Ocean acidification (OA) is a major threat to marine ecosystems, particularly coral reefs which are heavily reliant on calcareous species. OA decreases seawater pH and calcium carbonate saturation state (Omega), and increases the concentration of dissolved inorganic carbon (DIC). Intense scientific effort has attempted to determine the mechanisms via which ocean acidification (OA) influences calcification, led by early hypotheses that calcium carbonate saturation state (Omega) is the main driver. We grew corals and coralline algae for 8 to 21 weeks, under treatments where the seawater parameters Omega, pH and DIC were manipulated to examine their differential effects on calcification rates and calcifying fluid chemistry (Omega cf, pHcf, and DICcf). Here, using long duration experiments, we provide geochemical evidence that differing physiological controls on carbonate chemistry at the site of calcification, rather than seawater Omega, are the main determinants of calcification. We found that changes in seawater pH and DIC rather than Omega had the greatest effects on calcification and calcifying fluid chemistry, though the effects of seawater carbonate chemistry were limited. Our results demonstrate the capacity of organisms from taxa with vastly different calcification mechanisms to regulate their internal chemistry under extreme chemical conditions. These findings provide an explanation for the resilience of some species to OA, while also demonstrating how changes in seawater DIC and pH under OA influence calcification of key coral reef taxa.

In order to allow full comparability with other ocean acidification data sets, the R package seacarb (Gattuso et al, 2016) was used to compute a complete and consistent set of carbonate system variables, as described by Nisumaa et al. (2010). In this dataset the original values were archived in addition with the recalculated parameters (see related PI). The date of carbonate chemistry calculation by seacarb is 2018-07-26.

Supplement to: Comeau, Steeve; Cornwall, Christopher Edward; De Carlo, Eric Heinen; Krieger, E; McCulloch, Malcolm T (2018): Similar controls on calcification under ocean acidification across unrelated coral reef taxa. Global Change Biology