Ocean acidification affects coral growth by reducing skeletal density

Abstract

Ocean acidification (OA) is considered an important threat to coral reef ecosystems, because it reduces the availability of carbonate ions that reef-building corals need to produce their skeletons. However, while theory predicts that coral calcification rates decline as carbonate ion concentrations decrease, this prediction is not consistently borne out in laboratory manipulation experiments or in studies of corals inhabiting naturally low-pH reefs today. The skeletal growth of corals consists of two distinct processes: extension (upward growth) and densification (lateral thickening). Here, we show that skeletal density is directly sensitive to changes in seawater carbonate ion concentration and thus, to OA, whereas extension is not. We present a numerical model of Porites skeletal growth that links skeletal density with the external seawater environment via its influence on the chemistry of coral calcifying fluid. We validate the model using existing coral skeletal datasets from six Porites species collected across five reef sites and use this framework to project the impact of 21st century OA on Porites skeletal density across the global tropics. Our model predicts that OA alone will drive up to 20.3 ± 5.4% decline in the skeletal density of reef-building Porites corals.

Keywords: biomineralization; coral calcification; ocean acidification; skeletal density.

Fig. 1.

Coral skeletal parameters measured in representative Porites cores from four reefs across the Pacific. Coral calcification rates do not correlate with either Ω_sw or Ω_ECM (A and B). Instead, skeletal density exhibits a significant positive correlation with both Ω_sw and Ω_ECM (C and D), but extension does not (E and F; P = 0.14 and P = 0.09, respectively). Individual points represent annual averages of skeletal growth. Error bars denote 1 SD of Ω propagated from seasonal variability in seawater physicochemical parameters (for Ω_sw and Ω_ECM) and in boron isotope compositions of coral skeletons (for Ω_ECM).

Fig. 2.

Correlation between coral skeletal density and expected aragonite precipitation rate in the coral ECM (R_ECM) on both the annual (A) and seasonal (B and C) scales. Data in A represent the same cores as in Fig. 1. Error bars (A) and shaded areas (B and C) denote 1 SD in R_ECM as propagated from uncertainties in seawater parameters and in boron isotope measurements. Seasonal density profiles were retrieved parallel to the sampling track for boron isotope measurements.

Fig. 3.

Schematic representation of our Porites skeletal growth model (A) and comparison between model-predicted skeletal density and measured density (B). Also shown in A are a cross-section view of our model polyp geometry and a representative SEM image of a Porites calyx (orange dashed line). Porites cores in B were collected from reefs in the Pacific, Atlantic, and Indian Oceans reported in previous studies (, , –57). Data points from this study, the Caribbean, and the Andaman Sea represent densities of individual cores; data points from the Galapagos, the Great Barrier Reef, and the Andaman Sea represent site average densities for which error bars denote 2σ uncertainties. Vertical error bars represent uncertainties in model prediction propagated from uncertainties in model parameters α, λ, and w_o as well as measurements of in situ seawater conditions where available. Where seawater conditions were not reported, outputs from the CESM-BGC historical run were adopted.

Fig. 4.

Model-predicted decline in Porites skeletal density over the 21st century due to ocean acidification. Our model predicts an average 12.4 ± 5.8% (2σ) decline in density across global reef sites, with the largest decline in the western tropical Pacific coral triangle region (an average of ∼14% and a maximum of 20.3%) and the least in the Caribbean (∼6%). Simulations were conducted based on outputs from the CESM-BGC RCP 8.5 run for the years 2006–2015 and 2090–2099 (Methods). Skeletal extension, initial radius, and tissue thickness were held constant in these simulations. Error represents only that propagated from estimation of model parameters.