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

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.

Figure 1. Average area-normalized chlorophyll a ( = chl a) concentration and calcification ( = calc.) rate of (A, B) Porites divaricata, (C, D) Porites astreoides, and (E, F) Orbicella faveolata after 0 and 6 weeks on the reef.

Asterisks indicate significant differences between control and treatment corals at a specific time interval. Sample size ranges from 6–9. Calcification data reproduced from Grottoli et al. with permission of the publisher.

Figure 2. Average δ¹¹B, δ¹³C, and δ¹⁸O of (A–C) Porites divaricata, (D–F) Porites astreoides, and (G–I) Orbicella faveolata after 0 and 6 weeks on the reef.

Asterisks indicate significant differences between control and treatment corals at a specific time interval. Sample size ranges from 3–9.

Figure 3. Average Sr/Ca, Mg/Ca, U/Ca, and Ba/Ca of (A–D) Porites divaricata, (E–H) Porites astreoides, and (I–L) Orbicella faveolata after 0 and 6 weeks on the reef.

Asterisks indicate significant differences between control and treatment corals at a specific time interval. Sample size ranges from 3–7. Note that the Y-axis for Ba/Ca differs for P. astreoides and the other two species.