The distribution of phosphorus forms and bioavailability in sediments from Huang Dong Hai continental shelf

Phosphorus is the most important limiting factor in nutrient cycling. It is also the key element controlling primary production in aquatic environments. Sediments act as both a sink and a source of phosphorus because of continuous transport of phosphorus species across the sediment-water interface. Phosphorus is intimately involved in both the terrestrial and marine biogeochemical cycles. However, not all fractions of phosphorus in sediments are released to the overlying water. Therefore the effects of phosphorus in sediments in promoting aquatic environment eutrophication can be more efficiently evaluated by studying phosphorus fractions, rather than total phosphorus (TP). In this study, the forms and bioavailability of phosphorus in the sediment were analyzed by means of SEDEX (Ruttenberg KC) to determine the geochemical characteristics of phosphorus in the Huang Dong Hai continental shelf. According to this method, there are five types of phosphorus: iron-phosphorus (Fe-P), organic-phosphorus (Or-P), detritus-phosphorus (De-P), exchangeable-phosphorus (Ex-P), and autologous-phosphorus (Au-P). They differ from each other in occurrence, origin and geochemical features. The horizontal and vertical distribution of six forms is presented. The profiles of age deposition were also investigated to reveal changes in the contents of the different phosphorus forms over about two hundred years. The rank order of the contents of phosphorus fractions in the surface sediment was Au-P (140.72 mu g/g) > De-P (59.23 greater than or equal to g/g) > Or-P (32.69 mu g/g) > Fe-P (29.91 mu g/g)> Ex-P (5.92 mu g/g). The contents of Au-P and Or-P decreased from offshore to the open sea, and values for De-P were uniformly distributed over the whole sea area. High contents of TP and Ex-P were present in the Yangtze estuary area. TP, Fe-P, Or-P, De-P, Au-P and Ex-P contents in the sediments were in the range 310.23--941.78 mu g/g, 5.46--58.11 mu g/g, 4.87--80.57 mu g/g, 12.68--282.15 mu g/g, 31.86--271.38 mu g/g, and 1.32--20.95 mu g/g, respectively. The vertical distribution of the different fractions of phosphorus at four sampling stations varied. The distribution of different phosphorus fractions in different sediments reflected changes in the early diagenesis of phosphorus. The concentration of Au-P in HI-18 was much lower than at the other three stations. In the study area, the relative contributions of Fe-P, Ex-P, and Or-P were 5.96%, 1.18%, and 6.51% of TP, respectively. Fe-