Ocean Acidification

The increase in atmospheric CO2 presents a second major change for reef-building corals and other marine calcifiers. Nearly half of the CO2 that enters the atmosphere is absorbed by the ocean, where it reacts with water to form carbonic acid (Equation 1). Carbonic acid dissociates into bicarbonate and a proton (Equation 2). The protons released by the entry of CO2 into seawater then react with carbonate ions to form additional bicarbonate ions (Equation 3). The problem is that relatively small rises of CO2 in the atmosphere at the pH range of seawater will cause a large decrease in the concentration of carbonate ions.

The calcification rate of a range of marine organisms as diverse as microalgae (coccolithophores), molluscs (e.g. clams, pteropods) and corals is strongly dependent on the concentration of carbonate ions in seawater. Naturally, the concentration is highest in the warmer tropical regions due to the reduced solubility of CO2 in warm v. cold water (i.e. less CO2 dissolving into the ocean means more carbonate ions). The pH of the ocean has already decreased by 0.1 pH unit with the concentration of carbonate ions decreasing as much as 30 ^mol kg"1. Corals were among the first organisms identified as

Figure 10.4 Examples of diseases reported on Great Barrier Reef corals. A, White Syndrome on tabulate Acropora (photo: G. Roff); B, Black Band Disease affecting Pavona sp. (photo: G. Roff); C, Brown Band Disease on a branching Acropora (photo: O. Hoegh-Guldberg) and D, White Spot syndrome on Porites (photo: O. Hoegh-Guldberg). i, living tissue; m, advancing margin of disease; d, dead exposed coral skeleton; p, concentrations of ciliates.

Figure 10.4 Examples of diseases reported on Great Barrier Reef corals. A, White Syndrome on tabulate Acropora (photo: G. Roff); B, Black Band Disease affecting Pavona sp. (photo: G. Roff); C, Brown Band Disease on a branching Acropora (photo: O. Hoegh-Guldberg) and D, White Spot syndrome on Porites (photo: O. Hoegh-Guldberg). i, living tissue; m, advancing margin of disease; d, dead exposed coral skeleton; p, concentrations of ciliates.

having a major problem with the rising concentration of atmospheric CO2 and the decrease in the concentration of carbonate ions. Subsequent studies have shown that coral calcification is linearly related to the carbonate ion concentration. These studies have shown consistently that the calcification of coral reef communities effectively becomes zero at carbonate concentrations of 200 ^mol kg1 or less. Significantly, carbonate concentrations of 200 ^mol kg1 occur when atmospheric concentrations of CO2 rise beyond 450 ppm. Given that coral reefs represent a balance between calcification and erosion (Chapter 8), it would appear that atmospheric CO2 concentrations would need to remain well below

450-500 ppm if reef calcification (calcification minus erosion) is to remain positive against the forces of physical and biological erosion. Given that there is growing evidence that erosion (particularly bioerosion) is likely to increase under atmospheric CO2, these thresholds become even more real (see Chapter 8).

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