Declining calcification on the Great Barrier Reef

4 thoughts on “Declining calcification on the Great Barrier Reef”

  1. Koen, I don’t know. The little ice age? But given the size of the confidence intervals (spanning practically the whole range of calcification plotted) there is a good chance that is wasn’t.

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  2. John and Ove’s interpretation of De’ath et al.’s paper focuses mainly on the possibility of a causative link with increasing ocean acidification. The paper itself is much more circumspect, since the authors were unable to ascribe the declining calcification to any specific environmental signal (either temperature or seawater acidity). A subsequent study (Tanzil et al. 2009 Coral Reefs – DOI 10.1007/s00338-008-0457-5) from South Thailand on reefs in warmer waters closer to the equator (7 deg N) also demonstrated a similar decline in calcification, which like the GBR study was due to decreased linear extension and not to any changes in bulk density. Whilst the data available to Tanzil et al. for ocean acidification was sparse and incomplete, nonetheless no obvious link existed, but the authors were able to demonstrate that the decrease in linear extension was associated with a significant long term increase in sea temperature (R squared between 10 and 30%). The common finding in both studies, that there was no change in bulk density, may also be important, for it implies that whilst these corals may be growing less slowly, they are still producing robust skeletons, although an earlier study by Cooper et al. (2008 Global Change Biology 14:529-538) had found a small decrease (6%). As Atkinson & Cuet (2008 MEPS 373:249–256) point out, we are still a long way from understanding how changes in ocean acidity may affect coral calcification, particularly in natural conditions. At the moment whilst rising sea temperatures clearly seem to be reducing the growth of corals, there is still much work to be done to demonstrate whether the same is true for changes in ocean acidity.

    Abstract of Tanzil et al. paper: Of the few studies that have examined in situ coral growth responses to recent climate change, none have done so in equatorial waters subject to relatively high sea temperatures (annual mean >27degC). This study compared the growth rate of Porites lutea from eight sites at Phuket, South Thailand between two time periods (December 1984–November 1986 and December 2003–November 2005). There was a significant decrease in coral calcification (23.5%) and linear extension rates (19.4–23.4%) between the two sampling periods at a number of sites, while skeletal bulk density remained unchanged. Over the last 46 years, sea temperatures (SST) in the area have risen at a rate of 0.161degC per decade (current seasonal temperature range 28–30degC) and regression analysis of coral growth data is consistent with a link between rising temperature and reduced linear extension in the order of 46– 56% for every 1 degC rise in SST. The apparent sensitivity of linear extension in P. lutea to increased SST suggests that corals in this part of the Andaman Sea may already be subjected to temperatures beyond their thermal optimum for skeletal growth.

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