Role of vertical transport in the formation and maintenance of metalimnetic oxygen minimum in a stratified reservoir

• Vertical diffusion in metalimnion is one order lower than epi/hypolimnion. • DO fluxes converge towards MOM depth from above and below. • Above MOM, DO changes are governed by both vertical transport and consumption. • Below MOM, oxygen consumption dominates DO dynamics. Metalimnetic oxygen minima (MOM) are observed in many lakes and reservoirs, Typically, processes driving MOM formation include vertical transport and in-situ oxygen consumption, but the relative contributions of these two processes remain debated. This study investigates Panjiakou Reservoir, a deep-water drinking water source in northern China, combining high-frequency diel vertical monitoring (half-hour resolution) with a three-dimensional hydrodynamic-water quality model (EFDC) to quantitatively assess the role of vertical transport in MOM formation. Key findings: (1) The vertical turbulent diffusion coefficient within the metalimnion was significantly lower (by one order of magnitude) than in the epilimnion and hypolimnion. Its minimum value exhibited a strong positive correlation ( R 2 = 0.8) with the square of the buoyancy frequency ( N 2 ), confirming that thermal stratification stability inhibits vertical mixing and is a key factor in MOM formation.(2) The vertical flux divergence of dissolved oxygen (DO) in water layers above and below the MOM was negative, indicating that vertical transport acted as a source term in the oxygen balance, increasing DO concentration via vertical transport.(3) The contribution of vertical transport to MOM formation varied with depth. In the layer above the MOM, DO flux divergence ( FD ) was comparable in magnitude to the oxygen consumption rate ( P net ), indicating that both vertical transport and consumption processes governed DO changes. At and below the MOM depth, FD was relatively small (approaching zero), and oxygen consumption dominated. This study provides new insights into MOM formation mechanisms in stratified reservoirs and has practical implications for water quality management in drinking water sources.