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