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- Title
Porites Calcifying Fluid pH on Seasonal to Diurnal Scales.
- Authors
Knebel, Oliver; Carvajal, Carlos; Standish, Christopher D.; Vega, Elwyn de la; Chalk, Thomas B.; Ryan, Emma J.; Guo, Weifu; Ford, Murray; Foster, Gavin L.; Kench, Paul
- Abstract
Coral 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. Plain Language Summary: Ocean acidification and the associated decline in seawater pH has impacted the ability of many marine organisms to calcify. However, tropical corals do not precipitate their aragonite skeletons from seawater, but instead from a calcifying fluid located between skeleton and living tissue. The chemistry of this calcifying fluid is modified by physiological processes. This study examines the sensitivity of the calcifying fluid pH to variations in environmental conditions. Seawater pH, temperature, salinity, and water depth on a reef flat at Kiritimati Island in the central Pacific were monitored for 1 year. The calcifying fluid pH of a synchronously precipitated annual growth band of a Porites colony was determined using boron isotope analysis with dissolution of samples and novel laser ablation at enhanced resolution. Furthermore, a numerical model predicted calcifying fluid pH variability at a diurnal scale based on the environmental and geochemical data collected. Results indicate that on a seasonal scale, seawater temperatures rather than seawater pH primarily control the calcifying fluid pH. Further geochemical analysis suggests that this is related to the temperature dependency of the calcification rate. On a diurnal scale, model results suggest no elevated variability in calcifying fluid pH despite highly variable reef flat seawater conditions. Key Points: Porites calcifying fluid pH was investigated using conventional and laser ablation boron isotope analysis, and a calcification modelOn a seasonal scale, Porites calcifying fluid pH followed mainly seawater temperatures that control calcification ratesOn a diurnal scale, model estimates suggest limited calcifying fluid pH variability despite highly variable reef flat seawater conditions
- Subjects
KIRITIMATI (Kiribati); PORITES; OCEAN acidification; CORALS; BORON isotopes
- Publication
Journal of Geophysical Research. Oceans, 2021, Vol 126, Issue 3, p1
- ISSN
2169-9275
- Publication type
Article
- DOI
10.1029/2020JC016889