Salt Water - Oceans

THE MARINE CARBON CYCLE

Many marine organisms need carbonate—dissolved carbon dioxide—in order to build their shells. But too much carbon dioxide can upset the balance.

When carbon dioxide is absorbed by the ocean, it dissolves into carbonic acid and splits into two different types of ions, or charged atoms: hydrogen ions and bicarbonate ions.

The hydrogen ions combine with carbonate ions in seawater to form even more bicarbonate ions. As the amount of carbon dioxide in the atmosphere increases, the concentration of hydrogen ions in the water also increases, and more carbonate ions join to form bicarbonate ions. This means there is less carbonate available for organisms that use it to grow—corals, mussels, kina and starfish.

The concentration of hydrogen ions in seawater determines its pH, or ‘potential of hydrogen’.

The pH scale runs from zero (the strongest acids, such as hydrochloric acid) to 14 (the strongest bases, such as lye). Neutral solutions, such as distilled water, sit in the middle of the scale at 7. Seawater should be slightly basic, with a pH of around 8.2, but globally it has dropped below 8.1 over the past two centuries. While a change of 0.1 doesn’t sound like much, the pH scale is logarithmic, like the Richter scale, so this represents a 10-fold increase in acidity. The Munida time series shows that pH in New Zealand’s coastal waters has been decreasing at the same rates observed in the Northern Hemisphere. If this trend continues, the open ocean will reach pH levels of 7.8 by the end of this century, but in some areas along the coast, it already drops as low as 7.7.