The Role of Fronts in Horizontal Transports of the Changjiang River Plume in Summer and the Implications for Phytoplankton Blooms

Frontal processes play a significant role in estuarine and coastal ecosystem dynamics. In this study, a hydrodynamic model of the Changjiang River plume was developed based on the Regional Ocean Modeling System, and Lagrangian Coherent Structures (LCSs) were applied to the model simulations to analyze the horizontal transport characteristics of the river plume with the surface plume front, the tidal front, and the far-field plume front in summer. The results show that the plume currents during the ebb tide were supercritical upstream of the near-field surface plume front and tidal front and subcritical downstream of the fronts. In the LCSs results, transport barriers existing in the frontal regions indicated that the materials hardly crossed the fronts and tended to accumulate. The water parcels near the surface plume front and the tidal front in the near-field entirely originated from the Changjiang River Estuary. The surface East China Sea shelf water could not influence the near-field region due to the transport barrier formed by the far-field plume front. Momentum balance analysis results reveal that the plume currents were in geostrophic balance around the near-field fronts, rendering the flows mainly in the along-front direction and significantly weakening the cross-shore flows. These physical processes provided enough residence time of the plume water and growth time for phytoplankton in the frontal region and thus favored algal blooming with sufficient nutrients. Even though the frontal convergence induced by transport barriers also appeared near the far-field plume front, insufficient supply of nutrients limited the phytoplankton growth.