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Marine Photosynthesis and Oceanic pH
Volume 10, Number 34: 22 August 2007

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.

So what did they find?

The two researchers report, in line with what one would expect if photosynthesis tends to increase surface-water pH, that (1) "daytime pH was significantly higher in spring, summer and autumn than in winter at both study sites," often reaching values of 9 or more during peak summer growth periods vs. 8 or less in winter, that (2) "diurnal measurements at the most exposed site showed significantly higher pH during the day than during the night," reaching values that sometimes exceeded 9 during daylight hours but that typically dipped below 8 at night, and (3) that "diurnal variations were largest in the shallow water and decreased with increasing water depth."

In addition to their own findings, Middelboe and Hansen cite those of (1) Pearson et al. (1998), who found that pH averaged about 9 during the summer in populations of Fucus vesiculosus in the Baltic Sea, (2) Menendez et al. (2001), who found that maximum pH was 9 to 9.5 in dense floating macroalgae in a brackish coastal lagoon in the Ebro River Delta, and (3) Bjork et al. (2004), who found pH values as high as 9.8 to 10.1 in isolated rock pools in Sweden. Noting that "pH in the sea is usually considered to be stable at around 8 to 8.2," the two Danish researchers thus concluded that "pH is higher in natural shallow-water habitats than previously thought."

Adding to these findings the fact that rising atmospheric CO2 concentrations tend to stimulate marine photosynthesis (see Aquatic Plants (Marine - Macroalgae and Microalgae) in our Subject Index), it can be appreciated that doom-and-gloom stories of impending extinctions of earth's marine calcifying organisms due to a CO2-induced decrease in oceanic pH are merely that - stories, without any basis in fact.

Sherwood, Keith and Craig Idso

References
Bjork, M., Axelsson, L. and Beer, S. 2004. Why is Ulva intestinalis the only macroalga inhabiting isolated rockpools along the Swedish Atlantic coast? Marine Ecology Progress Series 284: 109-116.

Brussaard, C.P.D., Gast, G.J., van Duyl, F.C. and Riegman, R. 1996. Impact of phytoplankton bloom magnitude on a pelagic microbial food web. Marine Ecology Progress Series 144: 211-221.

Buddemeier, R.W., Kleypas, J.A. and Aronson, R.B. 2004. Coral Reefs & Global Climate Change: Potential Contributions of Climate Change to Stresses on Coral Reef Ecosystems. The Pew Center on Global Climate Change, Arlington, VA, USA.

Caldeira, K. and Wickett, M.E. 2003. Anthropogenic carbon and ocean pH. Nature 425: 365.

Gnaiger, E., Gluth, G. and Weiser, W. 1978. pH fluctuations in an intertidal beach in Bermuda. Limnology and Oceanography 23: 851-857.

Hansen, P.J. 2002. The effect of high pH on the growth and survival of marine phytoplankton: implications for species succession. Aquatic Microbiology and Ecology 28: 279-288.

Kleypas, J.A., Buddemeier, R.W., Archer, D., Gattuso, J.-P., Langdon, C. and Opdyke, B.N. 1999. Geochemical consequences of increased atmospheric carbon dioxide on coral reefs. Science 284: 118-120.

Lindholm, T. and Nummelin, C. 1999. Red tide of the dinoflagellate Heterocapsa triquetra (Dinophyta) in a ferry-mixed coastal inlet. Hydrobiologia 393: 245-251.

Macedo, M.F., Duarte, P., Mendes, P. and Ferreira, G. 2001. Annual variation of environmental variables, phytoplankton species composition and photosynthetic parameters in a coastal lagoon. Journal of Plankton Research 23: 719-732.

Menendez, M., Martinez, M. and Comin, F.A. 2001. A comparative study of the effect of pH and inorganic carbon resources on the photosynthesis of three floating macroalgae species of a Mediterranean coastal lagoon. Journal of Experimental Marine Biology and Ecology 256: 123-136.

Middelboe, A.L. and Hansen, P.J. 2007. High pH in shallow-water macroalgal habitats. Marine Ecology Progress Series 338: 107-117.

Pearson, G.A., Serrao, E.A. and Brawley, S.H. 1998. Control of gamete release in fucoid algae: sensing hydrodynamic conditions via carbon acquisition. Ecology 79: 1725-1739.

Santhanam, R., Srinivasan, A., Ramadhas, V. and Devaraj, M. 1994. Impact of Trichodesmium bloom on the plankton and productivity in the Tuticorin bay, southeast coast of India. Indian Journal of Marine Science 23: 27-30.