The Contribution of Opal-Associated Phosphorus to Bioavailable Phosphorus in Surface and Core Sediments in the East China Sea

To improve the burial flux calculations of bioavailable phosphorus (P) and study opal-associated P (Opal-P) in the East China Sea (ECS), surface and core sediments were collected in the Changjiang Estuary (CE) and the south of the Cheju Island. In this study, sedimentary P was operationally divided into seven different forms using modified sedimentary extraction (SEDEX) technique: LSor-P (exchangeable or loosely sorbed P), Fe-P (easily reducible or reactive ferric Fe-bound P), CFA-P (authigenic carbonate fluorapatite and biogenic apatite and CaCO3-bound P), Detr-P (detrital apatite), Org-P (organic P), Opal-P and Ref-P (refractory P). The data revealed that the concentrations of the seven different P forms rank as Detr-P > CFA-P > Org-P > Ref-P > Opal-P > Fe-P > LSor-P in surface sediments and CFA-P > Detr-P > Org-P > Ref-P > Fe-P > Opal-P > LSor-P in core sediments. The distributions of the total phosphorus (TP), TIP, CFA-P, Detr-P are similar and decrease from the CE to the south of the Cheju Island. Meanwhile, Org-P and Opal-P exhibit different distribution trends; this may be affected by the grain size and TOM. The concentrations of potentially bioavailable P are 9.6-13.0 μmol g⁻¹ and 10.0-13.6 μmol g⁻¹, representing 61%-70% and 41%-64% of the TP in surface and core sediments, respectively. The concentrations of Opal-P are 0.6-2.3 μmol g⁻¹ and 0.6-1.4 μmol g⁻¹ in surface and core sediments, accounting for 5.3%-19.8% and 4.2%-10.6% of bioavailable P, respectively. The total burial fluxes of Opal-P and bioavailable P are 1.4×10⁹ mol yr⁻¹ and 1.1×10¹⁰ mol yr⁻¹ in the ECS, respectively. Opal-P represents about 12.7% of potentially bioavailable P, which should be recognized when studying P cycling in marine ecosystems.