The Burning Issue of Ocean Acidification

One of the stand out scenes in the movie Dante’s Peak, is the grandmother jumping from the row boat into the lake – her legs dissolving in the acidic water as she pulls the boat to shore.  This scene is etched into my mind, so when I first heard about ocean acidification – this was what came into my mind.  Nothing beats heading down to the beach on a hot sunny day to dip your toes into the cooling water. Did this new threat mean that one day, that toe being dipped would be pulled back with a squeal of pain as it was burned by acidic seawater? 

Admittedly it seemed fairly far-fetched, so I didn’t pay too much attention.  That was until I went to a Café Scientifique meeting run by the local museum.  It featured a marine biologist who had seen shellfish stocks completely decimated on the west coast of America due to ocean acidification, and who was now in New Zealand trying to prevent the same thing happening to our mussel farms.  He talked about the small decrease in pH that was caused by the increased carbon dioxide levels, and the big impact this had to anything with a calcium structure in the ocean – including my favourite takeaways! 

Now I was concerned… but I was also pretty confused.  How did the increase in carbon dioxide in the air make our oceans more acidic?  How was a decrease of only 0.1 pH unit since the early 1800s (American Chemical Society, 2012), considered so worrying?  The answers came later that year while studying environmental chemistry – at last I had my a-ha moment – and my uh-oh.

The oceans absorb around 22 million tonnes of carbon dioxide each day – about a quarter of what is released by the burning of fossil fuels (Ocean Portal Team, 2015).  When the carbon dioxide in the air dissolves into the oceans, it combines with water molecules to produce carbonic acid (H₂CO₃), then disassociates into hydrogen ions and bicarbonate (HCO₃^-).  This increase in hydrogen ions is what lowers the pH (potential hydrogen) value. 

Creation of Carbonic Acid

CO₂ + H₂O → H₂CO₃

Disassociation of Carbonic Acid

H₂CO₃ → H⁺ + HCO₃^-

The oceans natural buffering system attempts to neutralise the excess hydrogen ions by pulling carbonate away from the calcium carbonate in shells and coral, and forming more bicarbonate. 

Disassociation of Calcium Carbonate

CaCO₃ → Ca²⁺ + CO₃^2-

Creation of Bicarbonate

H⁺ + CO₃^2- → HCO₃^-

This is what is causing the deterioration in the coral reefs, and the reduction in shellfish numbers.  It also has the potential to reduce populations of any other plant or animal species that requires calcium carbonate for their development, particularly in the early formation stages. The risk to fish who have calcium based skeletons could be potentially devastating to the world’s fisheries.

So while we may not ever reach the point of not being able to dip our toe into the water, and feel that cooling sensation, we do need to be concerned that the fish in our fish ‘n’ chips, or our fresh mussels that we tuck into on the beach after our swim, could well become a thing of the past.

References:

Middlecamp, C.H., Keller, S.W., Anderson, K.L., Bentley, A.K., Cann, M.C., Ellis, J.P. (2012). Chemistry in context: applying chemistry to society (7th ed.) New York, NY: McGraw-Hill.

Ocean Portal Team.  (2015). Ocean Acidification.  Retrieved from http://ocean.si.edu/ocean-acidification