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Scientific Significance Statement: Metabolic stoichiometry predicts that dissolved oxygen (O2) and carbon dioxide (CO2) in aquatic ecosystems should covary inversely; however, field observations often diverge from theoretical expectations. Here, we propose a suite of metrics describing this O2 and CO2 decoupling and introduce a conceptual framework for interpreting these metrics within aquatic ecosystems. Within this framework, we interpret cross‐system patterns of high‐frequency O2 and CO2 measurements in 11 northern lakes and extract emergent insights into the metabolic behavior and the simultaneous roles of chemical and physical forcing in shaping ecosystem processes. This approach leverages the power of high‐frequency paired O2–CO2 measurements, and yields a novel, integrative aquatic system typology which can also be applicable more broadly to streams and rivers, wetlands and marine systems. [ABSTRACT FROM AUTHOR]

Extended severe dry and wet periods are frequently observed in the northern continental climate of the Canadian Prairies. Prairie streamflow is mainly driven by spring snowmelt of the winter snowpack, whilst summer rainfall is an important control on evapotranspiration and thus seasonality affects the hydrological response to drought and wet periods in complex ways. A field-tested physically based model was used to investigate the influences of climatic variability on hydrological processes in this region. The model was set up to resolve agricultural fields and to include key cold regions processes. It was parameterized from local and regional measurements without calibration and run for the South Tobacco Creek basin in southern Manitoba, Canada. The model was tested against snow depth and streamflow observations at multiple scales and performed well enough to explore the impacts of wet and dry periods on hydrological processes governing the basin scale hydrological response. Four hydro-climatic patterns with distinctive climatic seasonality and runoff responses were identified from differing combinations of wet/dry winter and summer seasons. Water balance analyses of these patterns identified substantive multiyear subsurface soil moisture storage depletion during drought (2001-2005) and recharge during a subsequent wet period (2009-2011). The fractional percentage of heavy rainfall days was a useful metric to explain the contrasting runoff volumes between dry and wet summers. Finally, a comparison of modeling approaches highlights the importance of antecedent fall soil moisture, ice lens formation during the snowmelt period, and peak snow water equivalent in simulating snowmelt runoff. [ABSTRACT FROM AUTHOR]

Summary 1. Diel variation in metabolism contributes to variation in oxygen (O2) concentrations in streams. This variation in O2 and other parameters (e.g. pH) can in turn affect the rates of microbial nitrogen (N) processing, concentrations of nitrogenous solutes and production of the greenhouse gas nitrous oxide (N2O). We investigated diel variability in emissions of N2O and the magnitude of short-term variability in N solutes across 10 streams. 2. Nitrous oxide fluxes varied on average 2.3-fold over diel cycles. Concentrations would be underestimated by sampling around noon, but N2O fluxes would not show a consistent bias. Time-weighted mean daily N2O flux was strongly related to nitrate concentration ( r2 = 0.58). Diel patterns in N2O and dissolved N species were often complex (rather than simple sinusoidal curves), probably reflecting complex underlying processes. 3. Reliance on samples obtained around noon would overestimate daily mean nitrate concentrations by 5% and underestimate ammonium by 32% (average bias across all streams and dates). 4. Dissolved organic N did not show consistent day-night variation. However, the magnitude of diel variability was similar to that observed for dissolved inorganic N. Organic and inorganic N concentrations were often similar. Both appear to be dynamic components of stream N budgets. 5. The Intergovernmental Panel on Climate Change (IPCC) relies upon an emission factor to estimate indirect agricultural N2O emissions from streams and ground water. The measured emission factor (defined as the ratio of concentrations of N2O-N to -N) was typically below the recently revised IPCC default figure. Measured values varied on average 1.8-fold over approximately 24-h periods and were slightly higher at night than by day. The emission factor was actually highest in streams that were net sinks for N2O, highlighting a conceptual problem in the current IPCC method. 6. Typical sampling programmes rely on daytime-only sampling, which might cause bias in results. In our study streams, the bias was generally small. Diel variation in nitrate concentrations was related to mean temperature; variation in ammonium and N2O concentrations was greatest at low concentrations of nitrite and ammonium. [ABSTRACT FROM AUTHOR]

The current Intergovernmental Panel on Climate Change (IPCC) method for tabulating agricultural nitrous oxide (N2O) emissions suggests that aquatic ecosystems may be important N2O sources. However, estimates are highly uncertain, and the method to estimate emissions is rarely tested. The default IPCC emission factor for groundwater and surface drainage (EF5-g; defined as N2O-N:nitrate-N) has been lowered recently. Our data support further reduction. Notably, we present the first EF5-g data under ice, which reflects groundwater inputs not confounded by gas exchange. These under-ice data suggest an EF5-g of 0.13%, approximately half the current IPCC default value. Our data from the open-water season also suggest a reduction to 0.11% (based on annual means). The data highlight major problems with the IPCC method. EF5-g was extremely variable, with the highest values observed in a stream that was a net sink of N2O. Testing the overall method for freshwater emissions is more problematic because of the multidecade transit time from headwaters in our study region to the Atlantic Ocean and limited information on emissions from lakes. [ABSTRACT FROM AUTHOR]

This special issue focuses on the effects of ice cover on surface water bodies, specifically rivers and lakes. Background information on the motivation of addressing this topic is first introduced with some selected references highlighting key points in this research field. A summary and synthesis of the eleven contributions is then provided, focusing on three aspects that provide the structure of the special issue: Physical processes, water quality, and sustainability. We have placed these contributions in the broader context of the field and identified selected knowledge gaps which impede our ability both to understand current conditions, and to understand the likely consequences of changing winters to the diversity of freshwater ecosystems subject to seasonal ice cover. [ABSTRACT FROM AUTHOR]