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Sea butterflies show signs of acidification


Limanicina helicina

Small sea snails (Limanicina helicina) account for some 12 per cent of the entire flux of carbonate on our planet (Source: NOAA)

On the last two occasions, I've talked about two aspects of climate change. First, I spoke about how in the Arctic, the warming climate has melted about 80 per cent of the volume of the late summer ice since 1980.

Then, I spoke about the thawing of the permafrost that covers one quarter of the land area in the northern hemisphere, and how it has begun to release greenhouse gases — once again due to climate change.

Let me finish by talking about a third aspect of climate change — its evil twin, ocean acidification.

That's right, we humans are making the oceans more acid. About one quarter of the carbon dioxide we have dumped into the atmosphere has been soaked up by the oceans. As the carbon dioxide dissolved into the seawater, it made it slightly more acid. Now I don't mean that the ocean will burn your skin next time you go for a swim. No, the oceans have shifted from being slightly alkaline to just a little bit less alkaline.

When you measure acidity and alkalinity in numbers, you use the pH scale, which ranges from 0 to 14. Something really acid like stomach acid has a pH of 1, while something really alkaline like bleach has a pH of 13. Distilled water is neutral (neither acid nor alkaline) and has a pH of 7.

A hundred and fifty years ago, the oceans used to have a pH of 8.2, but now have a pH of 8.1 (a tiny bit closer towards the acid end of the pH scale). But, I hear you cry, surely a slight drift of 0.1 towards the acid end of the scale would have no effect on the oceans?

Well, it turns out that it does. When carbon dioxide dissolves in water, it makes carbonic acid, which is broken up into bicarbonate and a proton. A shift of 0.1 in the oceanic pH actually means a 30 per cent increase in the number of protons.

We get a problem because these extra protons like to combine with carbonate ions turning them into bicarbonate as well. The result is that adding carbon dioxide to seawater dramatically reduces the amount of carbonate that is available for sea creatures that use it to make calcium carbonate for their skeletons and shells. These creatures include coral, plankton, coralline algae, mussels, sea urchins, krill and pteropods.

Until recently, we really didn't think that having fewer carbonate ions would affect sea creatures for a century or more. Unfortunately, we were wrong.

Late in 2012, it was reported that one particular sea creature was actually having its shell dissolved by the increasing acidity of the ocean. It's the pteropod — a free-swimming sea snail that moves about thanks to wings like a butterfly. It lives for two years or longer and grows to have a shell about 1 centimetre in diameter.

Down in the Antarctic, it is the main sea creature that makes calcium carbonate. In fact, over the whole planet, these sea butterflies account for some 12 per cent of the entire flux of carbonate on our whole planet.

In the North Pacific, pteropods can make up to 60 per cent of the diet of juvenile pink salmon. But it was at the other end of the world, down in the Antarctic, just past the southern extremes of the Atlantic Ocean, that scientists found all kinds of changes to the shell of the sea snail. The acid ocean was dissolving its shell. One layer called the prismatic layer was sometimes completely dissolved and missing, while deeper layers were partially exposed with increasing porosity of the matrix — overall the shell was very fragile due to being dissolved by the more-acid ocean.

Now why did this was first happen in the cold Antarctic? The answer is that the chemical changes that lead to the unavailability of calcium carbonate are more extreme at low temperatures. So the coral reefs are safe for a while, although the scientists have been seeing less calcification on various coral reefs. One problem is that about one quarter of all the fish eaten by one billion people in Asia comes from coral reefs. Another problem is that ocean acidification will influence the biochemical dynamics of many other elements (such as iron, phosphorus, zinc, vanadium, chromium and so on) that are essential in the ocean's web of life.

Suppose we continue with business as usual, and continue to dump carbon dioxide into the atmosphere until the end of the 21st century. In this scenario, it's projected that the oceans will get more acidic, and drop another 0.3 or 0.4 points down the pH scale.

Perhaps this little sea butterfly is like the canary that the olden-days miners used to carry underground to warn them when the air was bad. The trouble is that when the canary started to croak, the miners back then could run back up and out of the mine to safety. In our case, if our sea butterflies die, where do we run?

Tags: climate-change, oceans-and-reefs

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Published 11 December 2012

© 2015 Karl S. Kruszelnicki Pty Ltd