Coral calcification under thermal stress and ocean acidification

Chair: Cliff Law

Juan Pablo D’Olivo Cordero (1)* and Malcolm T. McCulloch (1)

1 School of Earth and Environment, UWA Oceans Institute and ARC Centre of Excellence for Coral Reef Studies, The University of Western Australia, Crawley, WA, 6009, Australia

Background
Coral calcification, a vital process for the formation and preservation of coral reefs, is declining due to the synergistic effects of ocean acidification, thermal stress, pollution and disease. While there are some constraints on how the rates of coral calcification will likely decline as ambient waters become more acidic, little is known of how the processes controlling calcification will respond to multiple stresses during increasingly more frequent coral bleaching events. In particular, whether corals can maintain the pre-requisite conditions for calcification, that is, the ability to elevate the aragonite saturation state (cf) of their calcifying fluid.

Methods
Here we present high-resolution (~bimonthly) records of trace elements and boron isotopes (d11B) of four sample paths from a Porites coral subject to the global bleaching event that impacted the central Great Barrier Reef in the summer of 1998. This data is used to reconstruct changes in cf from the associated carbonate parameters (pHcf, DICcf) of the coral’s calcifying fluid, under conditions of severe thermal stress (bleaching) and ongoing ocean acidification.

Findings
A breakdown in the trace element systematics and reduced growth rates was observed along all sample paths indicating that the coral was severely affected by the thermal stress event of 1998. Interestingly, the d11B and B/Ca based reconstructions of pHcf and Ωcf at the site of calcification indicated that these remained significantly elevated with respect to seawater. Only along the most affected ‘bleaching-scar’ path was a loss in the seasonality and reduction in pH and DIC observed. Importantly cfwas higher during the stress period for all the other less affected sampled paths.

Conclusions
These results have important implications for the future of corals under predicted scenarios of ocean acidification and more frequent thermal stress, as coral calcification is closed linked to maintenance of pHcf and Ωcf above critical thresholds.