Position Shifts of a Bottom Density Front Control Hypoxia Exposure at the Tidal-Flat Margin in a Macrotidal Estuary

Bottom hypoxia in estuaries is usually attributed to local stratification and respiration beneath a pycnocline, but the role of estuarine density fronts in modulating hypoxia exposure at tidal-flat margins remains unclear. Repeated high-tide hydrographic surveys conducted during spring–neap cycles along a longitudinal transect extending seaward from the head region of a macrotidal embayment with extensive tidal flats were analyzed. Conductivity–temperature–depth profiles were used to identify a bottom density front separating a vertically well-mixed inner region from a more stratified offshore channel, and shifts in front position were tracked using the along-transect distance from the tidal-flat margin station. Bottom dissolved oxygen (DO) distributions at the offshore channel station and the tidal-flat margin station were compared, using 3.0 mg L⁻¹ as the primary hypoxia threshold. At the offshore channel station, hypoxia occurrence was primarily associated with strong local stratification. In contrast, at the tidal-flat margin station, the frequency of DO < 3.0 mg L⁻¹ was strongly associated with how far landward the bottom density front extended: even modest landward excursions of the front substantially increased hypoxia exposure, despite only small changes in local stratification. These results demonstrate that small shifts in bottom density-front position can strongly modulate hypoxia exposure at tidal-flat margins, with implications for habitat quality and the siting of monitoring stations and coastal infrastructure in macrotidal estuaries.