Coral growth and climate change
The growth and subsistence of coral depends on a number of requirements: temperature, irradiance, calcium carbonate saturation, turbidity, sedimentation, salinity, pH and nutrients. The level of these variables influences the physiological processes of photosynthesis and calcification, and also survival. In turn these requirements are affected by meteorological processes, which results in coral reefs occurring in only select areas of the world’s oceans. The growth rate of corals varies with species, location on the reef and age of the colony. There are many different morphologies of corals such as brain (or massive) coral, branching coral and plate coral. These all grow at different rates. For example the growth rates of the massive coral, Montastrea annularis, measured in the Caribbean, was 0.06 - 1.23 cm yr-1 , while the growth of the branching coral, Pocillopora eydouxi, measured in the Eastern Pacific, was 2.1 - 3.9 cm yr-1 . This article describes some of the factors that influence coral growth and survival, from climate change to coral mining, and mentions some of the approaches that may mitigate against coral reef destruction. Figure 1 summarizes the connections between different meteorological processes and coral requirements for growth and survival. These processes affect the distribution of corals on both global and synoptic scales.
Corals grow by the deposition of a calcium carbonate skeleton (calcification) in the form of aragonite by combining calcium ions with carbonate ions. The concentration of calcium ions in sea water is much higher than the concentration of the carbonate ion, therefore the rate of calcification is controlled by the saturation state of carbonate ions in the sea water. The saturation state of calcium carbonate is determined by the concentration of carbon dioxide (CO2), which dissolves in water to form an acidic solution consisting of three species of inorganic carbon; carbonic acid: H2CO3, bicarbonate ion: HCO-3, and carbonate ion: CO2-3. These are related by the following equilibrium equation:
The concentration of CO2 in water is largely controlled by the atmospheric concentration of CO2 and temperature. Therefore the concentration of calcium carbonate in the ocean is highly correlated with temperature.
Calcium carbonate also reacts with CO2 and water as described by the following equation:
This means that the more CO2 dissolved in the water, the more readily the calcium carbonate will dissolve. CO2 is more soluble in cold pressurized water and less soluble in warm non-pressurized water. Therefore the concentration of CO2 is smaller in shallow tropical waters, which reduces the solubility of the calcium carbonate, allowing corals to precipitate calcium carbonate skeletons in these conditions.
Dissolved CO2 also affects the pH of water. An increase in CO2 concentration causes a decrease in pH, which results in a decrease in the levels of the carbonate ion. As corals use the carbonate ion to form their skeletons, a decrease in the levels of carbonate ion will lead to a reduction in the calcification rate, less carbonate accumulation on average, and probably lower extension rates or weaker skeletons in some corals. The result of this would be a reduction in the ability of the coral to compete for space and to withstand erosion.
Many studies have been carried out to determine the influence of temperature and carbonate concentration on the growth rate of corals. Some studies have measured the growth rate directly through extension rate, or increase in weight, others measure the calcification rate. These rates can be measured directly from living corals or from cores drilled from the ground. Using the process of X-ray microanalysis, discrete seasonal banding can be detected and measured in the coral cores, and from this, coral growth rates over many decades can be measured. The calcification rate is a product of the growth rate and the linear extension rate. This means that a coral that has a lower calcification rate could have the same extension rate as a coral with a higher calcification rate, if the skeleton that was being formed had a lower density. It is the precipitation rate of calcium carbonate that determines the growth or accretion rate of the reef and therefore the net loss of calcium carbonate as well as net calcification rate needs to be considered.
From predictions of future atmospheric CO2 levels that the surface waters of the extra-tropics may well reach a level of undersaturation in the future. The tropical and warmest subtropical waters are unlikely to become undersaturated. Even though the waters are likely to remain supersaturated, the degree of supersaturation affects the rate of coral calcification. And when the saturation state of calcium carbonate is greater than one (supersaturated), the calcification rates of all calcifying organisms, including corals, decrease in response to the decreasing saturation state.
As we have seen, corals growth within very narrow limits of temperature, irradiance, salinity, pH and turbidity; all variables which are influenced by climate and weather. Corals are also influenced by direct human intervention – bomb fishing, coral mining, coastal development, marine pollution, overfishing and overexploitation of resources, and inland pollution and sedimentation. Such manifold stressors, together with climate change, make for a bleak outlook. It is clear that climate change will alter many aspects of what we know as coral reefs; what is less clear is exactly how, or what the results will be.
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