Phosphorus cycling in sediments of the Bohai and Yellow Seas

Using a box-sampler, surface and core sediment samples were collected from the Bohai and Yellow Seas in 1998–1999. The sediments were sequentially extracted to determine six phosphorus fractions, and non-sequentially extracted for total P (TP) and inorganic P (IP), where the difference between TP and IP was assumed to be organic P (OP). It was found that more than 70% of the total P is inorganic P. The concentrations of phosphorus were high near the river mouth, reflecting the influence from the Huanghe and Changjiang sediment loads. The phosphorus levels in the Bohai Sea are higher than those in the Yellow Sea, which is related to the sediment sources and distribution of chlorophyll a in these areas. Concentrations of TP in the Bohai and Yellow Seas are comparable to the other worldwide coastal areas. Among the six forms of phosphorus, phosphorus distribution ranges were 0.8–2% for loosely sorbed P, 3–10% for iron-bound inorganic P, 4–19% for leachable organic P, 3–15% for authigenic apatite, 35–66% for detrital apatite and 14–36% for refractory organic P. Potentially bioavailable phosphorus accounts for a small amount of TP, while phosphorus load associated with eroded soil may be larger than the anthropogenic load. The regeneration of dissolved P from sediments is a slow process, which could play a role in maintaining low phosphate levels in the seas. Most of the phosphorus was regenerated in a water column, and almost all of the remaining phosphorus was buried in the sediments after its accumulation. The high values of P burial efficiencies in the study areas were related to low benthic phosphate flux, which is due to the high percent of detrital apatite that is not bioavailable and the high sediment accumulation rate.