Marine photosynthesis and oceanic pH

Based on four theoretical constructs – a geochemical model, an ocean general-circulation model, an IPCC CO2 emissions scenario for the 21st century, and a logistic function for the burning of earth’s post-21st century fossil-fuel reserves – Caldeira and Wickett (2003) calculated that earth’s atmospheric CO2 concentration could approach 2000 ppm around the year 2300, leading to a concomitant surface oceanic pH reduction of 0.7 units, a change they describe as being much more rapid and considerably greater “than any experienced in the past 300 million years.”

What will be the result for earth’s coral reefs and other calcifying marine organisms if this unprecedented – but purely theoretical – surface oceanic pH reduction actually comes to pass? Kleypas et al. (1999) and Buddemeier et al. (2004) have claimed that the projected increase in the air’s CO2 content, together with its simulated decline in surface ocean water pH, will dramatically decrease coral calcification rates, which they say could lead to a major slow-down, or even reversal, of reef-building and the potential loss of reef structures. There are, however, some good reasons for believing otherwise.

First and foremost is the fact that calcification cannot be accurately modeled on a purely physical-chemical basis that is readily amenable to mathematical representation, for it is a biologically-driven physical-chemical process that behaves much differently than has been implied by the simplistic “lifeless” analyses of the researchers cited above, as has been repeatedly demonstrated by many of the studies we have reviewed and archived under the heading of Calcification in our Subject Index.

Second, and the subject of this Editorial, is the fact that marine photosynthesis tends to increase surface oceanic pH, countering the tendency for it to decline as the air’s CO2 content rises, as has been demonstrated by Lindholm and Nummelin (1999). This phenomenon has been shown to have the ability to dramatically increase the pH of marine bays, lagoons and tidal pools (Gnaiger et al., 1978; Santhanam, 1994; Macedo et al., 2001; Hansen, 2002), as well as significantly enhance the surface water pH of areas as large as the North Sea (Brussaard et al., 1996). And to this sizable body of research can now be added the findings of Middelboe and Hansen (2007), who studied a wave-exposed boulder reef in Aalsgaarde on the northern coast of Zealand, Denmark, and a sheltered shallow-water area in Kildebakkerne in the estuary Roskilde Fjord, Denmark.

CO2 Science, 22 August 2007. Web page.

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