Forms of sedimentary phosphorus in the South Yellow Sea and the implication to regional eutrophication trend

Eutrophication has been increasingly occurred in the South Yellow Sea (SYS), the main region of outbreak of green tide. However, the mechanism and evolution of eutrophication in this area remains not fully resolved. Concentrations and forms of the surface sediment phosphorus (P) in the SYS were determined by the modified sequential extraction method (SEDEX) to examine the bioavailability and potential contribution to the eutrophication. Results show that the mean value of total phosphorus (TP) in surface sediments was 514 mg/kg, being generally in a mildly contaminated condition. Inorganic phosphorus (IP) was the main form (76.39%), of which 30.17% was Ca-P. The average contribution of each P form was in the order of OP (organic P) ＞ Ca-P (Ca bound P) ＞ De-P (detritus P) ＞ Fe-P (Fe bound P ＞ Ex-P (exchangeable P). Ca-P and De-P are not bioavailable species, which are less easily released under normal hydrodynamic disturbance in sediments, and are generally enriched in coarse-grained sediments. Ex-P and Fe-P are easily adsorbed into fine-grained sediments, for which pH, temperature, hydrodynamics, and redox conditions are important environmental factors. Acidification of seawater in the SYS will elevate the release of Ex-P and Fe-P into seawater and worsen the degree of eutrophication. Distribution of OP indicated the contribution of biological processes in the open sea in addition to the terrestrial input in the nearshore. The mean value of bioavailable phosphorus including Ex-P, Fe-P and OP was 240.1 mg/kg, which accounted for 46.4% of TP, suggesting that sedimentary P in SYS is more bioavailable, showing a higher risk of P release into the water column, and an important long-term potential contributor to local eutrophication.