Transformation and source of nutrients in the Changjiang Estuary

The concentrations of phosphate (PO4 3−), ammonium, nitrite, nitrate, silicate, dissolved organic nitrogen (DON), dissolved organic phosphorus (DOP), particulate phosphorus (PP) and particulate nitrogen (PN) along the salinity gradient were measured in the Changjiang Estuary in April 2007. The behavior of nutrient species along the continuum from the freshwater to the coastal zone is discussed. In the mixing zone between the riverine and marine waters, nitrate and phosphate behave non-conservatively, while silicate behaves conservatively. Nutrient import was quantified from the river load. Nutrient export to the sea was quantified from river discharge and from the salinity-nutrient gradient in the outer estuary. Using these data, a nitrogen and phosphorus budget was made. The internal estuarine fluxes played an important role in the nutrient estuarine fluxes, which accounted for approximately 41% of the nitrogen flux and 45% of the phosphorus flux. The mixing experiments in the laboratory generally reproduced well the inorganic process affecting nutrient dynamics in the Changjiang Estuary, indicating that the primary P and N transformation processes were phosphate and nitrate desorption along the salinity gradient.