Reply to Ridd et al.’s Technical Comment to Science: “Have coral calcification rates slowed in the last twenty years?”

7 thoughts on “Reply to Ridd et al.’s Technical Comment to Science: “Have coral calcification rates slowed in the last twenty years?””

  1. Is it possible to put a link to where Peter Ridds comments/ paper?/whatever is available so we can see what he said as well as De’ath et al response?




  2. You say: “We maintain that this decline in calcification, probably due to synergistic effects of prolonged and repeated temperature stress and ocean acidification in tropical waters”.

    Is there in fact any evidence to support any independent link between calcification and acidity in the fairly narrow pH range in question?

    Since corals make calcium carbonate from the bicarbonate ion, and bicarbonate concentrations are not much affected by pH in the pH range in question (8.0 to 8.5 approx), there is no obvious reason why acidity should be a significant factor in coral deposition of calcium carbonate.

    Indeed increased ocean acidity caused by increased CO2 from the atmosphere will surely lead to higher levels of bicarbonate – which is known to be a coral ‘fertiliser’ which – other things being equal – results in a higher rate of calcification .

    As for temperature, there is of course no dispute that corals are stressed by higher temperatures and that this would be expected to reduce calcification. But again, direct experimental evidence in support of this entirely believable hypothesis would be desirable.

    Re the important question that temperature rise and acidification might act synergistically to reduce calcification, is there any evidence in support of this?

    Another question that it would be intriguing to investigate further is whether raising bicarbonate concentrations would have any effect in reducing the adverse impact on coral of heat stress. If it does, then that would offer a possible route for protecting coral reefs against temperature increase.


    1. Oliver – you need to do a bit more reading as your comments here are incorrect. There is abundant evidence of impacts from ocean acidification alone (through the affect on the carbonate ion concentration). I refer you to Kleypas and Langdon (2006).

      Kleypas JA, Langdon C (2006) “Coral reefs and changing seawater chemistry”, Chapter 5 In: Phinney J, Hoegh-Guldberg O, Kleypas J, Skirving W, Strong AE (eds) Coral Reefs and Climate Change: Science and Management. AGU Monograph Series, Coastal and Estuarine Studies. Geophys. Union, Washington DC, pp 73-110.


  3. Hi and thanks for your reply. In fact my comments were very much in the interrogative, not the declamatory. I will certainly try to follow up the reference you provide, together with other more accessible sources. However I am a little confused by your reply. Carbonate is, as I understand it, supersaturated in all surface waters and down to a considerable depth. Does the increase in ocean acidity to date threaten to end this condition of supersaturation? That is not something I have ever seen reported. And provided that carbonate does remain supersaturated, it is hard to see how calcium carbonate from corals or shells will be forced into solution. Or have I missed something important? OT.


    1. Thanks Oliver. The issue about supersaturation is twofold. Firstly, the presence of other irons in seawater inhibit the precipitation of calcium carbonate and help maintain seawater supersaturation. Secondly, the precipitation of calcium carbonate is a biological process and depends on the ability of corals to accumulate calcium and carbonate ions at the site of calcium carbonate deposition. That is, it does not simply depend on the physical and chemical state of sea water.


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