Great Barrier Reef: 'a massive chemistry experiment gone wrong'

It has long been known that pollution is having a devastating impact on the Great Barrier Reef but now scientists are warning that it may also be dramatically increasing the rate of ocean acidification in inshore areas of the region.

Dr Sven Uthicke, a senior research scientist at the Australian Institute of Marine Science, has with colleagues this week published a paper in the journal PLOS one, on ocean acidification in the reef. The study compares the reef’s inshore and offshore waters, and information on present-day water quality with 30-year-old data.

He said there was a complex interplay between chemistry and biology in the ocean, and the team suspected that the increased pollution in inshore areas decreases the light available for organisms to photosynthesise and thus “absorb” excess CO2.

“Because it’s darker there might be less productivity and the carbon dioxide levels go up,” he said.

The study, Coral Reefs on the Edge? Carbon Chemistry on Inshore Reefs of the Great Barrier Reef, reads as though a massive chemistry experiment has gone wrong on one of the country’s most precious ecosystems.

As the researchers explain: “When carbon dioxide from the atmosphere dissolves in water, it causes ocean acidification, slightly lowering the pH of the water and changing its carbonate chemistry. This in turn makes it harder for a range of marine animals to form their carbonate shells and skeletons.”

Acidification of seawater is especially harmful to coral reefs and shells but, as scientists have been learning in recent years, its impacts are also far more subtle, changing the homing instincts of fish and even their ability to detect predators.

While atmospheric CO2 is changing the ocean’s pH on a planetary scale because carbon dioxide in the atmosphere and the sea has increased, on average by 30%, there are parts of the marine environment where the impact is much worse.

By testing water samples collected over the past three years at numerous inshore and offshore reefs Uthicke’s team discovered that, while offshore reef areas have increased their dissolved carbon dioxide at the same rate as the atmosphere, inshore reefs have have done so at nearly three times the rate.

According to the researchers, the results suggest that the increased rate of dissolved carbon dioxide is “a relatively recent phenomenon, and it is possible that these are caused by anthropogenically increased sediment and nutrient runoff”.

It also seems to be a recent phenomenon because the scientists were able to analyse water samples taken at the Australian Institute of Marine Science from three decades ago.

“These samples indicated that 30 years ago this trend of inshore waters being more acidic than offshore did not occur,” Uthicke said. “Something has really changed in these 30 years.”

What was happening on the inshore reefs was possibly a case where the acidification of the sea was being even further enhanced by human activities, he said.

Such a huge change in these marine environments close to shore favoured the growth of brown algae, which are feared to replace coral reefs in inshore areas for many decades because of increased nutrient runoff.

Increased ocean acidification may further advantage the algae and push reef from coral-dominated to algae-dominated systems.

Uthicke said this study highlighted the need for ongoing water quality work in the Great Barrier Reef so scientists can understand exactly why the rate of acidification is worsening close to shore.

“We don’t have the answers to all the questions yet,” he said. “But we are seeing a trend and that is very worrying.”