The variation of the floc population in the Changjiang Estuary has been studied for both winter and summer season as a function of the presence of living (micro)algae. The influence of algae has been characterized through the use of the chlorophyll-a concentration to suspended sediment concentration (CC/SSC) ratio. Two whole tidal cycle sampling campaigns were carried out and a full set of parameters (particle size distribution, particle concentration, salinity, velocities, chlorophyll-a concentration) was recorded as function of time for 6 vertical depths. It is found that the floc population can be described by three particle classes. The two most dynamic classes (microflocs and macroflocs) co-exist in the water column. It was nonetheless found, due to the correlation between CC/SSC and particle sizes that the system is at steady state, both in summer and in winter. This can be explained by the limited flocculation ability between the classes due to their segregation in the water column. In winter, macroflocs are found at the top of the water column but their amount and size are very reduced with a mean CC/SSC value of 13 ± 11 μg g−1. In summer, algae-rich macroflocs are abundant at the top of the water column with a mean CC/SSC value of 21 ± 18 μg g−1, especially at flood tide. Microflocs, on the other hand, have a higher density and are generally found deeper in the water column. At high water slack, both macroflocs and microflocs will settle but will never catch-up. The fact that the flocs are at steady-state in terms of flocculation is of importance for sediment transport modelling.@en

The interactions between organic and inorganic particles in the context of flocculation is an on-going topic of research. Most current researches do not distinguish between the effects of EPS (produced by microorganisms) and living microorganisms (like algae). In this study, the effect of salinity, EPS and living algae on sediment flocculation are investigated separately. Several types of measurements were performed, which can be divided into the following categories: sediment at different salinities, sediment in the presence of EPS at different salinities, sediment in the presence of living algae at a given salinity. Results show that increasing salinity enhances slightly sediment flocculation. In the presence of EPS there was hardly any flocculation in demi-water, but the flocculation was significant in saline water. The living algae cells were shown to flocculate with themselves and form large flocs. These algae flocs can bind to sediment particles to form larger flocs, both in demi-water and sea water. Size-wise algae-sediment flocs were largest, EPS-sediment flocs came second, and salt-sediment flocs were smallest.@en

Flocculation is an important process in fine cohesive sediment transport that still requires investigation as it is a complex process, involving the aggregation, break-up and reconformation of mineral and biological particles. Fine cohesive sediments are found in rivers, estuaries and their adjacent areas. With the rapid economic development in the Changjiang (Yangtze River) estuary, human activities have intensified, including the development of shipping, and the exploitation of water and soil resources. The erosion and deposition problems in waterways and tidal flats have become increasingly prominent, affecting the evolution of geomorphology and ecological environment....@en

The flocculation process and modelling of suspended sediment transport in estuarine regions is a hot topic in estuarine science[Winterwerp, 1999]. The flocculation is greatly influenced by biochemical parameters[De Lucas Pardo, 2014]. In this paper, we analyse the impact of algae on the processes of flocculation in the Yangtze Estuary, and we identify the mechanisms which are responsible for their changes. The amount of algae (phytoplankton biomass) is linked to the chlorophyll α concentration which we measured[Uncles et al., 1998]. The measurements were performed in the maximum turbidity zone. The seasonal variations of phytoplankton lead to changes in the flocculation dynamics and the composition of suspended particle matter. We recorded the floc size changes in the Yangtze Estuary and we found that: (1) the flocs are significantly influenced by tidal dynamics, as the floc size during slack water is larger than ebb tide and flood tide, (2) there is a correlation between the Chlorophyll concentration and sediment concentration, (3) the floc size is correlated to the algae-sediment mass ratio, (4) in winter, with high salinity and small river discharge, the region has a significant stratification and the floc size was smaller than the one in summer@en