Ocean acidification and coral reproduction on the Great Barrier Reef


Taking the acid test on the Great Barrier Reef“, National Times, 9th November 2009

Unlike some sexual processes in the animal world, coral reproduction remains a rather magical and mysterious event. And Dr Selina Ward loves it. The thousands of little red bundles of eggs and sperm are, she says, “beautiful”.

But at Heron Island Research Station, as she waits patiently for her corals to spawn, there’s now something more to Dr Ward’s research than simply untangling the mysteries of how corals release their egg and sperm bundles to the ocean currents.

Dr Ward, from the University of Queensland’s Centre for Marine Studies, is looking at how changes in the ocean’s chemistry – driven by increasing greenhouse gases – will affect the reproduction of corals and their ability to “settle” and build new reefs.  And her preliminary results are not looking good.

When coral scientists first looked at the impact of global warming on reefs, they focused on rising sea temperatures and bleaching. This is still a concern and likely to impact large parts of the Great Barrier Reef, but the scientists now believe ocean acidification could be the process that will push the world’s reefs to the edge.

The oceans act as a big sponge for carbon dioxide produced by human industry. Since the industrial revolution, the oceans have soaked up about half of the greenhouse gases produced by humans. That carbon dioxide has reacted with the water, making the ocean more acidic. As the oceans’ pH levels drop, life becomes harder for organisms that rely on making calcium carbonate – such as corals and shell fish, and even tiny but important creatures such as krill.

Coral scientists are scrambling to understand what this process will mean for reef systems. Dr Ward’s colleagues, including well-known reef scientist Ove Hoegh-Guldberg, wrote a paper in 2007 showing that, as carbon dioxide levels increase, the worldwide area reefs can grow will shrink dramatically. Even at carbon dioxide levels of 450 and 500 parts per million (the atmosphere is now at 378 ppm) the area is very small, and does not include the Great Barrier Reef.

Last year, at the International Coral Reef Symposium, Dr Ward said about half a dozen papers were presented on the impact that ocean acidification has already had.  “They found that in the last 20 to 30 years, growth rates have suddenly dropped about 20 per cent. That could be a number of things – it could be water quality issues or that we have reached the temperature peak and it’s too hot. Or it could be that ocean acidification is kicking in.”  Scientists are also discovering that more acidic oceans leave corals even more vulnerable to bleaching.

Dr Ward’s research, which is not yet published, is one corner of this big picture. Her work involves exposing spawning coral to water at different pH levels. She is finding that the early life stages of corals are adversely affected by ocean acidification, with reductions in fertilisation and settlement (difficulty laying down the first skeleton). “If a coral can’t lay down a skeleton, they can’t build a reef, and that is a fundamental problem,” she says.

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