Nutrient element distributions in the mesotidal Shuangtaizihe estuary from north China

The Shuangtaizihe estuary is characterized by N/P ratios of 10²-10³, which are likely due to fertilizer applications and sewage discharges in the drainage area. In the estuary, phosphate shows a broad maximum in the mixing zone, presumably due to remobilization from solid phases. Silica and ammonia behave conservatively, although the data are scattered. Nitrate provides a distribution of mixing among three end-members owing to the variabilities of riverine concentrations. A simple mass-balance model was used to estimate the water exchange rate in the Shuangtaizihe estuary, indicating that 45-50% of the effluents in ebb tide returns to the estuary during the following 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 Northwest Pacific Ocean. The estuary is also a transit zone for bioproduction from phosphate limitation in fresh waters (e.g. river and lake) to nitrogen limitation in coastal oceans.