Intermittent oxygen flux from the interior into the bottom boundary of lakes as observed by eddy correlation

Brand, Andreas, Daniel F. McGinnis, Bernhard Wehrli, Alfred W├╝est

Limnol. Oceanogr., 53(5), 2008, 1997-2006 | DOI: 10.4319/lo.2008.53.5.1997

ABSTRACT: Turbulent oxygen transport from the overlying stratified water column into the bottom boundary layer (BBL) on the slope of a medium-sized lake was investigated using the eddy correlation (EC) technique. The seicheinduced oscillatory flow of the BBL, with a period of ~1 d, was identified as the mechanism driving turbulent oxygen transport. Sporadic short-term EC vertical oxygen fluxes exceeded the sedimentary oxygen uptake of 13 ± 2 mmol m-2 d-1 calculated from sediment oxygen profiles by more than a factor of three. The average EC flux over half of a seiching period was 9.2 mmol m-2 d-1 similar in range to the flux into the sediment; however, these two fluxes do not have to coincide spatially and temporally. The EC oxygen flux was only significant when the deep basin-scale currents exceeded a velocity of 2 cm s-1 and the corresponding bottom shear was sufficient to produce active turbulence. Below this threshold, decaying turbulence resulted in oxygen fluxes lower than 3.5 mmol m-2 d-1, with an even lower average flux of 0.8 mmol m-2 d-1 observed during reversals of the seiching. At low velocities, the weak turbulence is insufficient to transport dissolved oxygen through the stratified top of the BBL (stability N2 ~ 2.4 × 10-4 s-2), even though turbulence was found in the inertial subrange and periodical bottom convective mixing was still present. The EC technique provided valuable data on the temporal variability of oxygen transport related to the BBL hydrodynamics and flux pathways.

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