||The suspended particles always play an important role as a carrier of organic carbon and heavy metals in the nearshore area and around the river mouth. Therefore, to understand how the carriers control the fate of these carried substances, we need to understand the variation of suspended particle characteristics. |
According to previous studies, the nature of suspended particle properties would be influenced by three factors, which are water-particle interaction, boundary layer effect, and biotransformation. The feature of these factors also would be changed by the river plume, under different hydrodynamic conditions. The physical process would affect the particle characteristics directly or indirectly, making the physical and biogeochemical process more complex. Consequently, we heired R/V Ocean Researcher III and conducted a field experiment from July 6 to July 9, 2012 to focus on the temporal variations of the suspended sediment characteristics and fators that influence them at a fixed location off the Zhuoshui River mouth, Taiwan.
The CTD rosette was deployed with a laser in-situ scattering and transmissometry (LISST) to record the hydrographic profile (salinity, temperature, fluorescence, and volume concentration of suspended particle) and near-surface water samples (taken at 3 m depth). These samples were filtered onboard by Catnet (a nested filtration system) to separate the particle into four different grain-size classes. In addition, a floating platform was tethered to the research vessel on which a downward-looking ADCP (AQUAdopp, 1200 kHz) was mounted for measuring the current velocity during this experiment.
The results indicated the tidally modulated fluctuation of the river plume off the mouth of the Zhuoshui River is the most important factor controlling the water column stability and characteristics of the suspended sediment. In the ambient seawater condition, the surface particle consisted of the fine-grained size-classes (< 10 μm、10 ~ 63 μm). The SSC value was about 10 mg/l, and sometimes increased via the resuspension process by the tidal current. In the river plume regime, the SSC value could reach 40 mg/l ~ 60 mg/l, and the surface particles presented a status of fluffyness which means heavily large volume but less weight and that was consisted of bio-particle and flocs. In addition, the flocs were likely broken due to the turbulence in the convergent zone of the river-plume front whose water column was unstable, and resulting in particles of high bulk density values.The EOF (Emperical Orthogonal Function) results showed the river plume affect the hydrographic condition and suspended particle characteristics within three different physical processes. The time series result of settling velocity of different size classes was calculated according to the Stokes Law, and they showed the value of > 153 μm class was larger than 0.314 mm/s, and the values of 153 ~ 63, 63 ~ 10, and < 10 μm classes were smaller than 12.001, 1.139, and 0.138 mm/s respectively. The average particle settling velocity is 0.54 ~ 30.93 mm/s