Nutrient element distributions in the mesotidal Shuangtaizihe estuary fromNorth China

The Shuangtaizihe estuary is characterized by NIP ratios of 10(2)-10(3), wh ich are likely due to fertilizer applications and sewage discharges in the drainage area. In the estuary, phosphate shows a broad maximum in the mixin g zone, presumably due to remobilization from solid phases. Silica and ammo nia behave conservatively, although the data are scattered. Nitrate provide s a distribution of mixing among three end-members owing to the variabiliti es of riverine concentrations. A simple mass-balance model was used to esti mate the water exchange rate in the Shuangtaizihe estuary, indicating that 45-50% of the effluents in ebb tide returns to the estuary during the follo wing flood tide, the net exchange between the estuary and coastal oceans (i .e. Bohai) is limited to 10% for a tidal period. The Shuangtaizihe estuary acts as a source on the pathway of continental nutrient species to the Nort hwest Pacific Ocean. The estuary is also a transit zone for bioproduction f rom phosphate limitation in fresh waters (e.g. river and lake) to nitrogen limitation in coastal oceans.