Distribution characteristics of phosphorus speciation and potential bioavailable phosphorus in surface sediments of the central area of Bohai Sea

Phosphorus is an important biogenic element necessary for the growth and reproduction of marine phytoplankton, and it is also the basic element of marine primary productivity and food chain. The potential content of bioavailable phosphorus (BAP) in sediments that can participate in interface exchange depends on the form of phosphorus in sediments, while there are few studies in the central region of the Bohai Sea. In this study, five areas in the central Bohai Sea (Tangshan coast, Qinhuangdao coast, northeastern Bohai Sea, Bohai Strait, and southwestern Bohai Sea) were selected as the research objects. The composition and content of phosphorus in the sediment samples at 23 sampling points were determined by the hierarchical leaching method, and the potential BAP content was calculated. The distribution characteristics and influencing factors were analyzed. The results showed that the inorganic phosphorus (IP) in the surface sediments of the central Bohai Sea was the main form of total phosphorus (TP) in the surface sediments of the central Bohai Sea. The IP contents in the sediments of Tangshan coast, Qinhuangdao coast, northeastern Bohai Sea, Bohai Strait and southwestern Bohai Sea accounted for 78.39%, 79.06%, 71.46%, 84.60% and 81.46% of the TP content, respectively, while the organic phosphorus (OP) accounted for a small proportion. Detrital phosphorus (De-P) was the main occurrence form of IP, accounting for 54.02%, 52.12%, 33.33%, 69.41% and 57.28% of TP on average, respectively. The content of phosphorus in each form of IP was in the order of De-P > Oc-P > Fe/Al-P > Ca-P > Ex-P. River input, sediment grain size and sedimentary environment are the main factors affecting the content and distribution of different forms of phosphorus in surface sediments in the central Bohai Sea. According to the analysis of potential BAP content in sediments, the potential BAP content in the surface sediments of Tangshan coast, Qinhuangdao coast, northeast Bohai Sea and southwest Bohai Sea accounted for 44.77%, 46.94%, 64.87% and 40.54% of TP, respectively, which had strong potential to release phosphorus into water. This study provides a theoretical reference for further study of nutrient cycling and supplement mechanism in the central Bohai Sea and the upper limit of sediment contribution to phosphorus in marine water.