Seasonal and Spatial Variation in Suspended Sediment Characteristics off the Changjiang Estuary

On the 12th of December 2004 and 26th of June 2006, a LISST-100 B in-situ laser diffraction particle sizer combined with a SeaBird 25 CTD (Conductivity, Temperature, Depth), was used to measure in-situ particle size spectra, mean grain size and volume concentration of suspended sediment, temperature and salinity of water along two sampling lines off the Changjiang estuary. Water samples also were filtered during each sampling period. Suspended sediment concentration, effective mean density, settling velocity and potential vertical mean flux were studied based on the observed data. Results demonstrate that there are significant seasonal and spatial variations in particle size spectra and mean grain size. These variations are mainly due to flocculation, influenced by the spatial distribution of salinity and suspended sediment concentration in June by the distribution of salinity in December. Flocculation in the study area does not dominate the settling velocity; instead, the effective mean density does. Settling velocity and vertical mean flux do not display significant seasonal and longitudinal change. The suspended sediment concentration was relatively higher in December 2004 than in June 2006 due to the significant reduction in sediment discharge and the deposition of suspended sediment in the turbidity maximum zone in June. Variation in suspended sediment concentration in surface water can be used to trace the dispersal range of suspended sediment, while vertical change in suspended sediment concentration implies a prevailing resuspension of bottom sediment in December.