Flocculation under combined effects of hydrodynamic and subaqueous biomass in the bottom boundary layer (BBL) of a microtidal estuary

Abstract

The flocculation dynamics within the bottom boundary layer (BBL) of tidal estuaries constitute a pivotal and intricate aspect entwined with hydrodynamics and morphodynamics. In microtidal estuaries, where saltwater intrusion occurs, the ensuing impacts on ecosystems, biological habitats, and human activities underscore necessity for comprehensive understanding. In pursuit of elucidating flocculation dynamics within estuarine BBLs, an extensive 25-hour survey was conducted throughout a complete tidal-cycle in the Huangmaohai estuary, China. This investigation encompassed the collection of data pertaining hydrodynamics, biochemical characteristics of suspended flocs within the BBL. The observed irregular semidiurnal tide was delineated into six distinct stages: I) Weak flood, II) Flood slack, III) Strong ebb, IV) Ebb slack, V) Strong flood and VI) Flood slack. The amalgamation of empirical findings and theoretical analyses has facilitated the development of conceptual model delineating the intricate processes and interactions of multiple factors within each stage (I-VI) in the BBL of a prototypical micro-tidal estuary. Notably, it reveals biological factors exhibit a significantly higher efficacy in estuarine flocculation dynamics within the BBL compared to the chemical ion attractions, attributable to variations in salinity. Further nuances emerged, indicating that semi-liquid extracellular polymeric substance (EPS) plays a substantial role in the formation of high-density flocs, particularly during periods of heightened turbulent shear conditions in flood and ebb times (I, III, V). Conversely, particulate biological debris predominantly contributes to low-density flocs characterized by a low settling velocity, particularly for large flocs >200 μm during tidal slacks (I, IV), and smaller median-sized flocs (70–200 μm) during flood or ebb times (III, V) due to turbulent induced breakage of bio-particles. This study underscores the significance of quantitative investigations into the biological components within individual flocs under estuarine hydrodynamics as a pivotal step towards comprehending flocculation mechanisms and predicting cohesive sediment transport within the BBLs of estuaries.