Ocean hypercapnia and the future of ‘intoxicated’ fish

Today we have published a paper in the journal Nature which has potential global implications for fish and other marine organisms. Click here to see a short 1-minute explainer video on my Thinkable profile and to download the open-access pre-print.

Given our mission at Thinkable is to allow anyone to learn & engage with researchers, I thought it would be timely to also write a more general piece on what we have found and what it means.

What is ‘Ocean Hypercapnia’?

Humans have pumped a staggering amount of carbon dioxide into the atmosphere from energy use: 1.46 trillion tonnes of CO2. Fortunately about 38% of this pollution has been soaked up by the magic of carbonate chemistry (see the reaction below) and vastness of the ocean. The good news is that this CO2 (72ppm equivalent) can’t cause any climate damage — great! The bad news is that CO2 concentrations increase in the ocean.

Once seawater CO2 levels reach an excessively high level, known as ocean hypercapnia (generally regarded as >1000ppm), neurological and behavioural effects have been shown to occur in a range of marine species. The majority of research shows ocean hypercapnia to interfere with fish brain function in particular, causing all sorts of detrimental behavioural/sensory problems like an inability to find home or know where predators are. CO2 becomes an ‘intoxicant’, impairing their behaviour and leaving them vulnerable to predation & inability to re-settle/populate.

Here is a great short video animation created for one of Thinkable’s competitions from Tulio Rossi, a marine biologist, explaining how fish get ‘Lost at Sea’ under high CO2 conditions. Here’s one of the earlier papers on hypercapnia on juvenile fish species from my colleague, Phillip Munday. Since then, many studies from a range of species including reef fish, temperate fish, Cod, Salmon, sharks and even open ocean squid show various degrees of vulnerabilities.

Fish account for almost 17% of the global population’s intake of protein — in some coastal and island countries it can top 70 percent. Combined with fish directly supporting the livelihoods of up to 12% of the worlds population, the potential implications of hypercapnia are large.

How is this different from “Ocean Acidification”?

Ocean acidification relates to the seawater pH and carbonate ion decreases that are occurring as a consequence of increasing CO2 in the ocean. Ocean acidification has direct consequences for calcifying (shelled) organisms and coral reefs. Ocean hypercapnia however, relates to the direct effects of the CO2 concentration itself within non-calcifying organisms (fish, squid, sharks etc). Of course their is a direct link between CO2 and pH, however it is non-linear and think it’s time we clearly separate between acidification and hypercapnia from a science communication perspective.

What did we find?

Hypercapnia effects have been shown when exposing organisms to high CO2 from a few days to weeks. The objective for our research was to understand how future CO2 levels varied over the entire annual cycle, which would give us clues as to the timing and magnitude of ocean hypercapnia.

Even under the worst-case scenario for anthropogenic emissions (RCP8.5), atmospheric CO2 never reaches 1000ppm by 2100. However what we discovered is that the natural annual CO2 cycle in the ocean gets amplified due to chemistry changes caused by our increasing fossil fuel emissions. In some regions the annual natural CO2 cycle is amplified by up to 10-fold if we continue our emissions unabated, driving extreme fluctuations in CO2.

What this means is that instead of ocean hypercapnia being a distant thought, 1000ppm in the ocean is projected to begin in the Pacific Ocean once atmospheric levels hit 650ppm, spreading to up to half of the ocean by 2100. More worrying is the finding that hypercapnia occurs in major fishery zones with potential significant implications.

The evolution of our unexpected discovery

Previous work lead by a previous student of mine, Emily Shaw, showed this future CO2 amplification to induce extreme CO2 conditions within a shallow coral reef, with CO2 concentrations approaching 2100ppm by the end of the century.

Tristan Sasse, another previous student, wanted to come up with a better way of understanding natural variability of CO2 throughout the entire ocean. It was ambitious, but he did it for his PhD. Here is the underlining method and paper that outlines our approach important for this new discovery.

We then took Tristan’s global data-set for the two state variables needed to calculate CO2 in the ocean and projected future CO2 levels under a high-emissions scenario (RCP 8.5). I was waiting, expecting to see an amplification signal since we had found it locally, but we didn’t expect some regions around the world to undergo up to 8-fold amplification!! I sent it back to Tristan as I didn’t believe the results. There must have be an error but there wasn’t.

The figure below is an example of the amplification we found for a small region in the North Pacific Ocean. The blue and green dots represent present-day levels of carbon dioxide over the entire annual cycle, while the red dots are the likely future levels that we predict. Notice the very large future increase between the low point and high point throughout the year — that level of increase (or amplification) was entirely unexpected.

What do you mean by ‘Amplification’?

Any piece of the ocean exhibits natural variability over the annual cycle. Summer months have high biological production that lower CO2 concentrations, whereas winter months are less productive and bring in CO2 from water below. So lets assume that the annual cycle of CO2 for a given parcel of the ocean remains constant into the future. What would the response be when combining with long-term increases in CO2?

Linear Amplification Response:

If ocean carbon dioxide exhibits a linear coupling, like what happens with temperature or sea-level rise under climate change, then the natural variations are simply superimposed over the increasing trend. Click on the 5-second video above to see how linear coupling works for CO2.

Non-Linear Amplification Response:

However what we found was a non-linear amplification, whereby the future natural cycle was skewed to higher concentrations and much more variable. Click on the example video to see the actual response, with a 4-fold amplification by the end of the century. When you have a threshold like 1000ppm, the result is much earlier and more frequent exposure of that threshold. When looking at these future levels of CO2 across the ocean, we find CO2 hotspots where organisms will have to endure much higher and more extreme CO2 conditions that we previously thought.

What don’t we know about ocean hypercapnia?

ALOT! Although there has been solid evidence of hypercapnia in many species, we haven’t even brushed the surface of understanding if and when hypercapnia occurs for many marine species and how this relates to ecosystem functioning and fisheries. The implications are large, but the biological unknowns are even larger at this stage. Stay tuned.

Ben McNeil, Thinkable, 20 January 2016. Article.

1 Response to “Ocean hypercapnia and the future of ‘intoxicated’ fish”


  1. 1 Jean-Pierre Gattuso 28 January 2016 at 07:33

    It is weird to use the term hypercapnia in the context of ocean chemistry as its definition relates to physiology rather than oceanography “ocean acidification is a condition of abnormally elevated carbon dioxide (CO2) levels in the blood”. I think that it just confuses the picture. The argument that so-called hypercapnia is different than ocean acidification is rather weak as it is now well accepted that ocean acidification covers all changes of the carbonate system generated ocean CO2 uptake: increase in CO2, dissolved inorganic carbon and bicarbonate and decrease in pH, carbonate ion and saturation state of CaCO3.


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