Why are the oceans becoming more acidic and how does that threaten biodiversity? Human activities produce excessive carbon dioxide and much of it is absorbed by the oceans, where it is converted to an acid.
Instead of global warming, these days we talk about global change. And ocean acidification kind of worked its way into the fabric of this change, making it possible for us to look at the problem of global change in a more expansive way than the more simplistic idea of global warming.
But what is ocean acidification? And how exactly does it affect our topic of interest, biodiversity? First we need to look at the cause of ocean acidification in order to understand what it actually is and what it means to biodiversity.
Essentially, if you talk about this big cloud—this puff of CO2 that’s produced by human activity—30 to 40 per cent, almost half of that cloud ends up dissolved in the ocean. The rest stays in the atmosphere, or it’s incorporated into living things in some form or another, usually as plant material or the bodies of other producers. But the huge amount of CO2 that gets dissolved in the oceans is definitely going to be doing something.
The added CO2 in the ocean causes an increase in acidity. Acidity is measured by something called pH. And it’s worth talking a little bit about pH for a moment. A couple of things to note about the pH scale: it goes from 0 to 14, where 0 is highly acidic and 14 is highly alkaline, also called highly basic. A pH of 7 is neutral, like distilled water. So if you increase acidity, the pH is dropping. And if you increase alkalinity, the pH is going up.
Note that the pH scale is logarithmic, which means that each step is a factor of 10. If you go from a pH of 6 to something slightly less alkaline, that is, more acidic, at a pH of 5, you’re actually increasing the acidity 10 times. Going from pH 6 to pH 4, it’s more acidic by a factor of 10 times 10, which means 100 times more acidic.
Most importantly, pH is a measure of potential. That’s where the p in pH comes from. The power, or potential, of a liquid to make charged hydrogen atoms or ions. Think of pH as potential H, or power of hydrogen ions. We’ll see in a moment that the power of such a tiny thing as a hydrogen ion is so crucial.
First, let’s look at this problem of introducing carbon dioxide to sea water. Over the industrial period between about 1751 and the early 1990s, the surface ocean pH decreased from about 8.25 to 8.14. That doesn’t sound like a lot, but remember we’re talking about an average for the entire global ocean over the globe, and it’s logarithmic. So we’re actually talking about a 30 per cent increase in hydrogen ion concentration in the ocean.
We’ve got CO2 in the atmosphere that’s going into the water. The CO2 breaks down, we have a chemical reaction, where the CO2 plus the water leads to carbonic acid. The process looks like this. When you have the carbonic acid in the water, a couple of things happen. Each carbonic acid molecule can release one of its hydrogen ions to make something called a bicarbonate. And a bicarbonate molecule can further break down into a carbonate ion. The big issue here is, you get both of these molecules—bicarbonate and carbonate—by losing hydrogen ions, which are now zipping around freely in the water. And remember what we said about hydrogen ions—they’re going to increase the acidity of the water. That’s the key point. Through the addition of CO2, you set up a chain of events that results in these powerful little hydrogen ions being set free as the active ingredient, or culprit, in the damage that acids can do.
It’s worth talking about this global process in terms of rate. It’s not so much that the pH levels are changing, but they’re changing faster than anything we’ve seen for a very long time. The current rates of acidification are very similar to those during an enormous greenhouse event that occurred at the Paleocene–Eocene boundary 55 million years ago. And that time was marked by huge extinctions at very fundamental levels of ecosystem production, particularly in the deep sea. Geologic history tells us that biodiversity can be threatened by exposure to increased acidity in the oceans. There’s a huge range of harmful consequences, including drops in metabolic rate, or drops in immune response to other organisms such as parasites or bacteria that are in the environment, and we know that drops in pH can cause destruction of coral by triggering chemical reactions that result in an overall drop in the amount of carbonate ions available.
Okay, so what does that mean? Well, it means a bit more chemistry. Many organisms that live in the ocean use a very special building material, calcium carbonate, which is dissolved in sea water. And it’s made by this reaction. Add calcium atoms to carbonate ions, and you make calcium carbonate—a material that goes into the skeletons of organisms that live in the sea such as corals, and molluscs, and crabs. They’re very dependent on calcium carbonate.
Unfortunately, these free carbonate ions are also recombining with those busy, very reactive hydrogen ions to make more bicarbonate. So this reduces the available calcium carbonate that organisms would otherwise be able to use, and that means that organisms with a calcium carbonate skeleton are going to have trouble maintaining their skeletons simply because they can’t get enough of the calcium carbonate to grow, or repair their shells and skeletons.
It turns out that it’s not just corals, molluscs and crabs that are affected. Single-celled organisms called foraminifera and coccolithophores, which are close to the base of the food web and terribly important in marine ecosystems, are among the most affected. If you put a foramniferum, or foram, under a microscope, they look like little spirals and funny-shaped boxes. They’re fantastic things to look at. Forams are like little single-celled amoebae that make shells. Their metabolism and ability to make those shells is deeply affected by pH levels in the ocean.
Now, coccolithophores are really interesting, somewhat mysterious single-celled algae that also take up calcium carbonate from the ocean to make a coccolith. Lith means rock, and cocco roughly means berry-shaped. So these organisms are shaped like tiny fruit, but with a rocky covering. Not everyone knows about these, but now you do! Because they’re plants, they are really important as phytoplankton producers in ocean ecosystems. No one is too sure why they make their calcium carbonate coverings, but the mere fact that they are making their calcium carbonate shells means that they’re also going to be deeply affected by decreasing oceanic pH. And there’ve always been lots and lots of coccolithophores. The White Cliffs of Dover are made up almost entirely of fossil coccoliths. Coccolithophores produce a chemical that contributes to the formation of clouds. Some scientists even think that threatening the existence of coccolithophores could result in a reduction of cloud cover over the oceans, reducing the reflectivity of the Earth, and thereby increasing the rate of global warming.
As I mentioned, bigger things like corals and crabs and snails and clams will also have some issues with their ability to secrete calcium carbonate. They depend so much on that. Scientists have run experiments in which increasing the amount of CO2 in the air above a tank of sea water can actually increase the rate at which the skeletons of some of these forms will dissolve. Now, notice that I said some. It’s variable. But we’re seeing some effects on almost every major group of organisms we’ve looked at so far, even in starfish and sea urchins, which have protective skin over their entire bodies, they actually have an internal skeleton (like fish, or you, or me), but even those have problems. Particularly in larval stages. And these larvae form a huge part of the plankton, and remember how crucial plankton are in food webs in the sea.
Even for organisms that don’t have calcium carbonate skeletons and shells, increased acidity can be a problem. Hypercapnia, which is an actual excess of CO2 in the body fluids of organisms, can happen in things like fish, or squid, and mess with their immune responses. Excess CO2 can even make it difficult for baby clownfish to distinguish among the odours of friends and foes, and interfere with sensory mechanisms or even the ability to hear predators coming.
The latter is kind of interesting, because you can get changes in the acoustic properties of sea water by changing its chemistry. And that has huge implications for any animal that uses echolocation, for example. CO2 increases ocean noise, which is already getting noisier all the time through other human activities. More acidic environments can interfere with the construction of things like ear bones, and balance organs, such as what are known as statoliths, tiny little stones that squid make and hold in special chambers in their bodies. Statoliths allow squids to sense pressure and changes in direction and movement. And this just illustrates how little we know. Things have unusual kinds of cascading effects that you might not think of, over and above this inability to make calcium carbonate skeletons from sea water.
Here’s another example that has a complicated story. Like land plants, sea grasses do a bit better in building their bodies when CO2 levels are increased. And sea grasses are really important. They’re valuable feeding and spawning sites for a variety of species. So if you enhance the growth of sea grasses, maybe you’re doing something good? Well, what we don’t know is if those local benefits of better sea grass growth will be outweighed by the wider distruption to the marine food chain as a whole, and what that means for biodiversity. These are all pretty complicated things. We don’t really know what the long-term or even short-term interplay of all these different factors is going to be. We definitely need some focused research on these topics. But we do know that ocean acidification is certainly mostly bad news. It’s a global problem, and we’re going to need to start talking about global solutions as soon as possible.