What carbon dioxide (CO2) is doing to our atmosphere is common knowledge, but the effects it has on our oceans are not nearly as well studied. This is about to change. Scientists will simulate a future ocean floor under the sea ice off the Australian Antarctic station at Casey this summer to observe the potential impact of ocean acidification on seafloor communities.

Ocean acidification is caused by increasing amounts of CO2 dissolving into seawater as carbonic acid. Dissolved CO2 causes the pH of the seawater to drop and become more acidic. This affects the ability of some marine organisms, including corals and bivalves, to form shells and other hard structures.

A bivalve (Poromya adelaidis) with calcium carbonate shell that will be effected by ocean acidification in Antarctica, Credit: NIWA, IPY-CAML
A bivalve (Poromya adelaidis) with calcium carbonate shell that will be effected by ocean acidification in Antarctica, Credit: NIWA, IPY-CAML

The Southern Ocean around Antarctica absorbs 40 % of the global ocean uptake of CO2 as cold water is able to absorb more CO2 than warmer water. As a result, polar waters are acidifying at twice the rate of tropical waters. Over the past 300 million years ocean acidity, measured by its pH, has been slightly basic, and averaged about 8.2 (a pH of 7 is neutral). Current atmospheric CO2 concentrations are about 395 parts per million (ppm), while ocean pH has dropped to 8.1 since pre-industrial times – a 25 % increase in acidity over two centuries. By 2100, under business as usual emissions, atmospheric CO2 is predicted to be about 936 ppm and ocean pH 7.8. To test the effects of this increasing acidity, four semi-enclosed Perspex chambers the size of coffee tables are being deployed 10 to 20 m beneath the Antarctic sea ice off Casey between November 2014 and March 2015 as part of the Antarctic Free Ocean Carbon Enrichment experiment.

A team of scientists, divers, technicians and engineers will increase CO2 concentrations in the water within two of the chambers. This will decrease the pH of the water by 0.4 pH units, without changing light or nutrient concentrations. About 1500kg of CO2, or 69 gas bottles, will be used in the four-month experiment. A further two chambers will be used as controls to track natural changes in pH in the surrounding water. This will allow the team to compare the response of benthic (ocean floor) communities exposed to current seawater pH levels, and the more acidic pH levels predicted under future CO2 emission scenarios.

A diver tests one of the chambers in the cold waters of Tasmania before deployment in Antarctica, Credit: Jonny Stark
A diver tests one of the chambers in the cold waters of Tasmania before deployment in Antarctica, Credit: Jonny Stark

Ocean acidification disrupts the formation of calcium carbonate (CaCO3), which is a major structural component of shells and similar hard structures made by some marine organisms, including phytoplankton and coral. It also affects the development, growth and reproduction of marine organisms.

Small phytoplankton like this Ostracod (Gigantocypris) will suffer severely from ocean acidification since they are protected by a calcerous “shell”.
Small phytoplankton like this Ostracod (Gigantocypris) will suffer severely from ocean acidification since they are protected by a calcerous “shell”.

Source: Australian Antarctic Division (http://www.ecosmagazine.com/?paper=EC14271)