Phosphorus mobilization and availability across the freshwater to oligohaline water transition in subtropical estuarine marshes

Internal phosphorus (P) released from sediments, which has been verified to be affected by iron (Fe) and sulfur (S) reactions, strongly influences P cycling and water eutrophication. However, minimal research has been performed to determine how increased low-level salinity in estuarine tidal marshes affects P mobility and its coupled processes. Herein, in situ high-resolution distributions of labile P, Fe, and S in sediment-overlying water profiles were simultaneously measured along an estuarine freshwater-oligohaline gradient (MM River Estuary, China) utilizing newly developed diffusive gradients in thin films technique. Significant variations in the P, Fe, and S concentrations were observed with significantly higher sediment labile P concentrations in the freshwater-oligohaline transition. The observed labile P and Fe coupling confirmed the Fe redox-driven P release mechanism in the sediment, although this coupling was weakened in the freshwater-oligohaline transition. The diffusion flux results revealed that the sediments shifted from P sink to source with the transition from a freshwater to an oligohaline environment, probably caused by the release of Fe-bonded P stimulated by sulfate reduction. This study determined that increased salinity and the associated environmental responses alter the P remobilization capacity and internal P cycling by changing the sediment P pool and the P-Fe-S couplings across the sediment-water interface. Furthermore, saltwater intrusion into tidal freshwater wetlands caused by strong typhoons or sea-level rise may increase internal P release from sediments, which could have future implications.

Automated summary

This study investigated the impact of salinity gradient on phosphorus release from sediments in estuarine tidal marshes. Results showed significant variations in phosphorus concentrations along the freshwater-oligohaline transition, with a possible link to the release of iron-bonded phosphorus stimulated by sulfate reduction.