Phosphorus speciation and budget of the East China Sea

Abstract

Water samples collected from the inner (<50 m), middle (50–200 m) and outer (>200 m) shelves of the East China Sea (ECS) were analyzed to investigate the importance of dissolved inorganic phosphorus (DIP), dissolved organic phosphorus (DOP) and particulate inorganic phosphorus (PIP) in the water column. Concentrations of DIP, DOP and PIP ranged 0.05–3.01 μM, 0.01–0.54 μM and 0.18–13.94 ı̀mol g −1, respectively. Higher values were generally found in the inner shelf. Significantly higher concentrations of suspended particulate matter (SPM) (mean value 16.87 mg l −1) and PIP (mean value 11.59 μmol g −1) were found near the bottom along the inner shelf and was attributed to re-suspension of sediments. DIP was the major form of dissolved P, accounting for more than 70% of the total dissolved P (TDP) pool in inner and middle shelf waters. However, the DOP concentration generally exceeded the DIP concentration in the surface layer (<100 m) of the outer shelf. PIP was 10–20% of the total P pool (TDP+PIP) in the inner shelf and decreased markedly to less than 1% in the outer shelf. It was also found that the higher the concentrations of SPM and of particulate iron, the higher the percentage of P bound by particles. The phosphorus budget on the ECS was also calculated by employing a box model based on the balance of the water mass and phosphorus mass. The model results revealed that the Kuroshio Surface Water, the Kuroshio Tropical Water, the Kuroshio Intermediate Water and the Taiwan Strait Water annually import approximately 8.6±5.7×10 9 mol, 6.7±5.6×10 9 mol, 28.9±22.2×10 9 mol, and 22.8±13×10 9 mol P, respectively, into the ECS. These imports of P are primarily transported in the dissolved phase. In contrast, the input of P, 11.8±2.3×10 9 mol P yr −1, from the Changjiang River is chiefly transported in the solid phase. The Shelf Surface Water annually exports 44.5±27.3×10 9 mol P out of the ECS, predominately in the dissolved phase. The ECS is a phosphorus sink with approximately 34.3±21.5×10 9 mol P yr −1 deposited in the sediments. Finally, the results of the present study suggested that PIP and DOP could not be ignored when studying the geochemistry of P in the inner and outer shelf, respectively, of the ECS.