Distribution, mixing behavior, and transformation of dissolved inorganic phosphorus and suspended particulate phosphorus along a salinity gradient in the Changjiang Estuary

Surface water and suspended particulate matter (SPM) samples were collected along a salinity gradient in the Changjiang Estuary in June, 2011 to examine the distribution, mixing behavior and transformation of dissolved and particulate phosphorus (P) species. Particulate P was fractionated into five size fractions using a water elutriation method, followed by a sequential extraction procedure (SEDEX). Dissolved inorganic P (DIP) concentrations ranged from 0.53 to 3.02μmolL^-1 with a mean of 1.44μmolL^-1, while total particulate P (TPP) concentrations ranged from 20.56 to 26.90μmolL^-1 (24.21μmolL^-1). The variation of DIP and P species in both bulk and size fractionated particles along the salinity gradient indicated a significant transformation between different P phases, especially for labile P such as exchangeable P (Ex-P), organic P (Or-P) and Fe-bound P (Fe-P) in a particulate P pool. Among all P forms, detrital P (De-P) and Or-P in SPM were the most abundant, accounting for 43.9% and 26.1% of TPP in bulk particles. De-P increased with increasing particle grain size, with an average concentration of 8.92±2.20μmolg^-1 in the clay fraction (<8μm) and 12.6±1.49μmolg^-1 in the sand fraction (>63μm), whereas Or-P and other P forms decreased gradually with increasing grain size. The average concentration of Or-P in clay fraction was 6.93±1.92μmolg^-1 and it decreased to 1.59±0.61μmolg^-1 in the sand fraction. The flux of bioavailable P (BAP, sum of Ex-P, Or-P and Fe-P) in clay fraction (<8μm), defined as "truly" suspended particles, was estimated to be 0.21×10⁴tPyr^-1 and accounted for 29% of the bulk BAP flux. The particulate P to specific surface area ratios (P/SSA) was lower in fine suspended particles than in coarse particles, indicating lower preservation of P in these particles. When compared with the bed sediments of the Changjiang Estuary, the P/SSA loadings in fractionated suspended particles were maintained at the same level for De-P and Fe-P, slightly higher for authigenic P (Au-P) and refractory P (Re-P) and obviously higher for labile P such as Ex-P and Or-P, further indicating the differential behavior of P forms during particle settlement in this dynamic estuary.