What is ocean acidification? (a really brief summary)

Again, as part of my teaching assistant job, I’m going to try to do a little overview of ocean acidification (OA).

Nuts and bolts

Yes, I’m going to talk about chemistry.  Wait!  Don’t run away!  It won’t be too scary at all.  Let’s start with the sea surface, where CO2 is exchanged between the atmosphere and the ocean.  It’s constantly trying to reach equilibrium which means that if that if CO2 levels in the atmosphere are high, then CO2 gets “pulled” into the ocean.

When CO2 dissolves into the sea water, it can take on one of four forms:  dissolved (aqueous) carbon dioxide (CO2 (aq)), carbonic acid (H2CO3), bicarbonate (HCO3), and carbonate (CO32−).

carbon species in water

What determines the relative concentrations of these different forms of carbon, you might ask?  Turns out it is strongly dependent on the temperature and the alkalinity of the water.  Have a look at this diagram, also known as a “Bjerrum plot”:

bjerrum
An aside: You may be wondering why carbonic acid doesn’t show up on the Bjerrum plot. That’s because it occurs in extremely low concentrations and is frequently lumped in with the CO2.

Looks kinda wacky.  Let’s go through it a step at a time.  The x-axis is indicating the pH, which is a measure of how acidic (or basic) the solution is.  In other words, it’s a measure of the concentration of hydrogen ions (H^+) hanging around in the water (that’s what the H in pH stands for). It’s worth noting here that pH is, by definition, a logarithmic scale.  If you move down one number on the pH scale, you’re decreasing by a factor of 10.  If you move up one number, you’re increasing by a factor of 10.  In case you’re curious, here’s the equation:

pH = -log([H^+])

So: the pH is the negative log of the hydrogen ion concentration, [H+]. A pH of 7 is neutral (fresh stream water would have a pH of about 7).  If it’s higher, it’s basic, and if it’s lower it’s acidic.

I know I said I wouldn’t use too much chemistry, but for those who might be interested, here’s what’s going on:

CO2 (aq) + H2O \leftrightarrow H2CO3 \leftrightarrow HCO3 + H+ \leftrightarrow CO32− + 2 H+.

As more CO2 is dissolved into the ocean, more hydrogen ions are released, causing that vertical blue line in the Bjerrum plot to move to the left. More hydrogen ions means decreased pH, aka increased acidity. Which is what people are talking about when they say “ocean acidification”.  And when that happens, the equilibrium ratios of the different chemical species of carbon shifts too – carbon dioxide increases, bicarbonate doesn’t change much, and carbonate decreases fairly quickly with even small changes in pH.

Magnitude of OA

How much has the pH of the oceans changed since the industrial revolution?  It has decreased by approximately 0.1 pH units.  Maybe it sounds small, but remember that pH is a logarithmic scale – so it’s actually a 30% increase in hydrogen ion concentration.

Impacts of ocean acidification

Calcium carbonate shell dissolution

I’ll start off with this:  it’s complicated.  And no one can really say exactly what is going to happen.  But we are able to ascertain certain things.  There have been, and continue to be, many studies focusing on exactly what the impacts of OA will be.  This is often done by taking certain organisms that are believed to be susceptible to acidification, and conducting controlled experiments where they are exposed to increased acidity.

The animals that are likely to be affected are those with shells or plates made of calcium carbonate (CaCO3).  These shells and plates are formed when dissolved ions in seawater precipitate to form CaCO3. In order for those shells to remain intact (ie. not dissolve!) the surrounding seawater needs to be saturated with respect to CO32−.

carbonate saturation

Detrimental effects have been shown for many organisms, for example: shellfish, foraminifera, coccolithophores, pteropods (e.g. [1],[2],[3]). It’s well worth noting that in certain cases, and for certain species, studies have shown surprising effects in the opposite direction (e.g. [4],[5]).  Like I said:  It’s complicated!

There’s a lot more you can learn about ocean acidification, but it’s more than I want to cover here.  As always, if any of my (7 or so) readers finds errors in this or other posts: let me know so I can fix them!

 

[1] Orr, James C., et al. “Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms.” Nature 437.7059 (2005): 681-686.
[2] Beaufort, L., et al. “Sensitivity of coccolithophores to carbonate chemistry and ocean acidification.” Nature 476.7358 (2011): 80-83.
[3] Kuffner, Ilsa B., et al. “Decreased abundance of crustose coralline algae due to ocean acidification.” Nature Geoscience 1.2 (2007): 114-117.
[4] Ocean Acidification: A risky shell game (WHOI Oceanus) https://www.whoi.edu/oceanus/viewArticle.do?id=52990
[5] Range, P., et al. “Seawater acidification by CO< sub> 2 in a coastal lagoon environment: Effects on life history traits of juvenile mussels< i> Mytilus galloprovincialis.” Journal of Experimental Marine Biology and Ecology 424 (2012): 89-98.