Ocean Acidification – The Ocean’s other problem with CO2

This guest post is from Elliot Scanes, a PhD candidate from Western Sydney University who runs cool experiments to understand the impact of climate change on our molluscs. Elliot has a great Instagram research account with Dr Vicky Cole called seao2, so give them a follow to keep up to date with their work.

Ocean Acidification – The Ocean’s other problem with CO2 

by Elliot Scanes

Humans are currently emitting CO2 faster than has occurred on Earth for millions of years. Currently, atmospheric CO2 concentrations are at their highest point in 800,000 years and don’t look like slowing any time soon. Inevitable global warming as a consequence of the excess CO2 and other pollutants causing the “greenhouse effect” is well established among scientists (despite what politicians might say). This warming of the earth will eventually also cause warming of the oceans, most notably affecting species ranges. But this is not the only way excess CO2 is going to change the world’s oceans. The oceans have already absorbed 40% of the CO2 emitted by humans, and will keep continuing to do so. As CO2 dissolves in seawater, it forms carbonic acid, which, in turn causes the oceans to acidify. So far, oceanic pH (the measure of acidity) has fallen 0.1 units, and is predicted to fall 0.3 – 0.4 pH units by 2100 unless drastic global action is taken to curb emissions.


Collecting Sydney rock oysters (Saccostrea glomerata) for experiments in Port Stephens.


Opening flat oysters (Ostrea angasai).









This small decrease in pH may seem insignificant, however pH is measured on a natural logarithmic scale (for those mathematicians) which means that each unit is an order of magnitude larger than the previous. A 0.3 unit drop in pH means that calcium carbonate, the mineral that all shelled animals shells are composed of, is now soluble in seawater. Predictably, scientists are most concerned about the shelled animals of the ocean like molluscs, crustaceans and cnidarian (corals). Investigations by our lab at Western Sydney University have shown that under these scenarios Sydney rock oysters will have difficulty forming their shell, especially in their juvenile stages. These difficulties waste vital energy, which is especially important in an environment where you always need an edge on your competitor. Similar studies have found comparable effects in sea urchins, corals, scallops and almost every shelled animal in the ocean that you can think of.

Many marine ecosystems support hidden biodiversity, those animals too small to see or are just good at hiding. Such communities are vital as a source of food for larger predators. We have found that these entire communities can change under ocean acidification. While shelled animals are the focus of a lot of research, studies have shown that ocean acidification’s effects aren’t limited to only them. Research conducted on the Great Barrier Reef has found that under ocean acidification scenarios, clown fish can no longer use their sense of smell to find a suitable place to reproduce. Fish behaviour and their ability to hear predators has also been shown to be altered by ocean acidification. Marine invasive species are also more likely to become more established as other competing species are affected by ocean acidification.

These experiments have all been conducted over a relatively short time frame, surely marine animals will just evolve, right? Well, as previously stated, the rate of current CO2 emissions is unprecedented. The physiological changes that shelled animals will have to undergo to cope with ocean acidification are highly unlikely in the timeframe that they have. And even if whole scale extinctions don’t happen, changes in ecological functioning are almost inevitable.


Experimental set up at the Sydney Institute of Marine Science.


Experiments investigating elevated CO2 on Sydney rock oysters at Port Stephens Fisheries Institute.

From our research and that done by others, it appears that the biggest effects of ocean acidification will be on molluscs, crustaceans and other shelled animals. The oyster farming industry is one of the biggest single aquaculture industries in NSW. It is likely that oyster farming may become more difficult with ocean acidification, especially when these effects are combined with ocean warming. Oysters are expected to grow more slowly, have lower juvenile survival rates and be more susceptible to diseases. Already, oyster farms on the east coast of the USA are experiencing difficulties raising juvenile oysters, because of the decline in oceanic pH due to extra CO2. There, oyster farmers are adding calcium carbonate (lime) to the seawater to raise its pH. This only adds to the cost of farming oysters.

But there is some hope, especially for tolerant animals like oysters. Selective breeding techniques used by humans for thousands of years are now being applied to oysters and other aquaculture species to make them more tolerant to ocean acidification. Oysters that had already been selected for faster growth were better at coping with ocean acidification. We have now exposed successive generations of Sydney rock oysters to ocean acidification and found that each generation is better at coping than the last. However, like always, this adaptation comes at a cost, we just haven’t worked it out yet. It looks like the life span of oysters is likely to be shortened in this compromise.


Ocean acidification is just one of many challenges that our oceans will face in the future. Unfortunately without a cultural shift away from fossil fuels and deforestation, ocean acidification is going to happen. The best we can do, is to continue to study the effects on our marine life. Understanding how they may be affected is the first step in a larger process of trying to manage these changes for the future.

Elliot Scanes

PhD Candidate, Western Sydney University.

Further reading;

Caldeira K, Wickett ME (2003) Anthropogenic carbon and ocean pH. Nature 425: 365-365.

Parker LM, Ross PM, O’Connor WA, Borysko L, Raftos DA, et al. (2013) Adult exposure influences offspring response to ocean acidification in oysters. Glob Chang Biol 18: 82-92.

Parker LM, Ross PM, O’Connor WA, Pörtner HO, Scanes E, et al. (2013) Predicting the response of molluscs to the impact of ocean acidification. Biology 2: 651-692.

Scanes E, Parker LM, O’Connor WA, Ross PM (2014) Mixed effects of elevated pCO2 on fertilisation, larval and juvenile development and adult responses in the mobile subtidal scallop Mimachlamys asperrima (Lamarck, 1819). Plos one 0093649

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