An Introduction to Ocean Acidification

What is meant by aragonite saturation state (Ωarg)?

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Surface tropical seawaters are generally supersaturated with respect to the carbonate minerals (e.g. calcite, aragonite, and high-magnesium calcites) from which marine organisms construct their shells and frameworks. At deeper water depths, seawater becomes undersaturated and these minerals begin to dissolve, imparting an important control (amongst other factors) on the distribution of coral reefs. We refer to the degree to which seawater is saturated with respect to these minerals as 'saturation state' and denote it using the Greek term Ω (omega).

The effects of ocean acidification on calcification rate appears not to be directly related to changes in pH per se, but instead to corresponding changes in the degree to which seawater is supersaturated with respect to the carbonate minerals (e.g., aragonite) [Langdon and Atkinson, 2005]. A change in carbonate ion concentration results in a proportional change in Ωarg such that as ocean acidification continues, the surface ocean Ωarg-values will decline. As the saturation state declines, it is harder for marine calcifiers to precipitate the calcium carbonate they need to build their skeletons (see figure below). By the year 2065, rates could decline 60 ± 20% relative to preindustrial levels.

coral calcification rates and aragonite saturation state

A growing number of studies have now demonstrated a relationship between coral calcification rates and aragonite saturation state (Ωarg) The figure above was modified from Langdon and Atkinson, 2005). The figure shows that prominent coral reef ecosystems do not currently reside in waters exhibiting oceanic Ωarg<3, perhaps representing a critical threshold (Guinotte et al., 2003). The colors denote a convention employed by Guinotte et al. [2003]: surface waters exhibiting Ωarg > 4 are deemed 'optimal' (blue), 3.5 - 4.0 are 'adequate' (green), 3.0 - 3.5 are 'low' (yellow/orange), and less than 3.0 are considered 'extremely marginal' (red).



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