Short-term coral bleaching is not recorded by skeletal boron isotopes

Verena Schoepf; Malcolm T McCulloch; Mark E Warner; Stephen J Levas; Yohei Matsui; Matthew D Aschaffenburg; Andréa G Grottoli

doi:10.1371/journal.pone.0112011

Short-term coral bleaching is not recorded by skeletal boron isotopes

PLoS One. 2014 Nov 14;9(11):e112011. doi: 10.1371/journal.pone.0112011. eCollection 2014.

Authors

Verena Schoepf¹, Malcolm T McCulloch², Mark E Warner³, Stephen J Levas⁴, Yohei Matsui⁴, Matthew D Aschaffenburg³, Andréa G Grottoli⁴

Affiliations

¹ School of Earth and Environment, The University of Western Australia and ARC Centre of Excellence for Coral Reef Studies, Crawley, WA, Australia; School of Earth Sciences, The Ohio State University, Columbus, Ohio, United States of America.
² School of Earth and Environment, The University of Western Australia and ARC Centre of Excellence for Coral Reef Studies, Crawley, WA, Australia.
³ School of Marine Science and Policy, University of Delaware, Lewes, Delaware, United States of America.
⁴ School of Earth Sciences, The Ohio State University, Columbus, Ohio, United States of America.

Abstract

Coral skeletal boron isotopes have been established as a proxy for seawater pH, yet it remains unclear if and how this proxy is affected by seawater temperature. Specifically, it has never been directly tested whether coral bleaching caused by high water temperatures influences coral boron isotopes. Here we report the results from a controlled bleaching experiment conducted on the Caribbean corals Porites divaricata, Porites astreoides, and Orbicella faveolata. Stable boron (δ11B), carbon (δ13C), oxygen (δ18O) isotopes, Sr/Ca, Mg/Ca, U/Ca, and Ba/Ca ratios, as well as chlorophyll a concentrations and calcification rates were measured on coral skeletal material corresponding to the period during and immediately after the elevated temperature treatment and again after 6 weeks of recovery on the reef. We show that under these conditions, coral bleaching did not affect the boron isotopic signature in any coral species tested, despite significant changes in coral physiology. This contradicts published findings from coral cores, where significant decreases in boron isotopes were interpreted as corresponding to times of known mass bleaching events. In contrast, δ13C and δ18O exhibited major enrichment corresponding to decreases in calcification rates associated with bleaching. Sr/Ca of bleached corals did not consistently record the 1.2°C difference in seawater temperature during the bleaching treatment, or alternatively show a consistent increase due to impaired photosynthesis and calcification. Mg/Ca, U/Ca, and Ba/Ca were affected by coral bleaching in some of the coral species, but the observed patterns could not be satisfactorily explained by temperature dependence or changes in coral physiology. This demonstrates that coral boron isotopes do not record short-term bleaching events, and therefore cannot be used as a proxy for past bleaching events. The robustness of coral boron isotopes to changes in coral physiology, however, suggests that reconstruction of seawater pH using boron isotopes should be uncompromised by short-term bleaching events.

Publication types

Research Support, Non-U.S. Gov't
Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

Analysis of Variance
Animals
Anthozoa / physiology*
Bone and Bones / metabolism*
Boron / metabolism*
Calcification, Physiologic
Carbon Isotopes
Chlorophyll / metabolism
Chlorophyll A
Isotopes
Oxygen Isotopes
Time Factors
Trace Elements

Substances

Carbon Isotopes
Isotopes
Oxygen Isotopes
Trace Elements
Chlorophyll
Boron
Chlorophyll A

Grants and funding

This work was funded by the U.S. National Science Foundation (OCE#0825490 to AGG and OCE#0825413 to MEW). MTM was supported by a Western Australian Premiers Fellowship and an Australian Research Council Laureate Fellowship. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.