ORIGINAL Constraints of salinity- and sediment-induced stratification on the turbidity maximum in a tidal estuary Ting Lu 1 & Hao Wu 2 & Fan Zhang 1 & Jiasheng Li 2 & Liang Zhou 1 & Jianjun Jia 1 & Zhanhai Li 1 & Ya Ping Wang 1,2 Received: 18 February 2020 /Accepted: 31 August 2020 # Springer-Verlag GmbH Germany, part of Springer Nature 2020 Abstract The vertical density gradients of salinity and suspended sediment concentration (SSC) cause stratification in estuaries, which play a vital role in the turbulence structure, water mixing, and sediment transport. To investigate the effect of stratification, especially SSC-induced stratification, on maintaining the estuarine turbidity maximum (ETM), we conducted in situ measurements on sediment dynamics at the upper and central ETM sites in the South Passage of Changjiang Estuary in July 2018. The gradient Richardson number was estimated as a proxy for the stratification that is attributable to salinity or/and SSC. We found that salinity-induced stratification was observed mainly on the surface and in the middle layers, whereas SSC-induced stratification occurred mainly in the near-bottom layers. Furthermore, at the central ETM, the baroclinic effect was enhanced during the neap tide when the salinity-induced stratification was stronger than that during the spring tide. In the early phase of floods with minimum velocity during the neap tide, salinity-induced stratification suppressed the turbulence and vertical diffusion of sedi- ments. Moreover, the flocculation enhanced the settling process within the water column. Consequently, high concentrations of fine-grained sediments formed near the bottom and promoted SSC-induced stratification, thereby leading to the continuous accumulation and trapping of sediments. In conclusion, the interactions among the “salinity- and SSC-induced stratification” processes served as crucial constraints of the temporal and spatial variations of the ETM in the Changjiang Estuary. Keywords Suspendedsedimentconcentration . Salinity . Stratification . Flocculation . Changjiang . Estuarineturbiditymaximum (ETM) Introduction Stratification influenced by the interaction between freshwater discharge and tidal current is considered to be an essential indicator of estuarine classification. According to the salinity stratification scale, estuaries can be classified as salt wedges, strongly stratified, weakly stratified, and well mixed (Pritchard 1955). Stratification induced by vertical density gradients may affect both biological and physical processes by influencing the dissolved oxygen content in water columns (Officer et al. 1984), thereby impacting the primary produc- tivity of phytoplankton (Lucas et al. 1998), suppressing tur- bulent mixing (Geyer and Cannon 1982; MacCready and Geyer 2010; Scully and Geyer 2012), and inhibiting sediment diffusion (Geyer 1993). This is an indication of the paramount importance of understanding the stratification effect in estua- rine studies. Several studies have investigated the tidal asymmetry of stratification within tidal cycles in estuarine waters. Scully and Friedrichs (2003) found that stratification during the ebb tide was stronger than that during the flood tide and suggested the strong diffusion of suspended sediments from the bottom to the surface during flood tide, thus affecting suspended sed- iment concentration (SSC) distribution. However, this is an exception and only occurs when the effect of tidal straining (the oscillatory vertical shear of tidal flows acting on the hor- izontal density gradient) is significant (Schulz and Umlauf 2016; Scully and Geyer 2012; Simpson et al. 1990). More studies have suggested that the saltwater intrusion caused by baroclinic flow is the primary mechanism of stratification and * Ya Ping Wang ypwang@nju.edu.cn 1 State Key Laboratory of Estuarine and Coastal Research, School of Marine Sciences, East China Normal University, Shanghai 200241, China 2 Ministry of Education Key Laboratory for Coast and Island Development, School of Geographic and Oceanographic Sciences, Nanjing University, Nanjing 210093, China https://doi.org/10.1007/s00367-020-00670-8 / Published online: 8 September 2020 Geo-Marine Letters (2020) 40:765–779