The Influence of an Oscillating Bottom Boundary Layer on Oxygen Fluxes across the Sediment-Water Interface

We present synchronous observations of internal seiching, bottom boundary layer turbulence, diffusive boundary layer thickness, and small-scale diffusive oxygen fluxes from a eutrophic lake. Measurements were made during summer above a sloping bed at 6 m depth, where the thermocline intersects the bottom, which is a potential hotspot for biogeochemical activity. Oxygen microprofiles (0.1-mm resolution), measured over two days by a Clark-type oxygen probe (~24 profiles/day), revealed order-of-magnitude seiche-induced fluctuations in diffusive fluxes across the sediment-water interface. As expected, diffusive fluxes were correlated with diffusive boundary layer thickness. However, fluxes showed less correlation with turbulent dissipation rates. Turbulent velocities were measured along vertical profiles extending 0.15 to 2 m above the bed, using pulse-coherent Acoustic Doppler Profilers (1.5 cm and 0.5 s resolution). Through each seiche period (10–24 h), turbulent dissipation rates fluctuated by several orders of magnitude, and were often higher at z=1 m than adjacent to the bed. Temperature profiles, measured by 11 RBR Solo loggers within 2 m of the bed, and dissolved oxygen levels, showed seiche-induced fluctuations in stratification and oxygen (<2 to >8 mg l^-1). Strong stratification (Brunt-Väisälä frequency ~ 0.14 s^-1) often suppressed turbulence, even near the bed (between 0.18 and 0.28 m above bed, median Richardson number was 3.8, and median estimated buoyancy Reynolds number was 5). This suppression of turbulence often prevented supply of water column dissolved oxygen to the bed. However, a few periods of unstable stratification and intense turbulence (~10^-7 W kg^-1) associated with large seiche amplitude were observed followed by peaks in water column dissolved oxygen and diffusive boundary layer fluxes.