The Effects of Combined ocean acidification and Warming on the Calcifying Fluid Chemistry of Corals: A Geochemical Approach

Chair: Philip Boyd

Verena Schoepf (1), Malcolm T. McCulloch (1), Robert J. Toonen (2) and Christopher P. Jury (2)
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
2 Hawaii Institute of Marine Biology, SOEST, University of Hawaii, Kaneohe, HI, 96744, United States of America

Background
Coral skeletal boron isotopes are important proxies for seawater pH and ocean acidification (OA), and can provide critical insights into calcification mechanisms due to the process of pH-upregulation at the site of calcification. However, it remains poorly understood how elevated temperature and heat stress influence pH-upregulation and the carbonate chemistry of the calcifying fluid, especially in combination with elevated pCO 2. Further, it is largely unknown whether environments with naturally elevated pCO2 enhance pH-upregulation in corals.

Methods
We conducted a controlled culturing experiment using the Hawaiian coral species Montipora capitata and Porites compressa, which were collected from two environmentally different sites. Corals were maintained at three pCO2 levels (390, 600, 930 μatm) throughout the course of the experiment. During the first 5 weeks, corals from each of these pCO2 treatments were exposed to elevated temperature (26.8, 28.3°C), followed by 9 weeks of exposure to 1.5°C lower temperatures (25.2, 26.8°C). We analysed boron isotopes and trace element ratios (B/Ca, Sr/Ca, Mg/Ca, U/Ca, Ba/Ca) of these corals and used them to reconstruct the carbonate chemistry of the calcifying fluid.

Findings
Preliminary findings for M. capitata show that pH of the calcifying fluid continued to be up-regulated under OA conditions, but nevertheless significantly decreased with increasing seawater pCO2. In contrast, a sustained temperature difference of 1.5°C had no significant effect on pH-upregulation. Interestingly, corals adapted to naturally elevated pCO2 conditions had higher internal pH values than conspecifics from an open ocean reef setting. We will further present boron isotope and trace element data for P. compressa and reconstruct the carbonate chemistry for both species.

Conclusions
The findings from this study will improve our understanding of how combined OA and warming influence the carbonate chemistry of the calcifying fluid and thus the resilience of coral calcification to multiple climate change stressors.