Partitioning and behaviour of different forms of phosphorus in the Tanshui Estuary and one of its tributaries, northern Taiwan

Dissolved inorganic phosphorus (DIP), dissolved organic phosphorus (DOP), particulate phosphorus (PP) and sedimentary phosphorus (SP), were determined in northern Taiwan's Tanshui Estuary and one of its tributaries. Concentrations of DIP and PP (w/v) in the upper estuary were 15-20 times higher than those in the freshwater of the upper reaches in its tributaries, suggesting that the Tanshui Estuary is a seriously phosphorus-polluted environment. The fraction of DIP generally accounted for more than 85% of the total dissolved phosphorus (TDF, DIP plus DOP) within the salinity <30 region, and this decreased to c. 70% in the lower estuary (salinity >30). The fraction of PP accounted for about 60% of total phosphorus (TDP plus PP) in the salinity <5 region but fell to 40-50% in the lower estuary (salinity >30). The logarithm of distribution coefficient, log (K(D)), ranged from 4.36 to 5.61 within the whole estuary and was independent of the salinity. The total concentration of sedimentary phosphorus (TSP) ranged from 328 mg kg(-1) to 1240 mg kg(-1) within the estuary, and the inorganic form of phosphorus accounted for 87-97% of TSP. The distribution of DIP in the Tanshui Estuary showed slight addition in the upper estuary, removal in the mid-estuary and conservation in the lower estuary. The addition and removal of P appeared to be related to desorption from and adsorption onto PP. The concentrations of PP(w/w) and TSP exhibited addition in the salinity 5-10 region, strongly suggesting the removal of DIP onto the sediment. This removal of DIP probably resulted from co-precipitation or adsorption by Fe oxides/hydroxides. The upstream input of total P in the Tanshui River system was generally less than 0.3 tP day(-1) and the phosphorus flux from the discharge of domestic sewages input to the upper estuary was approximately 6.0 tP day(-1) during the study periods. A 2-D steady-state multi-box model was employed to calculate the phosphorus sedimentary and estuarine export fluxes. The calculated results showed that approximately 4.0 tP day(-1) deposited in the upper estuary, and that the estuary also exported about 2.0 tP day(-1) to the coast.