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.

Most applications of laser induced fluorescence (LIF) for dissolved oxygen (DO) imaging in flowing water are based on luminescence intensity measurements of a dissolved indicator. A major limitation for applying the technique in the bottom boundary layer (BBL) is the sorption of the luminescent dye to organic surfaces at the sediment. Many sediment and soil studies have used planar optodes on transparent foils as an imaging technique for observing concentration distributions across the sediment-water interface. The presence of the foil, however, is restricting the free flow and therewith the DO concentration above the sediment surface. In this study, we applied the phosphorescence lifetime LIF (sLIF) technique in combination with nanoparticles coated with platinum complexes as DO indicators. By using a planar laser light sheet for fluorescence excitation, the three-dimensional flow field around the sampling area is not restricted. In contrast to the intensitybased LIF technique, this method extracts the lifetime of the luminescence by evaluating the exponential decrease of the phosphorescence intensity after a short excitation light pulse. The method is therefore independent of dye concentration and is not affected by spatial inhomogeneities of the excitation light intensity. We applied the technique to visualize the burrow-ventilation activity of the tube-dwelling Chironomus plumosus larvae. Sequences of the two-dimensional DO concentration distributions showed the intermittent outburst of highly-depleted plumes from the outlet of the burrows, which mixed rapidly with the ambient oxygen-rich water.

Burrow ventilation by tube-dwelling benthic animals affects solute exchange between sediments and water by 2 means. Drawing of O2-rich water into the burrow increases O2 availability in the sediment and stimulates biogeochemical and microbial processes, whereas flushing of the burrow creates a 3-dimensional flow field above the burrow, which induces mixing. Previous studies have revealed the role of the diffusive boundary layer (DBL) thickness on the exchange of solutes between the sediment and overlying water. Mapping the O2 gradient 24 within the DBL is a challenging task in the presence of benthic faunal activities. We used a novel 25 lifetime-based laser induced fluorescence (τLIF) technique that enables unobstructed 26 observations of spatial and temporal O2 dynamics above burrows inhabited by midge larvae 27 (Chironomus plumosus). We observed instantaneous plumes of O2-depleted water released from the outlet of the burrows and drawdown of O2-rich water above the inlet caused by peristaltic pumping of C. plumosus larvae. Vertical O2 gradients changed dynamically during burrow 30 ventilation relative to in a control tank without animals. The advective transport of O2 above the opening caused by burrow ventilation degraded the O2 concentration gradient. For a range of larvae densities that is frequently observed in ponds and lakes, the advective transport caused by burrow ventilation was the dominant transport mechanism.