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Rationale Investigations of the isotope ratios of dissolved oxygen (δ18ODO) provide valuable information about the oxygen cycle in aquatic systems. However, oxidation of Fe(II) may change pristine δ18ODO values during storage and can lead to a misinterpretation. We sampled an Fe(II)‐rich spring system and measured δ18ODO values at various time intervals in order to determine influences of Fe‐oxidation. Methods Water samples were collected from an Fe‐rich spring and related stream and the δ18ODO values were measured in fresh, 4‐ and 13‐day‐old samples with an isotope ratio mass spectrometer. Three replicates were measured for each sample with a 1σ of ± 0.2‰. On‐site parameters and Fe(II) contents were also measured over the course of the spring system by multi‐parameter probes and spectrophotometry. Results The δ18ODO values over the course of the spring system in fresh, 4‐ and 13‐day‐old samples revealed differences of up to 8‰. We explain this increase by the consumption of DO by Fe(II)‐oxidation. After a flow length of 85 m the differences in δ18ODO values between fresh and older samples decreased because most of the Fe(II) was consumed. Conclusions False interpretations of δ18ODO values are possible if Fe‐rich water samples are measured after too long storage, and we recommend measurement immediately after sampling.

<p>Dissolved oxygen (DO) in the hyporheic zone (HZ) is a crucial parameter for the survival of many stream organisms and is involved in a multitude of aerobic chemical reactions. However, HZ DO budgets are easily perturbed by climate change and anthropogenic processes that have caused increased deposition of fine sediments (< 2 mm) in many stream beds. The fine sediment fraction hampers exchange of DO-rich stream water with the HZ. In this study we performed a raster sampling approach (0.90 cm length x 1.50 cm width; 30 cm distance between sampling points) at sediment depths of 10 and 25 cm with a focus on DO and its stable isotopes (δ<sup>18</sup>O<sub>DO</sub>). The aim was to analyze small-scale turnover patterns in a forested (site 1) and an anthropogenically influenced stream section (site 2) in a 3<sup>rd</sup> order stream in southern Germany. Grain size analyses showed similar average fine sediment fractions at site 1 (42.5 ±13.7 %) and site 2 (46.3 ±10.8 %). They increased with depth at both sites (38.5 ± 6.3 %, 0-15 cm; 46.5 ± 17.4 %, 15-30 cm at site 1 and 40.6 ±4.5 %, 0-15 cm; 52.0 ±12.2 %, 15-30 cm at site 2). DO concentrations in the HZ ranged from 1.4 to 4.5 mg L<sup>-1</sup> (2.0 ±0.7 mg L<sup>-1</sup>) and 1.5 to 1.8 mg L<sup>-1</sup> (1.7 ±0.1 mg L<sup>-1</sup>) at site 1 and from 1.2 to 2.9 mg L<sup>-1</sup> (1.6 ±0.5) and 1.0 to 2.4 mg L<sup>-1</sup> (1.6 ±0.4) at site 2 at 10 and 25 cm depth, respectively. The low DO concentrations in the HZ suggest high DO consumption rates and reduced exchange with stream water. This is possibly a result of increased fine sediment proportions. However, other factors such as organic carbon contents and increased respiration rates may also influence DO gradients. In contrast, the stream water had an average DO concentration of 9.8 ±0.2 mg L<sup>-1</sup>. Associated δ<sup>18</sup>O<sub>DO</sub> values of the open water (23.4 ±0.1 ‰) differed from those of sediment waters that showed averages of +22.5 ±0.5 ‰ and +22.4 ±0.3 ‰ at site 1 and +22.5 ±0.4 ‰ and +22.3 ±0.2 ‰ at site 2 at 10 and 25 cm depth, respectively. These sedimentary values indicated dominant photosynthesis, even though due to absence of light in the subsurface this process seems unlikely. Therefore, kinetically-driven processes such as diffusion, interactions with Fe or unknown DO sources within the HZ might have caused such <sup>16</sup>O-enriched values. Our findings suggest that the analyses of DO, δ<sup>18</sup>O<sub>DO</sub> and fine sediment gradients in the HZ should be combined with stable carbon isotope measurements to further our understanding of hyporheic processes relevant for stream biota.</p><p> </p>

Dissolved oxygen (DO) is crucial for aerobic life in streams and rivers and mostly depends on photosynthesis (P), ecosystem respiration (R) and atmospheric gas exchange (G). However, climate and land use changes progressively disrupt metabolic balances in natural streams as sensitive reflectors of their catchments. Comprehensive methods for mapping fundamental ecosystem services become increasingly important in a rapidly changing environment. In this work we tested DO and its stable isotope (18O/16O) ratios as novel tools for the status of stream ecosystems. For this purpose, six diel sampling campaigns were performed at three low-order and mid-latitude European streams with different land use patterns. Modelling of diel DO and its stable isotopes combined with land use analyses showed lowest P rates at forested sites, with a minimum of 17.9 mg m−2 h−1. Due to high R rates between 230 and 341 mg m−2 h−1 five out of six study sites showed a general heterotrophic state with P:R:G ratios between 0.1:1.1:1 and 1:1.9:1. Only one site with agricultural and urban influences showed a high P rate of 417 mg m−2 h−1 with a P:R:G ratio of 1.9:1.5:1. Between all sites gross G rates varied between 148 and 298 mg m−2 h−1. In general, metabolic rates depend on the distance of sampling locations to river sources, light availability, nutrient concentrations and possible exchanges with groundwater. The presented modelling approach introduces a new and powerful tool to study effects of land use on stream health. Such approaches should be integrated into future ecological monitoring.