Geochemical behavior of phosphorus and iron in porewater in a mangrove tidal flat and associated phosphorus input into the ocean

A thorough understanding of the distribution and geochemical behavior of phosphorus (P) and iron (Fe) across the sediment-water interface (SWI) is essential for estimating the exchange flux between sediment porewater and overlying seawater. In this study, in situ high-resolution diffusive gradients in thin films (DGT) technique, ZrO-Chelex DGT, were applied to investigate the depth profiles of dissolved reactive phosphorus (DRP) and ferrous iron (Fe2+) in sediment porewater in a mangrove tidal flat of the Jiulong River estuary, China. The concentrations of DRP and Fe2+, which ranged from 0.014 to 0.288 mg L−1 and 0.003-0.619 mg L−1, respectively, were highly heterogeneous across these profiles. The positive correlation between DRP and Fe2+ concentrations in most parts of the profiles verified the key role of Fe-redox cycling in controlling DRP variations. However, in some parts of the profiles, this coupling relationship was disturbed by the enrichment of mangrove organic matter or biological activity, such as that of macrobenthos and microorganisms. The recirculated seawater fluxes driven by tides and wave setups were calculated to be 23.15 m3 m−1 d−1 and 1.49 × 10−3 m3 m−1 d−1, respectively. Based on these calculated fluxes and end-member concentrations, the net DRP flux into the ocean was calculated to be approximately 1.57 mmol m−2 d−1, which is much larger than the molecular diffusion flux of DRP (1.79 × 10−3 mmol m−2 d−1). These results indicate that the advection of recirculated seawater is a significant pathway to deliver DRP into the ocean and that this kind of internal phosphorus from mangrove tidal flats or muddy coastal zones should be considered in future nutrient budgets of estuaries or coastal oceans.