Porites Calcifying Fluid pH on Seasonal to Diurnal Scales

Abstract

AbstractCoral resilience to ocean acidification is largely determined by the degree of physiological control corals can exert on their calcifying fluid carbonate chemistry. In this study, the boron isotopic composition (δ11B) of a Porites colony growing on a reef flat on Kiritimati Island in the equatorial central Pacific is examined to quantify the sensitivity of calcifying fluid pH (pHcf) to ambient environmental conditions. Skeletal δ11B along the growth axis of one annual growth band was determined with bulk analysis and by laser ablation (LA) MC‐ICP‐MS. Furthermore, the oxygen and carbon isotopic composition, trace element ratios, and skeletal density were quantified. Sclerochronological data were interpreted in the context of simultaneous recordings of reef flat seawater pH (pHsw), temperature, salinity, and water depth, and by measurements of these parameters on the fore‐reef. A recent model of pHcf upregulation, after optimization with seasonally resolved data, was used to simulate pHcf variability on a diurnal scale. Results showed that on a seasonal scale, Porites pHcf is upregulated compared to ambient seawater: both bulk and LA‐MC‐ICP‐MS derived δ11B resulted in a mean pHcf of 8.35 pH units. Calcifying fluid pH upregulation primarily followed variations in seawater temperatures, that is likely related to the control of temperature on calcification rate. On the reef flat, the diurnal range in pHsw was substantially higher (0.29 pH units) than on the fore‐reef (0.07 pH units). However, model results suggest that the high diurnal variability in reef flat pHsw resulted only in a limited variability in Porites pHcf.