Identification of flocculation during large-river estuarine mixing

The flocculation, triggered during estuarine mixing and having an important role on land-to-sea interactions, is a fundamental issue in near-shore oceanographic studies. Identifying the in situ flocculation in large-river estuaries is quite a challenging work, because of the complex seawater circulation and heterogeneous SPM composition in those 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 suspended particulate matter (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. The LISST data were used to successfully identify 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 the discontinuous variations of LISST parameters and δ¹³C from March to July. Although SPM size spectra involved in flocculation showed different patterns in different seasons, however, the 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 together greatly improves our understanding of SPM dynamics in estuarine and coastal areas, in which estuarine flocculation is a critical component.