Speeding towards an acid ocean

Ocean acidification is occurring faster than expected in Antarctic coastal waters, according to new research by PhD student Nick Roden from the Institute for Marine and Antarctic Studies (IMAS) and CSIRO.

The study, published in the journal of Marine Chemistry in August, is the first to observe changes in seawater acidity over decadal time-scales (16 years) in East Antarctica.

Ocean acidification results when atmospheric carbon dioxide (CO2) is absorbed by the ocean, causing chemical changes in the seawater. These changes affect the ability of some marine organisms to form shells or other hard structures made of calcium carbonate (CaCO3).

Nick’s research shows that changes in acidity over the past 16 years are nearly twice as large as those expected from atmospheric CO2 uptake alone.

‘The surprise was that the change in the acidity level was so large, indicating that natural and human induced changes have combined to amplify ocean acidification in this region,’ he said.

Nick spent a year at Davis station in 2010–11 collecting seawater samples from Prydz Bay. He then compared the chemistry of these samples with samples collected at the same location between 1993 and 1995 by former Australian Antarctic Division scientist Dr John Gibson.

To measure changes in acidity between the sample years, Nick compared winter pH values. Winter measurements were used because of the high variability in water chemistry in spring and summer, caused by rapid and large-scale phytoplankton growth as light increases and sea ice breaks up.

‘The mean winter pH value in 2010 was 0.11 pH units lower (more acidic) than the mean winter value in 1994,’ Nick said.

‘The expected decrease in pH from 1994 values, if the surface ocean CO2 uptake tracks the atmospheric CO2 increase, should be 0.04. This indicates that ocean CO2 uptake is not the only process affecting carbon cycle dynamics on the shelf environment and driving pH changes between 1994 and 2010.’

Because the coastal waters around Antarctica support iconic ecosystems of great conservation value, the change observed in this study is concerning, as it may accelerate the impact that ocean acidification will have on food webs in this environment.

Previous studies have suggested that if future CO2 emissions continue unabated, Southern Ocean waters could become corrosive to some calcifying organisms by the year 2030.

‘The study highlights the importance of long-term observational programs and the need to understand how future climate related changes will influence ocean chemistry in this area,’ Nick said.

Read more in Marine Chemistry Vol 155: 135–147 (2013).

Wendy Pyper1 and Craig Macaulay2

1. Australian Antarctic Division