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 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 species collected across five reef sites and use this framework to project the impact of 21st century OA on 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 corals.

The increase in atmospheric carbon dioxide (CO2) observed since the industrial revolution has reduced surface ocean pH by ~0.1 pH units, with further change in the oceanic system predicted in the coming decades. Calcareous organisms can be negatively affected by extreme changes in seawater pH (pHsw) such as this due to the associated changes in the oceanic carbonate system. The boron isotopic composition (δ 11 B) of biogenic carbonates has been previously used to monitor pH at the calcification site (pHcf) in scleractinian corals, providing mechanistic insights into coral biomineralisation and the impact of variable pHsw on this process. Motivated by these investigations, this study examines the δ 11 B of the high-Mg calcite skeleton of the coralline red alga Neogoniolithon sp. to constrain pHcf, and investigates how this taxon's pHcf is impacted by ocean acidification. δ 11 B was measured in multiple algal replicates (n = 4 to 5) cultured at four different pCO2 scenarios-averaging (± 1σ) 409 (± 6), 606 (± 7), 903 (± 12) and 2856 (± 54) μatm, corresponding to average pHsw (± 1σ) of 8.19 (± 0.03), 8.05 (± 0.06), 7.91 (± 0.03) and 7.49 (± 0.02) respectively. Results show that skeletal δ 11 B is elevated relative to the δ 11 B of seawater borate at all pHsw treatments by up to 18 ‰. Although substantial variability in δ 11 B exists between replicate samples cultured at a given pHsw (smallest range = 2.32 ‰ at pHsw 8.19, largest range = 6.08 ‰ at pHsw 7.91), strong correlations are identified between δ 11 B and pHsw (R 2 = 0.72, p < 0.0001, n = 16) and between δ 11 B and B/Ca (R 2 = 0.72, p < 0.0001, n = 16). Assuming that skeletal δ 11 B reflects pHcf as previously observed for scleractinian corals, the average pHcf across all experiments was 1.20 pH units (0.79 to 1.56) higher than pHsw, with the magnitude of this offset varying parabolically with decreasing pHsw, with a maximum difference between pHsw and pHcf at a pHsw of 7.91. Observed relationships between pHsw and calcification rate, and between pHsw and pHcf, suggest that coralline algae exhibit some resilience to moderate ocean acidification via increase of pHcf relative to pHsw in a similar manner to scleractinian corals. However, these results also indicate that pHcf cannot be sufficiently increased by algae exposed to a larger reduction in pHsw, adversely impacting calcification rates of coralline red algae. 1.0 Introduction Atmospheric CO2 has been increasing since the Industrial Revolution, from 280 ppm to more than 400 ppm today (Tans and Keeling, 2016). This increase has led to changes in ocean carbon chemistry, ultimately lowering seawater pH (pHsw) by 0.1 pH units. Climate models predict that by 2100, a high-end "business as usual" emission scenario (i.e. Intergovernmental Panel on Climate Change: Representative Concentration Pathway 8.5) will result in a global average surface pHsw of ca. 7.8, potentially reaching even lower levels at high latitudes. This large and rapid reduction in global pHsw will result in an environment that is potentially cha...

Coral reefs are important ecosystems that are increasingly negatively impacted by human activities. Understanding which anthropogenic stressors play the most significant role in their decline is vital for the accurate prediction of future trends in coral reef health and for effective mitigation of these threats. Here we present annually resolved boron and carbon isotope measurements of two cores capturing the past 90 years of growth of the tropical reef‐building coral Siderastrea siderea from the Belize Mesoamerican Barrier Reef System. The pairing of these two isotope systems allows us to parse the reconstructed pH change into relative changes in net ecosystem productivity and net ecosystem calcification between the two locations. This approach reveals that the relationship between seawater pH and coral calcification, at both a colony and ecosystem level, is complex and cannot simply be modeled as linear or even positive. This study also underscores both the utility of coupled δ11B‐δ13C measurements in tracing past biogeochemical cycling in coral reefs and the complexity of this cycling relative to the open ocean.