Observations of distribution and flocculation of suspended particulate matter in the Minjiang River Estuary, China

In this study, we described the distribution pattern of suspended particulate matter (SPM) and its influencing factors in the waters around the Minjiang River Estuary. Further, we investigated the flocculation process of SPM and its underlying mechanism in the study area. The objectives were achieved using water temperature, salinity, turbidity, fluorescence and particle size distribution of SPM data measured in August 2009. As influenced by dilution from the Minjiang River water, the water column structure was significantly stratified in the estuary, with an obvious thermocline and halocline. The turbidity was relatively low in the surface water (SSC < 5 mg/l) but sufficient to form nepheloid layer in the near-benthic (SSC > 15 mg/l). Fluorescence was high (up to 10 μl/l) in the surface water, which was markedly lower and more uniform in the lower water layer. The total volume concentration and particle size of the SPM in the upper water layer were significantly higher than those of the lower water layer. The in-situ particle size frequency distribution of SPM markedly changed in the different water layers, whereas the dispersed particles were fine, with little difference in frequency distribution. The mean effective density of SPM was generally < 600 kg/m3 in the surface and benthic water, and the values were slightly higher in the benthic water layer than in the surface water layer, with a significant increase in the middle water layer. The significant differences between the distribution trends of the total volume concentration and water turbidity were controlled by the size distribution of SPM. The concentration distribution of SPM was mainly influenced by source, re-suspension and thermocline, among other factors. Flocs widely occurred in the study area, and the flocculation mechanism of SPM varied in the different water layers. In the upper water layer, intense biological activity promoted the formation of stable macroflocs. In the benthic water layer, the biological activity was limited, the flocculation of sediment particles was mainly caused by physico-chemical attraction of inorganic particles, and the hydrodynamic conditions and high settling velocity were also unfavourable to the formation of macroflocs.