Short-term and seasonal variability of oxygen fluxes at the sediment-water interface in a riverine lake

In situ measurements of sediment-water oxygen fluxes conducted in a riverine lake during different seasons were analyzed with the aim of quantifying the combined effects of hydrodynamic forcing and seasonal changes in temperature on sediment oxygen uptake rate. Oxygen fluxes measured using the eddy correlation (EC) technique varied widely between -6.4 and -84 mmol m(-2) day(-1), while variations observed on hourly time scales were of comparable magnitude to seasonal variations. Oxygen fluxes were most strongly correlated to current speed in the benthic boundary layer and water depth, which both co-varied with discharge, temperature, and oxygen concentration. A direct correlation of measured fluxes with temperature and corresponding seasonal flux variations could not be observed. To explore the potential effect of temperature on oxygen fluxes, we applied a simplified analytical model, which couples the effect of hydrodynamic forcing with a temperature-dependent oxygen consumption rate within the sediment. The results suggest that the flux is a non-linear function of both variables and both can have comparable effects on the magnitude of the oxygen fluxes. The model confirms our observation that short-term variations of oxygen fluxes in response to hydrodynamic forcing can mask longer-term seasonal variations driven by temperature. The model further indicates that the magnitude and form of the temperature dependence of oxygen uptake and mineralization rates in freshwater sediments obtained from laboratory incubations can be strongly affected by flow conditions during incubations. We conclude that predictions of oxygen uptake and mineralization rates under changing climatic conditions should also take potential changes of flow conditions into account.