The phosphorus cycle in the Sanggou Bay

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The phosphorus cycle is studied during 2013–2014 in the Sanggou Bay (SGB), which is a typical aquaculture area in northern China. The forms of measured phosphorus include dissolved inorganic phosphorus (DIP), dissolved organic phosphorus (DOP), particulate inorganic phosphorus (PIP), and particulate organic phosphorus (POP). DIP and PIP are the major forms of total dissolved phosphorus (TDP) and total particulate phosphorus (TPP), representing 51%–75% and 53%–80%, respectively. The concentrations and distributions of phosphorus forms vary among seasons relative to aquaculture cycles, fluvial input, and hydrodynamic conditions. In autumn the concentration of DIP is significantly higher than in other seasons (P<0.01), and higher concentrations are found in the west of the bay. In winter and spring the phosphorus concentrations are higher in the east of the bay than in the west. In summer, the distributions of phosphorus forms are uniform. A preliminary phosphorus budget is developed, and shows that SGB is a net sink of phosphorus. A total of 1.80×107 mol/a phosphorus is transported into the bay. The Yellow Sea is the major source of net input of phosphorus (61%), followed by submarine groundwater discharge (SGD) (27%), river input (11%), and atmospheric deposition (1%). The main phosphorus sink is the harvest of seaweeds (Saccharina japonica and Gracilaria lemaneiformis), bivalves (Chlamys farreri), and oysters (Crassostrea gigas), accounting for a total of 1.12×107 mol/a. Burial of phosphorus in sediment is another important sink, accounting for 7.00×106 mol/a. Biodeposition by bivalves is the major source of phosphorus in sediment, accounting for 54% of the total.