Phosphorus Cycling\nin Intertidal Zones with Submarine\nGroundwater Discharge: Insights from Time-Integrated Phosphate Oxygen\nIsotope Analyses through Passive Sampling

In intertidal zones (crucial areas for biodiversity and\nblue carbon\nstorage), P cycling is essential because the amount of this element\nin the environment can limit primary productivity. However, owing\nto the significant spatiotemporal variations in water cycling driven\nby fluctuations in surface water, submarine groundwater discharge\n(SGD), and seawater, our understanding of P cycling remains inadequate.\nTherefore, we applied a passive sampling method using a Zr-loaded\nresin (Pas-Zir method) to analyze the phosphate oxygen isotope ratio\n(δ<sup>18</sup>O<sub>PO4</sub>) at a tidal flat dominated by\nSGD on Ikuchijima Island in western Japan. A Zr-loaded resin that\nselectively adsorbed phosphate ions was installed, and the δ<sup>18</sup>O<sub>PO4</sub> values were monitored at the tidal flat bottom,\nat 5 cm above the bottom, in SGD, and in seawater for a month. The\nrange of δ<sup>18</sup>O<sub>PO4</sub> values at the tidal flat\nbottom and above the bottom (17.0‰–18.8‰) exceeded\nthe analytical precision of the δ<sup>18</sup>O<sub>PO4</sub> analysis (±0.4‰) and deviate from the expected isotopic\nequilibrium by pyrophosphatase. The results indicate that the time-integrated\nδ<sup>18</sup>O<sub>PO4</sub> data can provide information on\nthe P cycling during tidal cycles. Our study demonstrates that the\nPas-Zir method is advantageous for understanding the P cycling in\nintertidal zones.