Fractions and spatial distribution of phosphorus in sediments of the Yangtze Estuary and adjacent marine zones

The fractions and distribution of phosphorus in sediments of the Yangtze Estuary and its adjacent marine and associated environmental variables were studied based on SMT phosphorus classification method. The results indicate that the total and organic phosphorus in sediments ranged from 476.91 to 980.04 μg·g^-1 and 15.84~139.21 μg·g^-1, respectively. Inorganic phosphorus was the predominate form of phosphorus, with content of 186.88~431.69 μg·g^-1, accounting for 72.9%~96.7% of total phosphorus. The P linked to calcium (HCl-P) was in the range of 186.88~431.69 μg·g^-1, accounting for 42.27%~57.79% of inorganic phosphorus. However, relative low concentration of P linked to Fe/Mn (NaOH-P) was in the range of 4.17~52.76 μg·g^-1, accounting for 2.00%~3.27% of inorganic phosphorus. The contents of different forms of phosphorus differed in space and time. Total phosphorus concentration in summer shows decreasing first and then increasing, while opposite variation trend was observed in winter from saltwater to freshwater zones. The organic phosphorus had no spatial variation in summer, while a decreasing trend from saltwater to freshwater zones was observed in winter. The content of NaOH-P was higher in freshwater than in saltwater zones. However, there was no obvious spatial variations of HCl-P in summer, and slight high contents near the freshwater zones were observed. The results of correlation analysis show that the particle size was the key factor affecting the forms of phosphorus in the sediments. Specifically, clay was significantly related to organic phosphorus in summer, while clay and silt show significant negative correlation with inorganic phosphorus and positive relation to organic phosphorus in winter. In addition, TOC, salinity and alkaline phosphatase were main factors affecting forms of phosphorus in sediments.