[B-BG21] Material cycles in tropical- subtropical coastal ecosystems

[BBG21-P02] Variation in the oxygen isotope ratio of dissolved orthophosphate induced by uptake process by hermatypic corals

Charissa M. Ferrera¹, Atsushi WATANABE¹, Kazuo NADAOKA¹, Yu UMEZAWA², Naoko MORIMOTO³, Takashi NAKAMURA¹, *Toshihiro MIYAJIMA³ (1.Tokyo Institute of Technology, 2.Faculty of Fisheries, Nagasaki Univ., 3.AORI, University of Tokyo)

Keywords:Phosphate, Isotope effect, Stable isotopes of oxygen, Hermatypic coral, Coastal marine ecosystem

The oxygen isotope ratio (δ¹⁸O) of dissolved orthophosphate (PO₄^3-) has been recognized as a promising tool to evaluate the contributions of both external sources and internal recycling of phosphorus (P) to the P budget in natural aquatic ecosystems. However, coexistence of many biological processes that can significantly alter the phosphate δ¹⁸O (δ¹⁸O_p) in a given system often complicates quantitative interpretation of this parameter. To use the information of δ¹⁸O_p effectively in biogeochemical researches, we have to know both the magnitudes of oxygen isotope effect and the reaction kinetics of major biological processes that take part in the P cycle of the concerned ecosystem. In this study, we conducted a model incubation experiment using natural hermatypic corals to evaluate the influence of uptake process of PO₄^3- by corals on the δ¹⁸O_p. Live coral samples (Porites cylindrica, Heliopora coerulea, Acropora digitifera) were collected from coral reefs around Ishigaki Island (Okinawa) and Bolinao (northern Luzon), acclimatized in incubation aquaria for a few days, and then incubated for 3 to 5 days under natural light conditions with elevated concentrations of NO₃^- and PO₄^3-. Subsamples of seawater were regularly collected and analyzed for the concentration and the δ¹⁸O of PO₄^3-. PO₄^3- was usually taken up by corals linearly with incubation time, and the uptake rate apparently depended on temperature. Difference in the uptake rate between coral species was not significant. The δ¹⁸O_p was initially approx. 3‰ lower than the equilibrium value with regard to oxygen-isotope exchange with ambient seawater. In a few cases, the δ¹⁸O_p remained unchanged during the incubation even though uptake proceeded. In the other cases, however, the δ¹⁸O_p gradually increased with time, and in some cases became even higher than the equilibrium value at the end of incubation. This observation suggests that kinetic isotope fractionation rather than simple equilibration operated during the uptake of PO₄^3- by corals and influenced the δ¹⁸O_p. The magnitude of isotope effect associated with uptake seemed to depend on coral species, being the largest with A. digitifera and the smallest with H. coerulea. In natural environments where the concentration of PO₄^3- is much lower than the incubation conditions we used, PO₄^3- is presumably turned over much faster and the δ¹⁸O_p is easily altered by corals and other major primary producers. This fact may limit the advantage of the δ¹⁸O_p as an indicator of external PO₄^3- sources.