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, pCO(2) ranged from 272 to 542 mu atm, and aragonite saturation state (Omega(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 pCO(2) and pH, respectively. Daytime ncp averaged 37 +/- 19 mmol Cm-2 h(-1) in summer and 33 +/- 13 mmol Cm-2 h(-1) in winter; nighttime ncp averaged -30 +/- 25 and -7 +/- 6 mmol Cm-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 Omega(arag) for both seasons, indicating that relatively small shifts in Omega(arag) may drive measurable shifts in calcification rates, and hence carbon budgets, of coral reefs throughout the year.