Remobilization and hypoxia-dependent migration of phosphorus at the coastal sediment-water interface

Sediment internal phosphorus (P) loading can be tightly associated with overlying water hypoxia. However, the effects of long-term seasonal hypoxia on the geochemical transition of P in P-poor coastal sediment and how this transition is linked to the early diagenesis of iron (Fe), sulfur (S) and carbon are still poorly understood. Here, we conducted a one-year monthly field investigation to study the (im)mobilization and migration of P among coastal sediment, porewater and overlying water. The coherent distribution of soluble Fe and mobile P and decoupled distribution of labile S (soluble sulfide) and mobile P in the depth profiles indicate that the redox cycling of Fe (but not S) dominates P mobility. Nevertheless, the monthly variation in the porewater soluble reactive P (SRP) presented significant positive correlations with that of the overlying water SRP. This finding highlights that hypoxia-fueled SRP migration from overlying water rather than weak diagenetic P mobilization due to deficient organic matter and solid labile P is the crucial factor responsible for internal P mobility over long time scales. Although SRP tends to migrate from overlying water to porewater, the potential risk of sediment labile P remobilization and reliberation to the overlying water is considerable.