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What Drives Coral Calcification on Heron Reef?

Paper Reviewed
Albright, R., Benthuysen, J., Cantin, N., Caldeira, K. and Anthony, K. 2015. Coral reef metabolism and carbon chemistry dynamics of a coral reef flat. Geophysical Research Letters 42: 3980-3988.

For many years now, scientists have expressed concern over the potential impacts of so-called ocean acidification on marine life, particularly with respect to coral reefs, where laboratory experiments and model-based studies predict these important ecosystems will suffer reduced rates of calcification -- if not altogether dissolution -- in the years and decades ahead as the atmosphere's CO2 concentration continues to rise. However, as noted by Albright et al. (2015) in a study published in the journal Geophysical Research Letters, "our ability to predict global-scale changes in coral reef calcification is limited by insufficient data relating in situ rates of calcification to seawater carbonate chemistry parameters," as current knowledge of the impacts of ocean acidification on coral reefs is almost solely derived from theoretical calculations (e.g., model-based studies) and controlled CO2-manipulation experiments conducted in the laboratory, which studies suggest calcification is primarily a function of the seawater aragonite saturation state (Ωarag). In an effort to remedy this data-deficient situation, Albright et al. set out to conduct a series of field-based measurements from which to discern and further constrain the primary factors influencing coral calcification out in the real world of nature.

The location of their study site was a 300 x 300 m section of Heron Reef (23.45°S, 151.92°E), a subtropical reef system approximately 72 km off the coast of Australia and at the southern end of the Great Barrier Reef, where every two hours for a period of 11 days (8 to 18 March 2012), they documented the reef's carbonate chemistry and estimated net community calcification (ncc) and net community production (ncp). Then, by further evaluating the relationship between ncc, ncp and other environmental parameters (light, temperature and Ωarag), they determined "which relationships best explain variability in ncc" so as to improve "our understanding of drivers of calcification in coral reef environments."

Among the several findings discussed in the paper, Albright et al. report a strong daily cycle in seawater carbonate chemistry at the study site in which pCO2 ranged from 281 ppm to 669 ppm (which values are 30 and 67% below and above the present-day atmospheric CO2 concentration of approximately 400 ppm, respectively), pHtotal from 7.8 to 8.1, and Ωarag from 2.3 to 4.2. They also found that "ncp (and factors that inherently govern ncp) is the strongest driver of ncc," which is "consistent with the hypothesis that photosynthesis is a strong driver of calcification." What is more, they determined that "the best linear regression model for hourly rates of net community calcification on the Heron Island reef flat is a simple linear function of net community production," adding that "despite the apparent dependence of calcification on Ωarag seen in a simple pairwise relationship, if the dependence of ncc on ncp is accounted for, knowing Ωarag does not add any substantial explanatory value."

In commenting on their findings, Albright et al. state "our results are not meant to challenge the previously documented relationship between coral calcification and Ωarag," though it is difficult to see how they do not. Clearly, out in the real world of nature, calcification is much less sensitive to changes in Ωarag than has long been assumed, which is essentially the same conclusion reached in a similar field study conducted one year earlier by Venti et al. (2014). Rather, as observed in this study, factors governing photosynthesis that drive net community production are the strongest drivers of net community calcification. And because marine photosynthesis tends to be enhanced under elevated pCO2 levels, concerns regarding the dissolution of corals in response to future ocean acidification appear to be well off base.

Venti, A., Andersson, A. and Langdon, C. 2014. Multiple driving factors explain spatial and temporal variability in coral calcification rates on the Bermuda platform. Coral Reefs 33: 979-997.

Posted 21 October 2015