Environmental and physiochemical controls on coral calcification along a latitudinal temperature gradient in Western Australia

Abstract

The processes that occur at the micro‐scale site of calcification are fundamental to understanding the response of coral growth in a changing world. However, our mechanistic understanding of chemical processes driving calcification is still evolving. Here, we report the results of a long‐term in situ study of coral calcification rates, photo‐physiology, and calcifying fluid (cf) carbonate chemistry (using boron isotopes, elemental systematics, and Raman spectroscopy) for seven species (four genera) of symbiotic corals growing in their natural environments at tropical, subtropical, and temperate locations in Western Australia (latitudinal range of ~11°). We find that changes in net coral calcification rates are primarily driven by pHcf and carbonate ion concentration [CO3normal2−]cf in conjunction with temperature and DICcf. Coral pHcf varies with latitudinal and seasonal changes in temperature and works together with the seasonally varying DICcf to optimize [CO3normal2−]cf at species‐dependent levels. Our results indicate that corals shift their pHcf to adapt and/or acclimatize to their localized thermal regimes. This biological response is likely to have critical implications for predicting the future of coral reefs under CO2‐driven warming and acidification.

Calibration of Sr/Ca, Li/Mg and Sr‐U Paleothermometry in Branching and Foliose Corals

“…These results indicate that there is a significant negative relationship between Li/Mg K D and [CO 3 2− ] at 25°C ( r 2 = 0.90, n = 19, p < 0.001; Figure a) and between the residual K D Li/Mg from the [CO 3 2− ] regression and temperatures ranging from 20 to 40 °C ( r 2 = 0.81, n = 29, p < 0.001; Figure b; Holcomb et al, ). While the same experiments have shown that Sr/Ca is insensitive to [CO 3 2− ] cf (DeCarlo et al, ), Sr/Ca may be indirectly influenced by [CO 3 2− ] cf via kinetic effects and Rayleigh fractionation assuming that higher [CO 3 2− ] cf drives more precipitation (DeCarlo et al, ; Ross et al, ). As such, higher [CO 3 2‐ ] cf could drive a direct decrease in the Li/Mg K D and a greater extent of Rayleigh fractionation, thereby simultaneously decreasing both Li/Mg and Sr/Ca.…”

“…As such, higher [CO 3 2‐ ] cf could drive a direct decrease in the Li/Mg K D and a greater extent of Rayleigh fractionation, thereby simultaneously decreasing both Li/Mg and Sr/Ca. This [CO 3 2− ] cf effect would explain why there is a relationship between the residuals (nontemperature effects) of Sr/Ca and Li/Mg (Figure a) while species‐specific differences in [CO 3 2− ] cf and calcification rates may account for the variance explained by species (Ross et al, ). Similarly, high variability in Sr/Ca‐temperature calibrations between different species has been attributed to [CO 3 2− ] cf or precipitation rate in a recent study based on Porites spp.…”

“…Thus, it is unlikely that location-specific differences in environmental conditions (Fowell et al, 2016;Tanaka et al, 2015) were the main driver of the significant relationship between the Sr/Ca and Li/Mg residuals observed here. Furthermore, growth rate differences among species (Ross et al, 2019) are not consistent with the species/genus offsets for Li/Mg and Sr/Ca (i.e., higher ratios for Acropora and Turbinaria and lower ratios for Stylophora and Pocillopora; Figures 6a and 6b).…”

“…Yellow circles denote the location of the Hobo temperature loggers deployed in situ at each site. See Ross et al () for more details.…”

“…There are species differences in the seasonally resolved skeletal trace element data for all three proxies and these are explored further in section 3.4. Raman spectroscopy analyses indicated that all samples were entirely aragonite with no contamination of other mineral phases (see Ross et al, 2019).…”