In a joint project (1997-2000), the University of Hamburg (Germany) and the Ocean University of Qingdao (China) have investigated the circulation, water mass structure, nutrient fluxes and phytoplankton dynamics in the Bohai Sea. A hierarchy of coupled circulation, transport and production models for the seasonal scale has been developed and tested against field data from two ship cruises in autumn and spring. The models are available now for scenario calculations under the impact of environmental change.

The transports of solutes and other tracers are fundamental to estuarine processes. The apparent transport mechanisms are convection by tidal current and current-induced shear effect dispersion for processes which take place in a time period of the order of a tidal eyele. However, as emphasis is shifted toward the effects of intertidal processes, the net transport is mainly determined by tide-induced residual circulation and by residual circulation due to other processes. The commonly used intertidal conservation equation takes the form of a convection-dispersion equation in which the convective velocity is the Eulerian residual current, and the dispersion terms are often referred to as the phase effect dispersion or, sometimes, as the "tidal dispersion." The presence of these dispersion terms is merely the result of a Fickian type hypothesis. Since the actual processes are not Fickian, thus a Fickian Hypothesis obscures the physical significance of this equation. Recent research results on residual circulation have suggested that long-term transport phenomena are closely related to the Lagrngian residual current or the Lagrangian residual transport. In this paper a new formulation of an intertidal conservation equation is presented and examined in detail. In a weakly nonlinear tidal estuary the resultant intertidal transport equation also takes the form of a convection-dispersion equation without the ad hoc introduction of phase effect dispersion in a form of dispersion tensor. The convective velocity in the resultant equation is the first-order Lagrangian residual current (the sum of the Eulerian residual current and the Stokes drift). The remaining dispersion terms are important only in higher-order solutions; they are due to shear effect dispersion and turbulent mixing. There exists a dispersion boundary layer adjacent to shoreline bound-anesAn order of magnitude estimate of the properties in the dispersion boundary layer is given. The present treatment of intertidal transport processes is illustrated by an analytical solution for an amphidromicsystem and by a numerical application in South San Francisco Bay, California. The present formulation reveals that the mechanism for long-term transport of solutes is mainly convection due tothe Lagrangian residual current in the interior of a tidal estuary. This result also points out the weakness in the tidal dispersion formulation, and explains the large variability of the observed values for tidal dispersion coefficients. Further research on properties of the dispersion boundary layer is needed.

Using a three-dimensional weakly nonlinear baroclinic shallow water model, the Lagrangian residual velocity associated with a multi-frequency tidal system has been analyzed. The first-order Lagrangian residual velocity, the mass-transport velocity, has been shown to be the sum of the mess-transport velocities derived from the respective constituencs of astronomical tides, and boch the wind-driven (barotropic) and the density-driven (baroclinic) componencs. The second-order perturbacion Lagrangian residual velocity, i.e., the Lagrangian drift velocity, has been shown to involve a series of nonlinear interactions between the products of the respective conscitutuents of tides, and reflects the periodicities of all the constituents of astronomical tides contained in the multi-frequency tidai system through the initial phases. AS an example, the Lagranglan drift velocity induced by an M2-S2 tidal system is analyzed in detail for a more thorough understanding of the mechanism of nonlinear interactions of the second-order dynamics. A coupled set of nonlinear field equations for the mass-transport velocicy and the zeroth-order apparent comcentration has been derived and used to describe and understand the shallow water residual circulation along with the intertidal transport processes coupled by the wind stress over the sea surface, the heat flux across the water surface, the horizontal gradient of water density, and the tidal body force resuleing from the nonlinear interaction among the multi-frequency astromomical tidal variables. An application of the model to the summer. The tide-induced component of the residual circulation in the Bohai Sea is more appropriately associated with and M2-K1 tidal system than an M2-tidal system alone.