Distribution and budget of nitrogen and phosphorus in the coastal area of Rushan Bay

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Based on the seven comprehensive surveys in the coastal area of Rushan Bay in summer of 2009 and 2014, the distribution of nitrogen and phosphorus nutrients and nutrient structure were analyzed, and the budgets of dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP) were estimated based on the water balance calculated by Princeton Ocean Model (POM). The results show that the high concentrations of DIN and DIP in adjacent area of Rushan Bay in summer appears in the mouth area of Rushan Bay. Their concentrations and distributions are influenced by the terrestrial inputs, tides and currents significantly. The DIN and DIP effluxes from the sediment to the overlying water at the sediment-water interface result in the higher concentrations of DIN and DIP in the bottom water than those in the surface water. The budget of DIN shows that internal cycling is the dominant source of DIN for the primary production, accounting for 86% of uptake by primary production, followed by water exchange (11%), and benthic efflux (3%); the removal of DIN in the water column is dominant by sedimentation (80%), export to the offshore (16%), and denitrification (4%). The DIP budget shows that internal cycling in the water column is the dominant source of DIP for the primary production, accounting for 91% of uptake by primary production, followed by water exchange (9%), and benthic efflux (lower than 1%); the removal of DIP in the water column is also dominant by sedimentation (67%) and export to the off shore by water exchange (33%). Based on the budgets of DIN and DIP, internal recycling is the dominant source for both of DIN and DIP supplies in the coastal water column, and the burial efficiency of P is higher than N into the sediment in the area off Rushan Bay. However, the different nutrient structure between external and internal sources of the study area would result in a long-term effect on the nutrient balance and primary production due to the differential budget of DIN and DIP.