Floc microcosms buffer hypoxia by reducing sediment-driven oxygen consumption in coastal waters

Coastal hypoxia is intensifying globally under anthropogenic pressures, yet the role of suspended particulate matter, especially in its aggregated form as natural flocs, remains inadequately quantified. While previous research has emphasized dissolved nutrients and dispersed particles, we demonstrate that floc size and architecture critically regulate oxygen consumption in coastal bottom waters. Integrating analysis of global hypoxia and suspended sediment datasets with field observations from a microtidal estuary and series of controlled laboratory experiments, this study reveals that the macroflocs (>200 μm) exhibit significantly lower mass-specific oxygen consumption rates than flocculi (<50 μm) and microflocs (50∼200 μm), despite containing higher organic matter content. This relationship emerges from intra-floc microenvironments that support coupled aerobic-anaerobic processes, reducing net oxygen demand. Field measurements further show that tidal hydrodynamics modulate floc size distributions semi-diurnally: high turbulent shear during peak flows favors small, oxygen-demanding aggregates, whereas slack water conditions promote the formation of oxygen-conserving macroflocs. These findings challenge the conventional view of suspended particles as uniformly oxygen-consuming substrates. Instead, we establish flocs as structured, size-dependent biogeochemical reactors. The derived three-dimensional relationships between floc size, concentration, and oxygen consumption offer a novel mechanistic framework for predicting hypoxia dynamics, with direct implications for coastal management in an era of changing sediment regimes and increasing climate extremes.