Dynamics of seawater carbonate chemistry, production, and calcification of a coral reef flat, central Great Barrier Reef

Abstract. Ocean acidification is projected to shift coral reefs from a state of net accretion to one of net dissolution this century. Presently, our ability to predict global-scale changes to coral reef calcification is limited by insufficient data relating seawater carbonate chemistry parameters to in situ rates of reef calcification. Here, we investigate diel and seasonal trends in carbonate chemistry of the Davies Reef flat in the central Great Barrier Reef and relate these trends to benthic carbon fluxes by quantifying net ecosystem calcification (nec) and net community production (ncp). Results show that seawater carbonate chemistry of the Davies Reef flat is highly variable over both diel and seasonal cycles. pH (total scale) ranged from 7.92 to 8.17, pCO₂ ranged from 272 to 542 μatm, and aragonite saturation state (Ω_arag) ranged from 2.9 to 4.1. Diel cycles in carbonate chemistry were primarily driven by ncp, and warming explained 35% and 47% of the seasonal shifts in pCO₂ and pH, respectively. Daytime ncp averaged 37 ± 19 mmol C m⁻² h⁻¹ in summer and 33 ± 13 mmol C m⁻² h⁻¹ in winter; nighttime ncp averaged −30 ± 25 and −7 ± 6 mmol C m⁻² h⁻¹ in summer and winter, respectively. Daytime nec averaged 11 ± 4 mmol CaCO₃ m⁻² h⁻¹ in summer and 8 ± 3 mmol CaCO₃ m⁻² h⁻¹ in winter, whereas nighttime nec averaged 2 ± 4 mmol and −1 ± 3 mmol CaCO₃ m⁻² h⁻¹ in summer and winter, respectively. Net ecosystem calcification was highly sensitive to changes in Ω_arag for both seasons, indicating that relatively small shifts in Ω_arag may drive measurable shifts in calcification rates, and hence carbon budgets, of coral reefs throughout the year.