What ocean acidification means: a question of chemical equilibria

More carbon dioxide changes several quantities at once

Dissolved carbon dioxide is in equilibrium with various carbon-containing species in seawater. These include bicarbonate and carbonate. When additional carbon dioxide is absorbed, these equilibria shift; the concentration of hydrogen ions increases and the pH falls. Because the pH scale is logarithmic, a small numerical change must not be equated with a correspondingly small relative chemical change. At the same time, seawater remains alkaline under many common conditions. Acidification here describes the direction of change. For calcifying organisms, changes in carbonate availability and the saturation state of certain calcium carbonate minerals are particularly relevant. The basic chemical principle is well established. How large the change is at a particular location, however, depends on water composition, exchange processes and biological activity.

Measurements and laboratory experiments answer different questions

Long-term observations show how ocean chemistry changes under real conditions. Spatial differences, seasons and short-term processes overlap. A laboratory experiment, by contrast, can deliberately alter individual factors and thereby investigate possible causes. If, for example, carbon dioxide levels are increased in a controlled way, an organism’s responses can be measured under defined conditions. Such experiments are important, but do not automatically represent the entire natural living environment. The way the chemistry is adjusted, the duration of the experiment and the accompanying variables monitored affect what it can tell us. Different life stages may also respond differently. A robust assessment therefore combines chemically well-characterised experiments with field observations. Neither a single pH value nor a single sensitive species is sufficient to infer all the consequences for a diverse ecosystem or to equate different locations.

Biological responses are not a uniform switch

Changes in carbonate chemistry can affect calcification, growth or other physiological processes. The response is not, however, the same for all species and conditions. Organisms regulate their internal environment differently and simultaneously face other stresses or have different resources available. The claim that all marine animals respond identically to a falling pH is therefore scientifically untenable. Equally, an insensitive species does not disprove the underlying chemical change. NOAA describes observation and biological research as complementary tasks. The shift in equilibria following additional carbon dioxide uptake is established. The magnitude of specific ecological consequences must be determined using appropriate data. Good research therefore identifies not only an average response, but also variability, adaptation over time and the conditions under which a result can reliably be applied to other habitats.

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