Dissolved oxygen isotope modelling refines metabolic state estimates of stream ecosystems with different land use background.

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.

Balance of carbon species combined with stable isotope ratios show critical switch towards bicarbonate uptake during cyanobacteria blooms.

Next to water quality deterioration, cyanobacteria blooms can affect turnover of aqueous carbon, including dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), and particulate organic carbon (POC). We investigated interactions of these three phases and their stable isotopes in a freshwater pond with periodic cyanobacterial blooms over a period of 23 months. This helped to map turnover and sources of aqueous carbon before, during, and after bloom events. During bloom events POC isotope values (δ13CPOC) increased up to -17.4‰, after aqueous CO2 (CO2(aq)) fell below an atmospheric equilibration value of 412 µatm. Additionally, carbon isotope enrichment between CO2(aq) and POC (εCO2-phyto) ranged from 2.0 to 21.5‰ with lowest fractionations observed at pH values above 8.9. The increase of δ13CPOC and decrease of εCO2-phyto values at low pCO2 and high pH was most likely caused by the activation of the carbon concentrating mechanism (CCM). This mechanism correlated with prevalent assimilation of 13C-enriched HCO3-. Surprisingly, CO2(aq) still contributed more than 50% to the POC pool down to pCO2 values of around 150 µatm. Only after this threshold the reduced εCO2-phyto suggested incorporation of 13C-enriched HCO3-.

Beware of effects on isotopes of dissolved oxygen during storage of natural iron-rich water samples: A technical note.

RATIONALE: Investigations of the isotope ratios of dissolved oxygen (δ18 ODO ) provide valuable information about the oxygen cycle in aquatic systems. However, oxidation of Fe(II) may change pristine δ18 ODO values during storage and can lead to a misinterpretation. We sampled an Fe(II)-rich spring system and measured δ18 ODO 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 δ18 ODO 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 δ18 ODO 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 δ18 ODO values between fresh and older samples decreased because most of the Fe(II) was consumed. CONCLUSIONS: False interpretations of δ18 ODO values are possible if Fe-rich water samples are measured after too long storage, and we recommend measurement immediately after sampling.