Seasonal and short‐term controls of riparian oxygen dynamics and the implications for redox processes

Riparian zones are highly‐dynamic transition zones between surface water (SW) and groundwater (GW) and function as key biogeochemical‐reactors for solutes transitioning between both compartments. Infiltration of SW rich in dissolved oxygen (DO) into the riparian aquifer can supress removal processes of redox sensitive compounds like NO3−, a nutrient harmful for the aquatic ecosystem at high concentrations. Seasonal and short‐term variations of temperature and hydrologic conditions can influence biogeochemical reaction rates and thus the prevailing redox conditions in the riparian zone. We combined GW tracer‐tests and a 1‐year high‐frequency dataset of DO with data‐driven simulations of DO consumption to assess the effects of seasonal and event‐scale variations in temperature and transit‐times on the reactive transport of DO. Damköhler numbers for DO consumption (DADO) were used to characterize the system in terms of DO turnover potential. Our results suggest that seasonal and short‐term variations in temperature are major controls for DO turnover and the resulting concentrations at our field site, while transit‐times are of minor importance. Seasonal variations of temperature in GW lead to shifts from transport‐limited (DADO > 1) to reaction‐limited conditions (DADO < 1), while short‐term events were found to have minor impacts on the state of the system, only resulting in slightly less transport‐limited conditions due to decreasing temperature and transit‐times. The data‐driven analyses show that assuming constant water temperature along a flowpath can lead to an over‐ or underestimation of reaction rates by a factor of 2–3 due to different infiltrating water temperature at the SW–GW interface, whereas the assumption of constant transit‐times results in incorrect estimates of NO3− removal potential based on DADO approach (40%–50% difference).


| INTRODUCTION
The notion that surface water (SW) and groundwater (GW) should be perceived as one entity rather than two separate components has been established over the last 20 years (Fleckenstein et al., 2010;Winter et al., 1998). Water and solute fluxes between SW and GW can significantly impact water quantity and quality of both compartments Brunner et al., 2017). These fluxes vary seasonally due to periodic fluctuations of stream stage and GW elevations (Bernard-Jannin et al., 2017;Ranalli & Macalady, 2010), but also at shorter, event-driven temporal scales (Shuai et al., 2017). Disentangling reaction and transport processes and their importance for subsurface reactions modulated by seasonal variations and short-term events is challenging due to highly dynamic bio-geophysical characteristics of riparian environments (Kolbe et al., 2019;Pinay et al., 2015;Song et al., 2018).
Stream stage variations can increase exchange magnitudes within hyporheic and riparian zones, and enhance subsurface solute-turnover (Trauth et al., 2015;Trauth & Fleckenstein, 2017). Trauth et al. (2018) showed that, particularly in riparian zones along losing stream sections, infiltrating stream water can improve the availability of dissolved organic carbon (DOC) as an electron donor in the subsurface and in turn enhance denitrification of agricultural nitrate (NO 3 − ). However, as denitrification requires low DO concentrations it also hinges on prior DO consumption in the infiltrating, oxygen-rich stream water, typically via aerobic respiration . Understanding the key controls and the spatio-temporal variations of these redox reactions is essential for defining appropriate measures to improve the natural attenuation of potentially harmful substances such as nitrate in river corridors (Abbott et al., 2016;Oldham et al., 2019).
The reactive potential along a subsurface flowpath depends on the time that a water parcel stays in contact with the reactive media (Oldham et al., 2013). The greater water travels through the riparian subsurface, the greater the exposure to potentially reactive zones.
However, discharge events can reduce GW transit-times in riparian zones due to sudden increase of SW-GW hydraulic gradients, limiting the exposure time to reactive zones and reducing net reactions (Sharma et al., 2012). In the riparian zone, subsurface transit-times have been quantified applying natural tracers like: heat, electrical conductivity (EC), stable water isotopes, and noble gases (e.g., 222 Rn,37 Ar; Schilling et al., 2017). Among them, cross-correlation analyses of natural EC variations is a simple, robust approach to derive transittimes in losing stream reaches by estimating time lags between the input (stream) and the output EC-signals measured in a monitoring well (Diem et al., 2013;Vieweg et al., 2016). However, Nixdorf and Trauth (2018) reported that the observed EC-signals in some riparian wells at losing stream sections could not only be attributed to direct surface water infiltration, but were instead also affected by stream water that had previously been stored in the banks. They found the estimated transit-times to be as much as two orders of magnitude lower compared to 'true' transit-times derived from introduced (e.g., natural gradient) tracer-tests. This indicates that transit-time estimates based on natural tracers such as EC may be uncertain under non-ideal conditions and that artificial tracers or a combination of both might be a more reliable alternative.
In addition to transit-times and DO concentrations, the biogeochemical reactions are temperature regulated (Pietikäinen et al., 2005;Sharma et al., 2012). Thus, seasonal and short-term temperature fluctuations driven by SW-GW exchanges may either fuel or hinder reactions depending on characteristics of infiltrating water in contrast to GW (Greskowiak et al., 2006;Zarnetske et al., 2011). Besides, water and heat propagate along subsurface flowpaths at different rates, with heat being strongly retarded due to interaction with the sediment matrix. There is a complex interaction between hydrological variations at different time scales, the associated heat and solute transport between SW-GW, and the biogeochemical reactions controlled by these variations. However, these complex interactions are difficult to characterize in the field and hence it is not surprising that to date only a few studies have attempted to explore them in field studies of GW-SW systems (Vieweg et al., 2016;Zarnetske et al., 2012).
This study extends these previous studies and aims to address major features controlling spatio-temporal variations of transit-times and DO consumption rates in riparian aquifers with a specific focus on the interplay between seasonal and event-scale variability. A fourth-order reach of the Selke Stream, a well instrumented and studied site (Nixdorf & Trauth, 2018;Vieweg et al., 2016) within the Bode Observatory of the TERENO initiative (Wollschläger et al., 2017) is used as a test case. At the site, we combined GW tracer-tests with high-frequency data of water levels, EC, and DO in the stream and in groundwater. We carried out a suite of well-to-well groundwater tracer-tests, employing salt (NaCl) and DO as tracers, in the riparian zone under a range of hydrologic conditions in terms of stream flow to: (i) derive GW transit-times; and (ii) acquire in-situ DO consumption rates (k DO ). A simple DO consumption model was parameterized based on the acquired data and subsequently applied to highfrequency observation data. We show how temporal variations of temperature and transit-times affect riparian DO reactions and transport processes. Finally, the relationship between DO consumption rates and transit-times was evaluated using the concept of Damköhler numbers in order to characterize the reactive state of the system.

| Study area
Our field site is located at the Selke River, central Germany (51 43 0 37.79 00 N, 11 18 0 51.0 00 E), Figure 1. Previous studies at the site covered different aspects of flow, transport and reactions at the SW-GW interface with focus on in-stream and near-stream hyporheic processes (Munz et al., 2017;Trauth et al., 2014;Vieweg et al., 2016), however the interplay between transit-time and temperature variations in regulating redox state and processes within the riparian zone has been faintly explored. Measurements of hydraulic heads in the streambed and in riparian wells reveal that the stream reach is characterized by predominantly losing conditions (Munz et al., 2016). Mean annual discharge at the site is 1.5 m 3 /s. During summer, baseflow can be below 0.2 m 3 /s, while bankfull discharge (about 15 m 3 /s) can occur during spring snowmelt.
The riparian aquifer consists of fluvial sediments of up to 8 m thickness, ranging from medium sands to coarse gravels, underlain by clay-silt deposits forming the bottom of the alluvial aquifer. Groundwater levels are generally shallow, with mean depths to groundwater of 0.5 and 1.8 m in the winter and summer, respectively. Drilling core samples from the vicinity of the stream showed a layered system (Figure 1), with a continuous less-permeable unit (fine sand) at around 3-4 m below ground separating the system into two connected aquifer compartments, referred to as shallow and deep groundwater. The hydraulic conductivity of the aquifer material, determined by salttracer and slug-tests in the riparian wells, ranges from 1.7 × 10 −3 to 1.2 × 10 −2 m/s (geometric mean: 3.72 × 10 −3 m/s).

| High-frequency data collection
Water levels, electrical conductivity (EC) and water temperature in stream (T SW ) and groundwater (T GW ) were measured by self-contained Solinst Levelogger Junior loggers (LTC). DO concentrations were monitored by self-contained DO loggers (HOBO Dissolved Oxygen Data Logger) in both stream and in groundwater. For capturing short temporal fluctuations, all loggers were set to a 10-min measuring interval from November 2017 until December 2018, comprising one full year of high-frequency measurements with few data gaps in between (10% of total dataset) due to probe malfunctioning and probe removal during maintenance. Stream discharge was calculated from a stream stage-discharge relation ( Figure S1), based on stage readings and monthly manual discharge measurements using an electromagnetic flow meter (MF pro, Ott, Germany).

| Discharge time-series decomposition
Discharge time-series were divided into baseflow and discharge events components to assess their different effects on subsurface processes. Baseflow separation was based on a rolling 5 days local minima (Gustard & Demuth, 2009); discharge events were defined according to the first derivatives of the discharge time-series using the functions findpeaks and local minima in MATLAB®. Significant events were defined as having a minimum peak prominence of at least 15% above the preceding baseflow value, with a minimum peak separation of 5 days imposed to avoid diminutive discharge variations and overlapping/double peak events that were counted as one large event.

| GW transit-times from tracer-tests in the riparian aquifer
We carried out eight natural gradient tracer-tests in the riparian aquifer adjacent to the stream between November 2017 and September 2018 (Table 1). The tracer-tests captured changes in main flow directions and transit-times of the infiltrating stream water. A total of 11 nested 2 00 PVC monitoring wells (including an 'injection-well'), screened at depths between 1 and 6 m below ground (mbg), were equipped with LTC-loggers to monitor tracer breakthrough curves in different depths, at a 1 min interval and over a minimum period of 7 days after injection.
The monitored locations were the same as for the long-term highfrequency data collection. Tracer (NaCl diluted in stream water) was injected below the groundwater table in the injection-well using a peristaltic pump. A packer isolated the well at 2.5 mbg to optimize tracer insertion by releasing it where maximum transport is expected. The tracer-test protocol was kept identical between different experiments with only the injection volumes and NaCl concentrations varying F I G U R E 1 Aerial view of the field site with average groundwater level contours (GWL) and detailed view of nested wells where groundwater tracer-tests were conducted; a driller's log (right) with observed lithology giving an indication of local heterogeneity and aquifer layering according to the hydrologic conditions. Injection durations varied, but were no longer than 20 min. Groundwater level variations in the injection-well were minimal during tracer-tests and considered to be negligible. In three of the experiments, a packer system was inserted in one of the monitoring wells to evaluate inner well mixing as the wells are fully screened. However, no significant differences between the wells with and without packer system were found. The specification of tracer-tests are summarized in Table 1.
Tracer concentration was measured in terms of EC values. Conservative transport of the EC signal was assumed so that the 1-D advection-dispersion equation without retardation and no sorption (Koestel et al., 2011) applies: where C is solute concentration [M 3 /L], V GW is the groundwater velocity [M/T], D h is the dispersion coefficient, t is time and x is distance to injection point [L]. With the initial condition that a slug mass M is injected at x = 0 and t = 0, at any given moment t and at x, the tracer concentration can be represented by:

| DO consumption rates
Although DO water solubility decreases with temperature (Weiss, 1970), groundwater DO variations at the site cannot be explained solely by temperature variations. Locally, groundwater DO consumption can mainly be attributed to organic matter degradation via aerobic respiration, while other processes such as iron-minerals oxidation and nitrification can be neglected Vieweg et al., 2016).
We derived DO consumption rates from DO concentration measurements in the observation wells during tracer-tests. Since half-saturation constants of aerobic respiration are typically small (0.03-0.3 mg O 2 /L) the reaction can be simplified to first-order kinetics instead of Michaelis-Menten kinetics (Diem et al., 2013;Greskowiak et al., 2006): where C is the measured DO concentration in the monitoring well is the first-order rate constant for respiration (1/T), and τ is the mean transit-time. We considered transit-times from the EC-BTC because first arrival times, peak times, and mean transit-times derived from DO breakthrough curves were similar (and not statistically different). The k DO for each measurement point was determined by plotting the natural logarithm of relative DO concentrations versus transit-times. Best-fit slopes (e.g., mean reaction rates) for each tracer-test were found by linear regression.
A stronger correlation between first-order rates and T GW further supported our use of first-order kinetics instead of 0th-order rates (R 2 = 0.78, R 2 = 0.32, respectively).
Empirical k DO derived from tracer-tests were correlated to measured T GW at the time of the experiment following Arrhenius equation (Pietikäinen et al., 2005), Equation (4). Thus, high-frequency DO consumption rates could be inferred from T GW values.
where k DO-TGW is the reaction rate, T GW is the GW temperature, and

| Simulation of riparian DO concentrations
In order to assess effects of short-term fluctuations of temperature and stream discharge on riparian DO dynamics, which are likely not captured by tracer-tests, we extended the analyses of GW transittimes and DO consumption rates to a high-frequency dataset. We simulated DO concentrations based on Equation (3)

| Effective temperature between stream and groundwater
As stream water infiltrates into the subsurface, water temperature is altered along transit from the stream to the observation wells. Thus, the temperature associated with the reaction is likely neither the stream (T SW ) nor the groundwater temperature (T GW ) but an effective temperature (T eff ) between the two. We implemented T eff as an optimization parameter in the simulations constrained between T SW and T GW . Best computed T eff resulted from minimizing the objective function (ε) for observed DO concentrations (DO obs ) in each time-step: where a and b are fitted parameters from Equation (4). Based on T eff , a temperature corrected consumption rate (k DO-Teff ) was derived for each time-step, and further used to compute riparian DO concentrations using Equation (3).

| Theoretical transit-times
To evaluate our transit-time and discharge relation we also computed the theoretical transit-times (τ*) that would be required to perfectly match observed DO concentrations based on k DO-Teff . τ* were calculated with k DO-Teff while allowing transit-time in Equation (3) to vary boundless in each time-step to match observed DO concentrations.
Thus, we can analyse the patterns and values of independently computed τ* and whether differences from empirical τ Q were physically plausible changes (i.e., realistic ranges for the field site) not captured by our streamlined model, or if other variables and processes influence the biogeochemical system.

| Damköhler numbers for dissolved oxygen
To compare the roles of reaction and transport processes and to evaluate whether the system was limited by the reaction rate or by the supply of reactants, we used dimensionless Damköhler numbers. They have been extensively applied in chemical engineering and hydrological studies to assess the balance between reactive and transport processes (Fogler, 2005;Oldham et al., 2013;Vieweg et al., 2016). The Damköhler number for oxygen (DA DO ) is computed as: where k DO(t) and τ t represent the first-order DO consumption rates   Figure 2a). We also observed that most of the discharge events (around 60%) resulted in reduced stream water temperatures (T SW ) and consequently under the prevailing losing conditions also in lower GW temperatures (T GW ) (up to 4.5 C lower in January) relative to the temperature prior to the event ( Figure S2).
T SW and T GW followed a seasonal pattern as imposed by ambient air temperature, with higher values in summer and lower values during winter. The variation of T GW is increasingly lagging behind variations in T SW with increasing distance from the stream (Figure 2b). Highest and lowest temperatures were slightly different for T SW and T GW , maximum and minimum water temperatures were 23 C and 0 C for T SW (μ = 8.7 C), and 20 C and 2.7 C for T GW (μ = 13.5 C), respectively. T GW and DO concentrations in Figure 2 are vertically averaged values over the entire saturated thickness of the aquifer since temperature differences between shallow and deep GW were generally below 2 C and DO concentrations differences were smaller than 2 mg/L at different depths.
Stream DO remained close to saturation (corrected for temperature) with only minor diurnal fluctuations linked to stream metabolism.
In groundwater, DO concentrations exhibited a high spatio-temporal variation, generally decreasing with increasing distance from the stream (Figure 2c). During summer, prolonged periods of anoxia (DO < 2 mg/L) occurred in all observation wells. DO concentrations in the stream and in groundwater increased during discharge events, especially during winter (Figure 2c; Figure S2).

| EC cross-correlation transit-times (τ EC )
The EC time-series between the Fs and Fd were not significantly different and, therefore, EC cross-correlation transit-times ( T GW explained 87% of k DO variations for Fs and 91% for Fd, indicating a strong temperature control of the reactions. There was a systematic increase of rates towards the summer, however differences between the shallow and the deep groundwater can be seen, Figure 4b. In the winter, T GW in the deep groundwater was slightly warmer than in the shallow groundwater. Therefore the resulting k DO values were slightly higher in the deep groundwater. In the summer the opposite was observed.

| Controls of riparian DO
In order to further constrain the interplay and major short/long-term controlling factors of riparian DO concentrations, we used different parameterizations of Equation (3)  The opt-τ found for Fs was 0.48 day (below the average τ of 0.

| DO simulations with effective temperatures
Although model fits of Section 3.4.1 were acceptable, subsurface temperatures in the riparian zone are affected by both the temperature of the freshly infiltrating stream water as well as the local groundwater temperature, Figure 2b. To better represent the true, yet unknown temperature that best represents the temperature dependent consumption rate (k DO-Teff ), we computed an effective temperature (T eff ) for the different transit-time scenarios of Section 3.4.1. Hence, we can jointly assess the sensitivity of DO concentrations to k DO-Teff and to different transit-times.
Since more degrees of freedom were added to the model, simulations employing k DO-Teff had better model fits in comparison to simulations relying on T GW . By employing k DO-Teff , the scenario with opt-τ was better than the simulation using τ Q for Fs (NSE = 0.926 and NSE = 0.897, respectively). However, the opposite was observed for Fd, where the model using τ Q had a slightly higher NSE than the model considering opt-τ (NSE = 0.953 and NSE = 0.938, respectively).
Comparing to k DO-TGW , the k DO-Teff exhibited earlier summer peak times, Figure 6b,d. There, the resulting k DO-Teff was two to three times higher than k DO-TGW , which can be attributed to a higher T eff caused by infiltrating warmer stream water. If this is not taken into account and k DO is purely based on T GW , consumption rates could be underestimated in summer, whereas they could be overestimated in the winter.
T eff showed a shift from being close or at T GW towards T SW especially in the summer when transit-times were generally longer and T SW > T GW ( Figure S4). We noted that T eff reached the warmer temperature boundary in winter when T GW > T SW and on average 47%

| Damköhler numbers for DO (DA DO )
To assess changes of the reactive regime, we derived Damköhler numbers for Oxygen (DA DO ) applying computed GW transit-times and DO consumption rates according to Equation (6).  the use of signal analysis methods for the EC data. Additionally, Nixdorf and Trauth (2018) had argued that for using EC timeseries to calculate riparian transit-times, EC variations and their propagation into the subsurface are a necessary but not sufficient condition to constrain travel times. Additionally, it is also required EC variations to be higher relative to stream discharge variations for successful application of EC-based transit-times analysis. At our site relative EC variations are smaller than relative Q variations and the transmission of EC signals to the observation wells may be affected by other processes that we could not account for.

| Low-frequency DA DO
Conceptually, the τ Q relationship is based on the dependency of infiltrating stream water transit-time on stream discharge, and the assumption that at Q ≈ 0 no water infiltrates into the subsurface, and therefore, infiltration time goes to infinity. The method is straightforward and conceptually simple, with the advantage of providing transient riparian transit-times for infiltrating water from a losing stream section directly from an independent and usually continuously measured hydrologic variable, while requiring only small field calibrations on the fitted function from tracer-tests. Computed theoretical transit-times based on k DO-eff allowed us to evaluate that the powerlaw seemed a good approximation of transit-times. We compared our relation with median transit-times from Munz et al. (2017), who simulated flow and heat transport at a losing reach a few meters downstream from our site. The relationship between their modelled median transit-times and stream discharge could also be well represented by a power-law of the same type (τ Q = 0.268Q −4.6 , R 2 = 0.87). The greater exponent in their study compared to ours seems to indicate that transit-times to their observations wells were more sensitive to stream discharge than at our observation well, in agreement with the greater distance of our well to the stream in comparison to their observation wells.
At our site, GW transit-times decreased with increasing stream discharge as it has also been reported based on modelling of a larger losing stream by Sharma et al. (2012). In their work, simulated V GW around 10 m/day were observed during discharge events, whereas at our site V GW was higher, but in the range of values acquired from tracer-tests. High values could be related to high conductive layers at the site leading to shorter transit-times, or along tree roots in the riparian zone (Tobella et al., 2014). Nevertheless, very short theoretical transit-times and thus very high V GW during or just after flood events (Q > 10 m 3 /s) are likely related to a shortening of the dominant flowpaths (e.g., by infiltration from the upper soil due to inundation of the riparian zone caused by flooding) rather than really meaningful V GW .
DO simulations using constant transit-times also yielded high NSE values, but we believe this is an oversimplification of the system since we observed differences in transit-times during tracer-tests.  Temperature was found to be the main driver of k DO variations in our system, explaining about 90% of its variations. Our k DO-TGW relation was close to the one derived by Vieweg et al. (2016) (Vieweg et al., 2016: k = 0.34 exp [0.2Temp] ). Our rather higher intercept values may be explained by the consumption of organic matter that is either stored in the aquifer or imported from other sources than stream water (Diem et al., 2013). This could be related to additional autochthonous DOC mobilized during high flow events (Sawyer et al., 2014;Trauth & Fleckenstein, 2017), or DOC produced within the unsaturated and root zones (Adeleke et al., 2017;Baker & Vervier, 2004). Due to high temporal dynamics of T GW near losing stream sections, a constant ambient groundwater temperature at some distance from the stream is not a good estimate of the effective temperature relevant for DO consumption, which can lead to incorrect estimations of solute turnover due to erroneous temperature-dependent rates (Munz et al., 2017). The k DO-Teff based on T eff was two to three higher than the empirical k DO-TGW in summer (Figure 6), leading to underestimations of DO consumption rates if only T GW is considered. Our results are in line with results from an analysis by Song et al. (2018) on temperature effects of high-frequency flow variations on thermal regimes and biogeochemical processes within hyporheic zones. Using a numerical model, they showed that temperature contribution to DOC consumption was greater within the fluctuation zone, where infiltrating cold water and its long-term storage led to locally reduced reaction rates (0.1C decrease resulted on about 1% decrease in reaction rates).

| Hydrologic and temperature controls on DO consumption rates
Our findings elucidate strong effects of temperature on reactions in the biogeochemical system of the near-stream riparian zone, which seem to be more pronounced than the effects of variable groundwater transit-times. In contrast to Sharma et al. (2012), who attributed short temporal changes in reaction patterns mainly to changes in transit-times, we found that discharge events did not significantly alter subsurface transit-times, but instead changed groundwater temperatures in the near-stream aquifer with effects on DO consumption rates. Furthermore our field data suggest that even for short transit-times, water temperature is not constant along a flowpath from the infiltration point at the streambed to the observation well, but instead changes according to stream dynamics and water temperature differences between the stream and ambient groundwater. This is different from Diem et al. (2013) who assumed constant water temperature along stream-groundwater flowpaths for all hydrologic conditions ranging from discharge events to baseflow conditions. In contrast, Vieweg et al. (2016) used T eff for computing temperature dependent DO respiration rates by shifting T SW time-series based on EC-derived transit-times. However, one expects that heat presents different transit-times from other solutes due to different transport mechanisms.
Accounting for temperature effects on reactive turnover in riparian groundwater requires a thorough characterization of temperature patterns. Methods as the use of heat as a tracer and direct temperature measurements in the streambed (Munz et al., 2011;Schmidt et al., 2007)

| CONCLUSIONS
In this study we combined GW tracer-tests with high-frequency data to assess spatio-temporal variations of GW transit-times (τ) and DO consumption rates (k DO ) in a riparian aquifer under different hydrological conditions. To evaluate the effect of different transit-times on k DO , we simulated riparian DO concentrations under different transittime scenarios based on acquired field data. Results demonstrate that τ and k DO in our near stream GW system are influenced by seasonal and short-term stream stage and temperature fluctuations. Our data showed that transit-times decreased with increased stream stage, whereas k DO was strongly temperature dependent. The variability of k DO was higher than the variability of τ, with coefficients of variation equal to 0.51/0.65 and 0.18/0.14, respectively, for the shallow/deep riparian aquifer.
At our losing stream reach, estimates of τ directly from stream discharge and of k DO from an effective temperature appropriately Finally, the calculated riparian DA DO values indicated distinct seasonal-shifts in the reactive regime of the system (from transport to reaction limited conditions in summer and winter, respectively), that superimpose effects of short-term event fluctuations, which can move the system to slightly less transport limited conditions (shorter transittimes and smaller consumption rates). During short-term events the variations of transit-times caused by discharge fluctuations on DO consumption were smaller than changes in consumption rates caused by changes in water temperature, which were generally lower than before the event. Under climate-change scenarios (e.g., greater/longer discharge events, higher temperatures), these effects might increase, and impact the reactive state of the system (oxic-anoxic conditions) ultimately leading to short-term transitions of the redox-state of the near stream aquifer. Our findings indicate that event-driven changes in SW-GW exchange not only affect riparian oxygen consumption via associated variations in subsurface transit times, but predominantly via induced changes in subsurface temperatures that affect the reaction rates. Also at seasonal time-scales, subsurface temperature is a major controlling factor for biogeochemical reaction rates. Short-term, event-scale as well as seasonal temperature variations are a common feature of most coupled SW-GW systems. Thus, assessments of F I G U R E 9 (a) Mean GW transit-times; (b) mean temperature corrected DO consumption rates; (c) resulting mean DA DO considering τ Q and k DO-eff , and computed NO 3 − fraction removal potential for shallow (orange) and deep (blue) groundwater distinctly for baseflow (Bf) and discharge events (Q). Shaded area in (c) represents DA DO < 1 (oxic conditions, net-nitrifying potential). NO 3 − fractional removal potentials in (c) are based on Zarnetske et al. (2012) riparian reactive potentials and solute turnover have to consider not only the hydrological variability, but also the variability of temperature conditions.

ACKNOWLEGDEMENT
Open access funding enabled and organized by Projekt DEAL.

DATA AVAILABILITY STATEMENT
The data that support the findings of this study are openly available in