River plume induced variability of suspended particle characteristics

- Tidal currents control the dispersal of the river plume and the sediment resuspension.
- There are the ambient seawater, statically stable plume, and unstable plume-front regimes.
- Suspended particles have different properties in these three regimes.
- The suspended particles in the river plume are composed of flocs and biogenic particles.
- EOF is used to decipher the co-variability of water-particle interactions.

A shipboard experiment using R/V Ocean Researcher III was conducted on July 6-9, 2012 to focus on the temporal variability of the suspended particle characteristics off the Zhuoshui River mouth in central Taiwan. The experiment included hydrographic profiling using a CTD rosette with a LISST-100X and collection of near-surface water samples. These samples were immediately filtered on-board by a nested filtration system Catnet to separate the suspended particles into four grain-size classes (> 153 μm, 63-153 μm, 10-63 μm, and < 10 μm). The filtered particles were analyzed for suspended sediment concentration (SSC), POC and PN. A downward-looking ADCP (Aquadopp, 1200 kHz) mounted on a tethered floating platform measured the current velocity profile. The results show that the river plume fluctuating at the semidiurnal tidal frequency was the most important factor affecting suspended particle characteristics. In the ambient seawater regime, surface suspended particles consisted of fine-grained size-classes (< 10 μm, 10-63 μm) with low SSC values (10 mg/l). In the statically stable river plume regime, SSC value reached 40-60 mg/l, surface particles were composed of biological particles and flocs with fluffy structures. In the unstable plume-front regime, floc breakage occurred, resulting in particles of high bulk density values. Also, biological particles were drawn by the convergent flows to the frontal zone. EOF (Empirical Orthogonal Function) analysis indicates that the three most important processes affecting the characteristics of the suspended particles at the study site include: 1) tidal currents that affect dispersal of the buoyant river plume in the surface water; 2) convergence and water-column instability in the frontal zone of the plume; and 3) resuspension of the seafloor sediment into the water column.