Sources, distribution and bioavailability of sedimentary phosphorus species in coastal areas off Changjiang Estuary

The Changjiang Estuary (CE), two transects along the Andong salt marsh (AD-A and AD-C) south-west of Hangzhou Bay, and three salt marshes on Zhoushan Island (ZS-A, ZS-B and ZS-C) were coastal zones receiving different flow regimes and human activities. In this study, sedimentary P species consisting of exchangeable P (Ex-P), iron P (Fe-P), authigenic P, detrital P (De-P) and organic P (OP) were determined in these sediments. The objectives of this study were to evaluate the degree of P pollution and the potential for these sediments to release P into the environment. Authigenic P was the largest fraction in AD-A, AD-C, CE, ZS-A and ZS-B (42–62%), followed by Ex-P (28–33%), Fe-P (4–19%), OP (2–8%) and De-P (1–2%); except in ZS-C where Ex-P was the most abundant (57.86%), followed by authigenic P (24.45%), Fe-P (12.34%), OP (2.85%) and De-P (2.49%). High portions of authigenic P indicate presence of apatite minerals; low levels of OP and De-P suggest low P pollution and low contribution from riverine input. The sedimentary total P (TP) in these locations (ranged from 91.30 to 324.74 mg/kg) indicates the risk of P pollution was very low. However, the bioavailable P (Ex-P, Fe-P and OP) constituted approximately 37–73% of TP. Thus, these locations are prone to release P from Ex-P fraction under oxic conditions and from Fe-P fraction under anoxic conditions.