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A Century-Long Proxy of Ocean Acidification and Coral Calcification on the Belize Mesoamerican Barrier Reef

Paper Reviewed
Fowell, S.E., Foster, G.L., Ries, J.B., Castillo, K.D., de la Vega, E., Tyrrell, T., Donald, H.K. and Chalk, T.B. 2018. Historical trends in pH and carbonate biogeochemistry on the Belize Mesoamerican Barrier Reef System. Geophysical Research Letters 45: 3228-3237.

Multiple laboratory-based studies have led many to conclude that ocean acidification (i.e., a decline in seawater pH that is predicted to occur in consequence of rising atmospheric CO2 concentrations) has the potential to harm marine organisms, particularly corals, by reducing their rates of calcification and/or increasing their calcium carbonate dissolution. What many of such studies fail to consider, however, is that calcification is a biologically-mediated process; and with life in the equation, all bets are usually off!

Consider, for example, the recent work of Fowell et al. (2018), who examined "annually resolved boron and carbon isotope measurements of two cores capturing the past 90 years of growth of the tropical reef-building coral Siderastrea siderea from the Belize Mesoamerican Barrier Reef System." Utilizing the two isotopic records, the eight researchers were able to successfully reconstruct estimates of seawater pH, net ecosystem productivity and net ecosystem calcification between the forereef and backreef of this barrier reef system.

And what did their reconstructions reveal?

In the words of the authors, they showed that "the forereef colony, which exhibits declining growth over the past century, recorded unexpectedly steady pH over the same interval," whereas the backreef colony "exhibited accelerating growth over this interval, while recording surprisingly rapid ocean acidification." Consequently, Fowell et al. say that "ocean acidification has not had an obviously detrimental effect on the calcification of S. siderea over the last 100 years in the [Belize Mesoamerican Barrier Reef System]." Indeed, not only has it not had a detrimental effect, it has had a positive effect on coral growth in the backreef!

In concluding their paper, Fowell et al. thus rightfully acknowledge that "the relationship between seawater pH and coral calcification is complex," while adding that "the evolution of the carbonate system on coral reefs over the last century has not been controlled solely by anthropogenic CO2." Like we said at the beginning of this review, when life enters the equation, all bets are off; and conventional wisdom is often turned upside down!

Posted 24 August 2018