Physical controls of oxygen fluxes at pelagic and benthic oxyclines in a lake

We compared oxygen fluxes measured simultaneously at the pelagic and benthic oxycline in a lake and analyze their relation to hydrodynamic forcing conditions. While the mean oxygen fluxes did not differ significantly among both sites, the fluxes were highly variable in time. Short energetic periods contributed disproportionately to the overall oxygen flux above both the benthic and pelagic oxycline. In the pelagic region, mean fluxes across the oxycline were limited by low diffusivities (7 3 1028 m2 s21) and were one to two orders of magnitude smaller than fluxes above the oxycline (0.5 and 32 mmol m22 d21, respectively). A one-dimensional transport model was used to estimate sources and sinks of oxygen potentially causing this imbalance. The model results indicate that
92% of dissolved oxygen transported into the oxycline is used by the respiration of organic material imported into the oxycline from the epilimnion; chemical oxygen consumption associated with the upward flux of reduced substances is negligible. Our findings indicate that under such conditions, dissolved oxygen consumption and therewith mineralization within the oxycline can be comparable with the corresponding rates occurring in the sediments of eutrophic lakes with an oxic hypolimnion.