Salt-wedge intrusion of seawater and its implication for phytoplankton dynamics in the Yura Estuary, Japan

We monitored hydrographic and biological conditions in the Yura Estuary and its coastal area from April 2006 to March 2008. Hydrographic conditions changed seasonally, and were mainly determined by river discharge and sea level, with tides playing a minor role. In summer, when river discharge was low and sea level was high, a salt-wedge regime was established until ∼18 km upstream from the river mouth. Seawater intrusion into the river was however restricted in winter, when the river discharge was high and sea level was low. Water stratification caused by the seawater intrusion played an important role in phytoplankton dynamics. Phytoplankton developed in the middle layer of the estuary utilizing riverine nutrients, corresponding to the seawater intrusion into the bottom layer in summer. In winter, however, the phytoplankton production remained low in both estuary and sea.