The influence of summer hypoxia on sedimentary phosphorus biogeochemistry in a coastal scallop farming area, North Yellow Sea

In situ field investigations coupled with laboratory incubations were employed to explore the surface sedimentary phosphorus (P) cycle in a mariculture area adjacent to the Yangma Island suffering from summer hypoxia in the North Yellow Sea. Five forms of P were fractionated, namely exchangeable P (Ex-P), iron-bound P (FeP), authigenic apatite (CaP), detrital P (De-P) and organic P (OP). Total P (TP) varied from 13.42 to 23.88 μmol g^-1 with the main form of inorganic P (IP). The benthic phosphate (DIP) fluxes were calculated based on incubation experiments. The results show that the sediment was an important source of P in summer with ~39% of the bioavailable P (BioP) recycled back into the water column. However, the sediment acted a sink of P in autumn. The benthic DIP fluxes were mainly controlled by the remobilizing of FeP, Ex-P and OP under contrasting redox conditions. In August (hypoxia season), ~0.92 μmol g^-1 of FeP and ~0.52 μmol g^-1 of OP could be transformed to DIP and released into water, while ~0.36 μmol g^-1 of DIP was adsorbed to clay minerals. In November (non-hypoxia season), however, ~0.54 μmol g^-1 of OP was converted into DIP, while ~0.55 μmol g^-1 and ~0.28 μmol g^-1 of DIP was adsorbed to clay minerals and bind to iron oxides. Furthermore, scallop farming activities also affected the P mobilization through biological deposition and reduced hydrodynamic conditions. The burial fluxes of P varied from 11.67 to 20.78 μmol cm^-2 yr^-1 and its burial efficiency was 84.7-100%, which was consistent with that in most of the marginal seas worldwide. This study reveals that hypoxia and scallop farming activities can significantly promote sedimentary P mobility, thereby causing high benthic DIP flux in coastal waters.