Acidification threatens the world’s oceans. Specifically, as CO2 levels go up, pH levels come down. Acidity depends on the presence of hydrogen ions (the H in pH) and more hydrogen ions mean, counter-intuitively, a lower pH. Expose the surface of the ocean to an atmosphere with ever more CO2, and the gas and waters will produce carbonic acid, lowering pH on a planetary scale. The declining pH does not make the waters acidic (they started mildly alkaline). But it makes them more acidic, just as turning up the light makes a dark room brighter.

Ocean acidification has further chemical implications: more hydrogen ions mean more bicarbonate ions and fewer carbonate ions. Carbonate is what corals, the shells of shellfish, and the outer layers of many photosynthesizing plankton and other microbes are made of. If the level of carbonate ions falls too low the shells can dissolve or might never be made at all. There is evidence that the amount of carbonate in the shells of foraminifera, microplankton is crucial to ocean ecology. However, how bad the change in pH will be for oceans is not clear, yet.

There is no doubt that a pH drop is underway. This is because the atmosphere does not have an iron grip on the CO2 level in surface waters. Increased photosynthesis will use up CO2; increased respiration produces more of it. Water coming up from below will often have a lower pH than the surface water because at depth there is no photosynthesis but plenty of respiration. In many places, natural variations in pH will be larger than long-term changes in its meaning. This is not to say that such changes have no effect. If peak acidities rather than long-term averages matter most, natural variability could worsen things.

The Mediterranean Institute of Advanced Studies recently analyzed data from a wide research sample into how individual organisms respond to increased CO2 in their seawater. The Mediterranean Institute found that the range of responses was wide, with some seeming to prefer the lowered pH. Also, the Mediterranean Institute found that the effects to be expected in the 21st century were on average comparatively modest.

If some creatures can tolerate lower pHs and others cannot, you might expect things to average out: the tolerant and adaptable prosper, the more persnickety perish. For the “primary producers” in the ocean – the mostly single-celled creatures that photosynthesize – this will probably be the case. However, changes in the relative prevalence of different photosynthesizers could still matter.

The ecology of the oceans is all about who eats what, and small changes in the population of certain creatures near the bottom of the web could have large effects on the larger ones that eat them. Some creatures may be a double whammy by having less of what they like to eat and by the pH itself, amplifying the disruption. Adaptation is not without costs: dealing with lower pH may divert a creature’s resources from other ends. Ocean ecosystems are beset by changes in nutrient levels due to run-off near the coasts and by overfishing, which plays havoc with food webs nearly everywhere. And the effects of global warming need to be included, too. Surface waters are expected to form more stable layers as the oceans warm, affecting the availability of nutrients and, increasingly feared, O2. Some suspect that these physical and chemical effects of warming may prove a greater driver of productivity change in the ocean than altered pH.

Georgios Ardavanis – 11/12/2023