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

Abstract

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 10 28 m 2 s 21 ) and were one to two orders of magnitude smaller than fluxes above the oxycline (0.5 and 32 mmol m 22 d 21 , 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.Anoxic conditions prevail in bottom waters of many stratified water bodies like lakes, reservoirs, and oceans. These conditions arise from an imbalance in the biogeochemical uptake and downward vertical transport of dissolved oxygen (DO), which can result from eutrophication (Wetzel 2001) and/or limited vertical mixing (Boehrer and Schultze 2008). While the former enhances the input of organic matter to the bottom waters and leads to an increased DO demand, the latter restricts downward fluxes and oxygen replenishment. Deep-water DO can be depleted either temporarily, during the seasonal cycle of density stratification (Turner and Erskine 2005), or permanently if meromixis inhibits seasonal mixing (Rodrigo et al. 2001). The so-called oxycline forms at the boundary between oxic surface waters and anoxic bottom waters. We define the oxycline as the layer ranging from the depth where the DO is 62.5 mmol L 21 to the depth where the DO becomes 0 mmol L 21 ; it is congruent with the hypoxic zone. Pelagic oxyclines exhibit strong vertical gradients not only in dissolved substances, but also in biodiversity and ecosystem functioning. The lack of DO results in drastic habitat alterations (Zhang et al. 2009) and affects chemical cycling (Davison 1981). Inland water bodies are likely to be more susceptible to anoxia more frequently in the future since climate warming leads to longer stratification periods and thereby promotes the additional formation and persistence of anoxic conditions (Adrian et al. 1995;Livingstone 2003;Foley et al. 2012). Therefore, a process-based understanding of vertical oxygen transport between the oxic surface water and anoxic bottom layers becomes increasingly important. From a physical point of view, the transport across the oxyclin...

Oxygen Ebullition From Lakes

“…For Wolfersgrün Reservoir, the measured mean oxygen saturation of 92 ± 27% and a typical k for German reservoirs of 0.9 ± 0.5 m d −1 (Saidi & Koschorreck, ) resulted in a diffusive O 2 flux of −18 ± 10 mmol m −2 d −1 . In the eutrophic lake Scharmützelsee the diffusive O 2 flux was 5.6 ± 27.8 mmol m −2 d −1 (Kreling et al, ). The mean bubble flux of O 2 during our study period was 1.6 ± 1.5 mmol m −2 d −1 .…”

The importance of physical transport and oxygen consumption for the development of a metalimnetic oxygen minimum in a lake

“…) at a particular depth can be described as the sum of the net mass flux towards this depth and the volumetric rate of oxygen production ( P net , negative values denote oxygen consumption) at this depth. To remove all reversible vertical motions resulting from internal waves, the DO mass balance will be analyzed in isothermal coordinates, i.e., measured DO concentrations and gradients are projected onto isotherms (De Szoeke and Bennett ; Kreling et al ): …”

Mixing efficiency in the thermocline of lakes observed from eddy correlation flux measurements

“…Turbulent diffusivities were found to be related to flow velocity and wave intensity ( Figure 6). Wave intensity in Lake Scharm€ utzelsee can be attributed to high frequency internal waves with periods of 5-7 min [Kreling et al, 2014] whereas in Lake Arendsee basin-scale internal waves with periods of 2.5 and 4.6 h prevailed. The latter were analyzed in detail in Bernhardt and Kirillin [2013].…”