kleypas says, and you’ll have some idea of how much we stand to
lose.

Of course, CO₂ build-up also means warmer temperatures, and this may
help offset some of the damage by helping corals grow at higher latitudes than
they can today. But when Kleypas and Buddemeier built a computer model to
simulate this trade-off, they found that the loss of coral reefs because of
reduced calcification far outweighs their expansion due to warmer sea
temperatures. Ominously, as CO₂ continues to rise, the area suitable
for coral reefs dwindles. At double the present-day CO₂
concentration—not much above what’s predicted for later this
century—”it practically disappears on the globe”, says Kleypas.

Not everyone agrees with Kleypas and Buddemeier’s conclusions (see “Swimming
against the tide”), but some researchers have turned to the fossil record to see
how reefs responded to earlier fluctuations in the levels of CO₂. The
relatively low levels of CO₂ that have prevailed since the last ice age
have in fact been unusually good for building reefs. So the reefs we view as
“normal” are actually quite exceptional. Millions of years ago there were many
periods when CO₂ levels rose and reefs disappeared from the fossil
record, only to reappear later once CO₂ levels fell.

The fact that corals survived these reef-less millennia, Buddemeier believes,
should teach us that coral communities can be healthy even when reefs are not.
Reef building is simply one option in the coral repertoire, something they turn
to when the environment is right. “Reefs come and go, and form themselves from
organisms that have an existence elsewhere,” Buddemeier says.

He even believes that reefless coral communities are key to the survival of
both corals and other reef-associated organisms. Such communities are common
even today, particularly at higher latitudes—in Hawaii or Florida, for
example, where corals grow on lava flows or carbonate rock rather than the
skeletons of their immediate predecessors. Indeed, reefless corals may occur
anywhere that erosion outpaces accumulation, as long as temperatures are right.
Even in the tropics, Buddemeier says, biologists have tended to focus on the
more spectacular reef-building communities and overlook more marginal areas
where reefs do not accumulate.

Like active reefs, reefless coral communities take many different forms. In
some, thickets of fast-growing corals forming a veneer over uneven rocks or
other hard substrates may form a community that, to the untrained eye, has a
three-dimensional complexity almost indistinguishable from a true reef. Though
less diverse than true reefs, they may still be ecologically stable and serve as
refuges for many species. “Many of these communities are very healthy in terms
of maintaining species diversity and organic production,” says Kleypas. But
other reefless coral communities are highly eroded, rubble-strewn wastelands
dominated by algae or other organisms. In other areas, a few abundant coral
species form flat “meadows” of coral with little biodiversity.

So what will coral’s future be? Will these communities look more like reefs
or rubble? The answer may depend largely on whether local factors push reefless
coral communities towards the impoverished end of the spectrum. As calcification
rates fall, corals tend to produce more fragile skeletons. This makes them more
vulnerable to erosion and less able to compete for space against other
organisms.

Reefless coral communities will probably provide refuges for some reef
species, but how many? Even in the tropics, Kleypas says, “we really don’t know
how important the excess calcium carbonate production is to the coral community
overall”. Scientists simply don’t know enough to predict how many of the animals
and plants now found on coral reefs could be packed into the smaller,
structurally simpler communities that will likely emerge over the next
century.

The fossil record does offer some clues, though. John Pandolfi, a
palaeontologist with the Smithsonian Institution in Washington DC, has found
that reef communities in Papua New Guinea remained remarkably constant for
nearly 100,000 years, until about 30,000 years ago. “There was essentially no
change in community structure through time, even though there was tremendous
variability in global climate parameters such as CO₂, sea surface
temperature, and sea level,” he says. Reefs waxed and waned during this period,
changing location but never disappearing completely from the region.

Pandolfi has also examined the history of Caribbean reefs through a series of
changes in climate and sea level. “These were probably difficult times for
reefs,” he says, “but in the Caribbean I have observed only two species
extinctions during the past 600,000 years, out of 50 or 60 coral species.”

Such robustness is encouraging, but it may not mean much for the immediate
future. “We are headed into a time when CO₂ concentrations will be
higher than they have been for at least half a million—and probably many
millions—of years,” Kleypas says. Conditions this century are likely to
change faster and more dramatically than they did in the period Pandolfi
studied. CO₂ levels for Pandolfi’s corals, for example, didn’t approach
the high levels that were seen 50 million years ago in the Eocene and are
predicted for later this century. And major extinctions did occur in the Eocene,
although corals as a group survived.

Scientists can’t say whether reefless refuges helped corals survive hard
times during the more recent Pleistocene, because many of the sediments from
this period are now far below the ocean surface. They cannot even tell how
common reefless communities were in the past, because they do not fossilise as
well as reefs. For these reasons, Pandolfi is cautious about emphasising the
importance of reefless coral communities.

While we cannot possibly foresee all the combinations of climate and local
influences that lie ahead, Kleypas and Buddemeier believe researchers and
conservationists should focus more on reefless coral communities. “Just because
they don’t build reefs doesn’t mean they are not important,” Kleypas says. And
many more coral ecosystems of all kinds need to be brought under the protection
of reserve networks. “If anything, the fact that these communities are under
attack from both global warming and global chemistry changes implies that we
should try to eliminate those stresses that we can do something about,” says
Kleypas.

No matter how hard we try to care for corals, though, our children or our
children’s children may be the last generation to see the awesome spectacle of
flourishing tropical coral reefs. The world should certainly mourn their
passing. But if the creatures that once inhabited the reefs survive in an
ecological diaspora to reunite far in the future, the tragedy of the coming
century may not matter much 50,000 years hence. In a strange way, that’s
comforting.

Not everyone agrees that reefs are shrinking. According to calculations by
Joan Kleypas of the US National Center for Atmospheric Research, increasing
CO₂ should already be making reefs grow more slowly. But exactly the
opposite may be true on the Great Barrier Reef, according to a study of old
coral skeletons by Dave Barnes of the Australian Institute of Marine Science.
Calcification rates for the major reef-building genus Porites, he found, were 4
per cent higher from 1930 to 1979 than for each of the three preceding 50-year
periods, perhaps because rising ocean temperatures gave the corals a growth
boost. So what happened to the calcification problem?

In follow-up experiments in the laboratory, Barnes found a possible
explanation: as CO₂ concentrations rise to levels approaching double
the pre-industrial norm, reef rocks release carbonate in a form known as
magnesian calcite. This has a buffering effect, shifting the acidity of ocean
water back towards its normal value and so stimulating calcification. At about
the same time, Robert Halley and his colleagues with the US Geological Survey in
St Petersburg, Florida, used an underwater chamber on an actual reef to monitor
chemical responses to raised CO₂ levels. They observed the same
buffering phenomenon. “We found it occurred at much less than a doubling of
atmospheric CO₂,” he says.

Many experts doubt this buffering mechanism will do much to offset the
expected decrease in calcification, though. Researchers led by Chris Langdon of
Columbia University saw no sign of buffering at doubled CO₂ levels in
the Biosphere 2 experiments, for example. Halley counters that this may simply
be due to the oversimplified nature of the Biosphere “reef”. For example, he
notes, in natural reefs the surf produces a soluble fine sediment that is absent
in Biosphere 2. But even if buffering takes place, mixing by ocean currents will
likely overwhelm any locally produced effects on water chemistry, says Kleypas.
“I think it might help some reefs in very calm, enclosed areas,” she says, “but
I don’t think it’s going to operate on a large scale.”