Acid test: monitoring ocean CO2 uptake

Glen Paul: G’day, and welcome to CSIROpod. I’m Glen Paul. Prior to the industrial revolution the earth’s oceans had for millions of years maintained a fairly stable acidity level, but since the late 18^th Century and the burning of fossil fuels the oceans have been absorbing large amounts of carbon dioxide. When carbon dioxide enters the ocean it combines with seawater to produce carbonic acid, which increases the acidity of the water, lowering its pH.

Australia contains many ecosystems that may be sensitive to ocean acidification, including the extensive coral reefs of northern Australia, right down to the Polar Regions. In an effort to assess how marine ecosystems might be impacted a new mooring has been deployed in the Tasman Sea near Maria Island, to provide data to determine the change in ocean acidification.

Leading the acidification research is CSIRO’s Dr Bronte Tilbrook, who joins me on the phone. Firstly, Bronte, how different are the oceans of today, compared to those of just 200 years ago?

Dr Tilbrook: Well since about the last 200 years there’s been about a 0.1 pH unit change in the ocean surface waters. That doesn’t sound like much, but pH is reported on a logarithmic scale, so it’s actually about a 30 per cent increase in the hydrogen ion concentrations in seawater, and that’s the real measure of acidity levels.

Glen Paul: So this acidity has proven to inhibit shell growth in marine animals, exactly how does it do that?

Dr Tilbrook: Well there’s been a lot of experimental evidence that shows that if you grow organisms that precipitate calcium carbonate shells or skeletons that they grow less under high CO₂ conditions, or conditions that have greater levels of ocean acidification. So what’s happening is that accompanying the change in pH that we just talked about, there’s also a decline in the dissolved carbonate ion concentration of seawater, so CO₂ gets taken up by the ocean surface waters and then it reacts in the water and it consumes carbonate ion and converts it to bicarbonate.

So all these organisms require carbonate plus calcium to grow their shells or skeletons, and if there’s less carbonate they have to put more energy into producing that carbonate in their internal fluids that precipitate the shells or skeletons, and that gives other organisms that don’t produce calcium carbonate shells or skeletons a bit of an advantage. And not all organisms respond in the same way. There are some corals that are more resilient to change than others.

Glen Paul: Now that’s obviously quite complex, but other impacts that I’ve heard which appear more simplistic are that the higher concentrations of carbon dioxide might make it harder for some larger marine animals to get oxygen from the seawater. How true is that?

Dr Tilbrook: Well there is some evidence that that may happen in the future, but it’s very difficult to do experiments on large fast moving organisms like tuna and sharks in the ocean. But it does bring up an important point, that there are a number of effects of ocean acidification on ecosystems. It’s not just about corals growing, or some shell growing in the ocean, but there are a whole series of issues.

For example, if the pH levels of the water change, then there’s a potential that nutrients that are now available to some plants which produce half of the earth’s oxygen may not be as available, so they could affect some of the plant life we have in the ocean, and that effect might be for example you get a change in the base of the food chain.

When you change the base of the food chain that cascades right up the food chain, so different sized phytoplankton might result, then you may get for example different organisms that feed on them, and that affects larger organisms and so on through the fish, and the kind of things that we normally look at in the ocean, so we may see shifts in ecosystems in ways we don’t know.

Glen Paul: Now, getting onto the new mooring itself, it’s pretty big, it reminds me of an old Apollo space capsule painted yellow, it’s sitting off Maria Island and forms part of a bigger network, and is equipped with the latest in environmental sensors, but what exactly is it sensing, and how does it do it?

Dr Tilbrook: It does look a little bit like an old spacecraft. It was designed in the U.S. and it’s actually a modified tsunami warning buoy that Australia and the U.S. use in their warning arrays, and one of the reasons it’s like that is it’s really robust and so it can stand some big seas and getting knocked around.

It contains sensors to measure carbon dioxide levels in the water, and temperature, the salinity, the oxygen content, and there are other sensors deployed around it as part of a national reference station in the Integrated Marine Observing System for Australia. And combined all those measurements give us estimates of the acidity level of the ocean, the carbonate chemistry, so how much dissolved carbonate ion there is, and it’s giving us some first time information on just what’s changing in this region.

It’s quite a significant area the South Tasman Sea, it’s the southern end of the East Australian Current, so we’re seeing large scale changes down there. There’s more subtropical fish moving in as the East Australian Current intensifies over the number of years. And we don’t know at present what effect that will have, first on how much carbon dioxide is taken up in the region by the ocean, and also how it will affect acidity levels.

Glen Paul: How do you actually access the data from the mooring? Do you have to row out in a boat, or is it sent via satellite?

Dr Tilbrook: No, it comes in each day via satellite. We have to get out every six months and pull the mooring up and clean it up because it gets barnacles growing all over it and we don’t want to get bad data, so it does need constant attention, but about once every six months there’s a flurry of activity and then we can get the data in our lab on land and make sure everything’s working alright. If there’s a problem we can get out there fast and fix it.

Glen Paul: And is the data publically accessible?

Dr Tilbrook: Yes, the data is publically available. It’s displayed by the Integrated Marine Observing System, on their website, and anyone can access it.

Glen Paul: And this mooring forms part of the Integrated Marine Observing System. Where are the other monitoring sites, and are you getting similar results from them?

Dr Tilbrook: Yeah. We have other sensors similar deployed off the Great Barrier Reef, off Townsville, which some work we’re doing in conjunction with AIMS(?). And we have another one off Kangaroo Island, which is on the west side of Kangaroo Island, and we’re hoping to build the array further. I might have a number of reference sites in all States around the country, and we’d like to eventually instrument all those sites with these sensors.

Glen Paul: And this isn’t some fashionable new initiative, measurement samples have been collected at Maria Island by CSIRO technicians monthly since 1944 in a variety of different ways, so you’ve got a great monitoring record to refer to, but what plans are in place for collecting data into the future?

Dr Tilbrook: Well I think people have realised the value of these long term records in recent years, they’re a key to understanding what’s changing in the ocean and to identify the causes of those changes, so there’s been an improvement in the observing system around Australia, particularly through the Integrated Marine Observing System rollout. We hope that will continue and grow into the future.

Six sites around Australia don’t provide a huge view of what’s going on in our waters, and those sites contain biological chemical and physical measurements, so we can understand the currents, how the chemistry of water’s changing, and how the biology is responding to those changes. So, yeah, I think there’s new technology coming onboard, there’s a lot of opportunities to really understand what’s happening in the ocean and what’s changing, and what’s driving that change.

We’re at the start of a really interesting time in oceanography, where we’ve got new technology and really can start to understand how the ocean’s changing and how it might affect us in the future.

Glen Paul: OK. Well we look forward to hearing more about it. Thank you very much for your time today, Bronte.

Dr Tilbrook: Good. Thank you.

Glen Paul: Dr Bronte Tilbrook. And to find out more about the research, or to follow us on other social media, just visit www.csiro.au.