Flocculation of suspended particles during estuarine mixing in the Changjiang estuary-East China Sea

Flocculation, triggered during estuarine mixing plays an important role in land-to-sea interactions and is a fundamental topic in near-shore oceanographic studies. Identifying in situ flocculation in large-river estuaries can be challenging due to the complex seawater circulation and heterogeneous suspended particulate matter (SPM) composition in these areas. In this study, three cruises were conducted in the Changjiang (Yangtze) River Estuary and the adjacent area in March, May, and July 2016. Vertical profiles of SPM total volume, mean size, and size spectra were determined using laser in situ scattering and transmissometry (LISST) measurements at 66–89 stations during the three cruises. Stable isotopic ratios of δ¹³C were also measured in the organic carbon contents of SPM collected at the surface, middle, and bottom layers of the sampling stations. LISST data were used to successfully identify the flocculation occurring in the field as well as to trace SPM size spectrum changes before and after the flocculation process. The δ¹³C values were utilized to study the response of biogeochemical parameters to the flocculation. Phytoplankton blooms occurring in May largely resulted in discontinuous variations in LISST parameters and δ¹³C from March to July. Although the pattern of SPM size spectra involved in flocculation differed among seasons, flocculation processes were always contributed by smaller particles with sizes of several tens of μm aggregating into larger ones >300 μm. Using LISST and δ¹³C measurements combined greatly improved our understanding of SPM dynamics in estuarine and coastal areas where estuarine flocculation was a critical component.