Speciation, bioavailability and preservation of phosphorus in surface sediments of the Changjiang Estuary and adjacent East China Sea inner shelf

• Physicochemical processes controlled the distribution and preservation of P.
• Mobile muds played a unique role in the remobilization of bioavailable P.
• P/SSA loadings were used to characterize the preservation of P in sediments.
• Preferential loss of labile organic P relative to OC occurred during transport.

The speciation, potential bioavailability and preservation of phosphorus (P) in surface sediments of the Changjiang (Yangtze River) Estuary and adjacent East China Sea (ECS) inner shelf were investigated through the analyses of P fractions and sediment bulk properties. A sequential extraction method (SEDEX) was used to separate and quantify the following six sedimentary P reservoirs: exchangeable P (Ex–P), authigenic P (Au–P), detrital P (De–P), organic P (Or–P), refractory P (Re–P) and Fe-bound P (Fe–P). Total P (TP) in surface sediments ranged from 15.0 to 21.4  μmol g−1 and was highest near the Changjiang river mouth. The average contribution of each form of P to TP was 55.6% (De–P), 17.8% (Re–P), 16.1% (Or–P), 5.5% (Au–P), 2.5% (Ex–P) and 2.5% (Fe–P), respectively. De–P showed relatively higher concentrations in the river mouth and the ECS shelf region, off the Changjiang Estuary. High concentrations of Or–P were found mainly in mud areas showing a similar distribution pattern with silt, sediment surface area (SSA), and total organic carbon (TOC). Re–P was mainly distributed near the estuarine area and the Zhe–Min coast. Bioavailable P (BAP), accounted for 9.5–32.0% of TP (with a mean of 21.2%) and showed a similar distribution pattern to that of Or–P. De–P/SSA and TOC/SSA loadings both decreased with increasing of SSA, while Or–P/SSA loadings varied little with SSA, indicating that Or–P may have reached an adsorption–desorption equilibrium on mineral surfaces. TOC to total organic P (TOP; sum of Re–P and Or–P) ratios less than the Redfield ratio (84 in average) may have indicated efficient remineralization of organic matter in mobile muds of the Changjiang Estuary and adjacent ECS inner shelf. Furthermore, the relatively high TOC/Or–P ratios (72–422 with a mean of 188) likely suggest a higher degree of preferential regeneration of labile Or–P over TOC in sediments.