Phosphorus Cycling in Intertidal Zones with Submarine Groundwater Discharge: Insights from Time-Integrated Phosphate Oxygen Isotope Analyses through Passive Sampling

In intertidal zones (crucial areas for biodiversity and blue carbon storage), P cycling is essential because the amount of this element in the environment can limit primary productivity. However, owing to the significant spatiotemporal variations in water cycling driven by fluctuations in surface water, submarine groundwater discharge (SGD), and seawater, our understanding of P cycling remains inadequate. Therefore, we applied a passive sampling method using a Zr-loaded resin (Pas-Zir method) to analyze the phosphate oxygen isotope ratio (δ18OPO4) at a tidal flat dominated by SGD on Ikuchijima Island in western Japan. A Zr-loaded resin that selectively adsorbed phosphate ions was installed, and the δ18OPO4 values were monitored at the tidal flat bottom, at 5 cm above the bottom, in SGD, and in seawater for a month. The range of δ18OPO4 values at the tidal flat bottom and above the bottom (17.0‰–18.8‰) exceeded the analytical precision of the δ18OPO4 analysis (±0.4‰) and deviate from the expected isotopic equilibrium by pyrophosphatase. The results indicate that the time-integrated δ18OPO4 data can provide information on the P cycling during tidal cycles. Our study demonstrates that the Pas-Zir method is advantageous for understanding the P cycling in intertidal zones.