Ocean research has shown that about half of all the carbon dioxide (CO2) released by human activities is now found in the world’s oceans and that the Southern Ocean absorbs about 40% of this. As CO2 continues to dissolve in the ocean it increases ocean acidity, making it harder for some marine organisms to form shells. These ecological changes in turn reduce the capacity of the ocean to absorb CO2.
The Southern Ocean contains more CO2 than other oceans because cooler water absorbs more CO2 than warmer water. Thus, the impacts of ocean acidification will appear first in the Southern Ocean.
How does the water change?
When (CO2) dissolves in water (H2O) it forms carbonic acid (H2CO3) — the same weak acid found in carbonated drinks. Increasing levels of carbonic acid interfere with the formation of calcium carbonate (CaCO3), a major structural component of the shells of many important planktonic organisms (free-floating marine plants, animals and microbes ranging in size from microscopic to several centimetres). Increasing acidity also affects the availability of nutrients in the ocean.
What effect will ocean acidification have?
As it becomes more difficult for calcium carbonate to form, it will become more difficult for some planktonic organisms to form shells. If their shells are thinner and/or deformed, the organisms may be unable to function properly. Many of these organisms are key components of the food chain — important in the diets of krill, fish, squid, penguins, seals and whales. They are also important in the removal of carbon from surface waters to the deep ocean and the release of oxygen into the air. Important metabolic processes, such as respiration in fish, may also be impaired by the acidity, as lowering the pH reduces the efficiency of oxygen exchange in their gills.
What organisms will be affected?
In the Southern Ocean and other open-ocean ecosystems, calcifying organisms affected will include snail-like molluscs called ‘pteropods', abundant, single-celled algae called ‘coccolithophorids’ and protozoan ‘foraminifera'. Changes in microbial populations are likely to flow on to dependent species throughout the food chain. In tropical coastal ecosystems coral reefs, comprised of colonies of small animals that secrete calcium carbonate skeletons, are also at risk.
How acidic will the oceans get?
The pH of seawater has historically remained at about 8.2, which is slightly alkaline (pure water is neutral — pH 7). However, CO2 from human activities has caused the pH of ocean surface waters to drop by 0.11 pH units. This might not sound like much, but it is equivalent to a 30% increase in acidity. Unless CO2 emissions are curbed the pH is expected to fall by 0.5 pH units by 2100; a 320% increase in acidity.
How can we stop it?
Even if all carbon emissions stopped today, we are committed to a further drop of 0.1–0.2 pH units and it will take thousands of years for the oceans to recover. However, action now can prevent conditions that are corrosive to calcifying organisms from becoming more widespread.
What research is being done?
In 2014–15 Australian Antarctic Division scientists created a ‘future ocean’ under the sea ice off Casey, using four underwater chambers to measure the impact of ocean acidification on seafloor (‘benthic’) communities. During the Antarctic ‘Free Ocean Carbon Enrichment’ (antFOCE) experiment, a team of scientific divers, technicians and engineers increased CO2 concentrations in the water within two of the chambers. This decreased the pH of the water by 0.4 pH units, without changing light or nutrient concentrations. A further two chambers were used as ‘controls’ to track natural fluctuations in pH in the surrounding water. A variety of observations and measurements were taken to allow the team to determine how benthic marine habitats respond to decreased seawater pH.
Scientists are also studying the effects of ocean acidification on Antarctic marine microbial communities, which include bacteria, phytoplankton and protozoa. These organisms sit at the base of the food web and directly, or indirectly, support all life in the Southern Ocean.
Six 650 litre ‘minicosms’ or incubation tanks have been installed in a shipping container on the shore at Davis, and filled with filtered seawater containing different concentrations of CO2. These concentrations range from ambient Antarctic concentrations in summer of 84 ppm (parts per million), to the maximum seawater CO2 concentration predicted for the year 2300 of 2423 ppm. For comparison, the current atmospheric CO2 concentration is just over 400 ppm.
Early results indicate that CO2 concentrations at or above those predicted by 2100 negatively affects phytoplankton, reducing their productivity and growth, and changing the community composition. Such changes at the base of the food chain will have flow-on effects up the food chain, including to krill, seabirds and whales.
Together, these research programs will assist governments, scientists, modellers and society to understand the emerging impacts of ocean acidification on marine ecosystems and to ensure that the most relevant information underpins decisions to manage the threat.