Anthropogenic ocean acidification (OA) has compromised the ability of marine organisms to calcify. However, many coastal environments naturally exhibit high variability in seawater pH (pHsw) and the impact of OA on these environments is unclear. For instance, sub-tropical corals can modify the pH of the calcifying fluid (pHcf) from which they precipitate their skeleton. This study examines the influence of OA on pHcf upregulation of Porites lutea microatolls inhabiting reef flat environments. Environmental measurements including pHsw and temperature were performed on reef flats and adjacent fore-reefs on Kiritimati Island (Kiribati), Arno Atoll (Marshall Islands), and Rarotonga (Cook Islands) to quantify the temporal and spatial variability of these parameters. Slabs were removed from microatolls to construct multi-decadal (1938 – 2018) records of their boron isotopic (δ11B) and geochemical composition. The sensitivity of microatoll pHcf upregulation to ambient pHsw was evaluated by comparing annual band δ11B with synchronously recorded pHsw and temperature, and microatoll records were compared to a fore-reef record of similar age. Although daily means in pHsw on reef flats and fore-reefs were relatively similar, large diurnal cycles in pHsw (ΔpHsw = 0.28) and temperature (ΔT = 2.0°C) were found on reef flats exceeding that on fore-reefs by far (ΔpHsw = 0.07, ΔT = 0.7°C). Furthermore, spatial variations in pHsw and temperature were observed that were linked to reef flat hydrodynamics. Microatoll pHcf revealed a higher correlation to ambient seawater temperatures than to pHsw and only the fore-reef core showed a long-term trend in pHcf (-0.0003±0.0009 year-1) that is indicative of OA, while microatoll records revealed variable long-term trends unlikely reflecting ocean conditions (-0.0030±0.0005 to +0.0007±0.0003 year-1). Corals from the three sites revealed similar mean pHcf ≈ 8.44 although the difference in pHsw between the locations (ΔpHsw = 0.17) noticeably exceeded the decline in pHsw due to OA (ΔpHsw = 0.10). In conclusion, Porites lutea microatoll pHcf appeared to be relatively insensitive to OA. This is likely a result of the large variability in seawater conditions on reef flats that supersede OA, and the strong modification of coral pHcf by physiological processes.
Knebel, O. 2020. Boron isotope records from Pacific microatolls: modifications in Porites lutea calcifying fluid composition under anthropogenic ocean acidification and natural pH variability. PhD thesis, University of Auckland, 239p. Thesis.
