Flocculation hysteresis, temporal asymmetry, and their impacts on sediment transport in the benthic boundary layer of Sansha Bay, SE China

Sansha Bay, a semi-enclosed, tide-dominated embayment in Southeast China with intensive mariculture activities. Sediment flocculation is a key process regulating coastal morphodynamics, biogeochemical cycling and ecological functions, yet intra-tidal flocculation dynamics and their impacts on sediment transport remain less studied in shallow macrotidal areas under anthropogenic pressure. Based on field observations in the semi-enclosed Sansha Bay, this study characterizes intra-tidal variations in sediment flocculation dynamics, quantifies the temporal hysteresis and flood-ebb asymmetry of flocculation processes, and assesses their effects on sediment transport and associated ecological processes in the bay. Temporal hysteresis between floc formation and breakup was observed to exhibit pronounced tidal asymmetry, which is regulated by tidal phase duration and vertical settling differences and amplified under large tidal ranges. Clockwise hysteresis during flood tides promotes landward sediment transport, facilitating the redistribution of particulate nutrients from fish aquaculture zones to nearshore macroalgal farming areas. In contrast, weaker hysteresis during ebb tides accelerates floc settling, leading to the retention and local accumulation of particulate pollutants in the bay. Results confirm that tidal-timescale flocculation is a continuous and cyclic process, whose evolution is governed by tidal phase duration and the carryover effects of floc populations across successive tidal stages. Therefore, accurate modeling of fine sediment and pollutant transport in shallow, tide-dominated coastal environments must account for both phase-dependent initial floc conditions and the coupled effects of flocculation temporal hysteresis and tidal asymmetry. • Tidal-timescale flocculation is a continuous, phase-controlled cyclic process. • Asymmetric temporal hysteresis in flocculation drives net landward transport. • Large tidal ranges amplify the flood-ebb asymmetry of flocculation dynamics. • Floc-transport interacts with aquaculture to alter nutrient and contaminant dynamics. • Models require phase-inherited floc states and hysteresis asymmetry for accuracy.