Worrying signs that warmer seawater combined with a
possible change in the ocean’s acid balance may be curtailing the
growth of an important reef-building coral species have been
documented by a research team from AIMS in Townsville.
The paper, published in the journal Global
Change Biology*, points to a 21 per cent decline in the rate
at which Porites corals in two regions of the northern Great
Barrier Reef (GBR) have added to their calcium carbonate skeletons
over the past 16 years.
The AIMS research team analysed a total of 38
Porites colonies from the two regions. Porites are a common
massive coral with a striking spherical appearance. They are
long-lived and distributed widely around the Indian and Pacific
oceans.
The researchers speculate that their results may be an
early signal that the corals, as well as being subjected to warmer
water, are being affected by a phenomenon known as ocean
acidification. This is a predicted consequence of climate change, in
which large quantities of carbon dioxide from the atmosphere dissolve
in the oceans, causing their alkaline/acid balance (their "pH") to
shift towards acidic.
AIMS climate change team leader, Dr Janice Lough, a
co-author of the paper, said that much more needs to be done to
understand all the implications of the increase in carbon dioxide
entering the oceans and to put these preliminary coral growth data
into context.
"We need more information about the chemistry of the
GBR and how that has, is and will change," Dr Lough said.
The paper outlines Porites growth at two sites
in the northern reaches of the GBR, about 450km apart. The project
examined calcification rates, which are a combination of coral
skeleton density and the linear rate at which the coral grow.
"The fact that the two sites are reasonably well
separated and have different general characteristics and different
average water temperatures but are still showing the same decline in
calcification is good evidence that something unusual is happening,"
Dr Lough said.
Reef-building corals create their hard skeletons from
materials dissolved in seawater. When large amounts of carbon dioxide
enter seawater, the resulting chemical changes effectively reduce the
ability of marine organisms to form their skeletons.
"It is basically an equation – change the background
conditions like temperature or pH and that shifts the equation," Dr
Lough said.
Calcification will not switch off completely,
according to Dr Lough, but marine creatures won’t be able to do it as
well as they have in the past.
Rates of coral calcification are known to be naturally
variable over long time scales. What is concerning now, according to
Dr Lough, is that the rate of change exceeds what would be expected in
natural cycles of coral growth.
Earlier analyses of coral growth data had shown that
calcification had been increasing during the 50 years up to around
1980, in line with a rise in average sea surface temperature over that
time. Dr Lough and colleagues had suggested back then that some corals
might initially respond to global warming by increasing their growth
rates.
However, it appears that the higher growth response
only goes so far and that a sharp decline now may indicate that the
greater acidity is overwhelming any growth effects caused by warming.
"A decline in coral calcification of this magnitude
with increasing seawater temperatures is unprecedented in recent
centuries based on analysis of growth records from long cores of
massive Porites," according to the paper.
If projections are correct that pH could decrease by
up to 0.4 by the end of this century, this would be "well outside the
realms of anything organisms have experienced over hundreds of
thousands of years," Dr Lough said.
Even as the ocean pH swings towards acidic, it will
still be essentially alkaline. At the extreme ends of the pH scale are
0 (for example, battery acid) and 14 (drain cleaner). Seawater sits on
the alkaline side of the scale at around the 8.2 mark. A shift
downwards towards acidity could mean a seawater pH or around 7.8,
which some scientists speculate would be a disaster for all the
planet’s oceans.
If the ocean is turning more acidic, it will affect
all sea creatures, not just coral. Marine scientists around the world
are marshalling their resources to try to better understand exactly
what is happening and what its consequences may be (see note below on
AIMS’ public forum on the topic).
*The Global Change Biology paper, written by
AIMS scientists Timothy Cooper, Katharina Fabricius, Glenn De’ath and
Janice Lough, is titled "Declining coral calcification in massive
Porites in two nearshore regions of the northern
Great Barrier Reef".
Go to:
http://www.blackwell-synergy.com/doi/abs/10.1111/j.1365-2486.2007.01520.x
Note to media: AIMS will hold a media briefing on
ocean acidification research plans at its headquarters near Townsville
on Tuesday 11 March at 10am and a public forum at the Museum of
Tropical Queensland in Townsville at 5.30pm on Friday 14 March. Please
contact Wendy Ellery, below, for further details.
