Phosphorus speciation and availability in sediments off the eastern coast of Hainan Island, South China Sea

• This is the first study concentrating on sediment P speciation in the study area.
• Detrital and authigenic P were the main forms of sedimentary P.
• Organic matter and Fe oxides played an important role in the P geochemistry.
• A significant part of the sedimentary P pool can be potentially bioavailable.

Surface and core sediments were collected off the eastern coast of Hainan Island (South China Sea), and a sequential extraction method (SEDEX) was used to separate and quantify five sedimentary phosphorus (P) reservoirs: exchangeable or loosely sorbed P (Ex-P), iron-bound P (Fe-P), authigenic P (Ca-P), detrital P (De-P) and organic P (OP). Total P (TP) in the surface and core sediments ranged from 7.96 to 22.34 µmol g−1, and was dominated by inorganic P (IP). De-P and Ca-P were the main chemical species of P in surface and core sediments. The distribution of the P species in surface and core sediments was governed by various factors, including terrigenous input, biological processes, Fe oxides/hydroxides, organic matter degradation, and sediment grain size. The organic carbon (OC)/OP ratio ranged from 37 to 293 (average, 107±58) in surface sediments, suggesting that the organic matter in sediments had been subject to degradation and comprised mostly refractory compounds. The OC/Preactive ratio (37–69) was relatively low in the core sediments, suggesting an excess of sedimentary reactive P compared with OC. The Fe/P ratio ranged from 2 to 19 in surface and core sediments, which indicates that the higher proportions of the crystalline phases occurred in estuarine areas relative to the offshore areas. The potential bioavailable P accounted for 20.3–54.2% of TP in the surface and core sediments.