Phosphorus speciation, transformation, and preservation in the coastal area of Rushan Bay

Phosphorus (P) speciation, burial, and transformation are poorly constrained under low-oxygen conditions. Sequential chemical extraction techniques, in-situ incubation, and laboratory incubation were employed to explore P cycling in the low-oxygen area of coastal Rushan. The study determined that the total P concentrations in the coastal area of Rushan Bay were higher than those of other China shelf seas, and largely affected by anthropogenic activities. The phosphate (DRP) fluxes in the study area calculated using an incubation method (0–1960 μmol m− 2 day−1) and measured based on pore water gradients (1.5–50.4 μmol m− 2 day−1) were both highly correlated with oxygen conditions. Sediment incubations showed that DRP diffusion from the sediment mainly originates from Fe–P and Auth-P dissolution and that Org-P recycling contributed only a small portion of the total released P pool. The benthic phosphate flux can be 60 times higher under low bottom-water oxygen levels of 63–150 μmol L− 1 than under oxygen levels exceeding 150 μmol L− 1 in the study area. The P accumulation rates and burial efficiencies in this study area ranged from 16.5–33.3 μmol cm− 2 year− 1 and 81.1–83.4%, respectively, and were regulated by the oxygen level and diffusive DRP flux. This study indicates that low oxygen levels between 63 and 150 μmol significantly govern P transformation and preservation in the sediment and P pools in the water column.