Headwater streams can be important sources of carbon dioxide (CO₂) and methane (CH₄) to the atmosphere. However, the influence of groundwater–stream connectivity on the patterns and sources of carbon (C) gas evasion is still poorly understood. We explored these connections in the boreal landscape through a detailed study of a 1.4 km lake outlet stream that is hydrologically fed by multiple topographically driven groundwater input zones. We measured stream and groundwater dissolved organic C (DOC), CO₂, and CH₄ concentrations every 50 m biweekly during the ice‐free period and estimated in‐stream C gas production through a mass balance model and independent estimates of aquatic metabolism. The spatial pattern of C gas concentrations was consistent over time, with peaks of both CH₄ and CO₂ concentrations occurring after each groundwater input zone. Moreover, lateral C gas inputs from riparian soils were the major source of CO₂ and CH₄ to the stream. DOC mineralization and CH₄ oxidation within the stream accounted for 17–51% of stream CO₂ emissions, and this contribution was the greatest during relatively higher flows. Overall, our results illustrate how the nature and arrangement of groundwater flowpaths can organize patterns of stream C concentrations, transformations, and emissions by acting as a direct source of gases and by supplying organic substrates that fuel aquatic metabolism. Hence, refined assessments of how catchment structure influences the timing and magnitude of groundwater–stream connections are crucial for mechanistically understanding and scaling C evasion rates from headwaters.

Stream temperature is one of the most critical factors controlling aquatic ecosystem health. Practitioners and researchers from a range of fields, including biology, ecology, hydrology, engineering, and watershed management, are concerned with how climate and environmental changes are impacting stream thermal regimes. This primer provides an introduction to the various energy and water exchange processes that underpin stream temperature patterns from small headwater streams to large river systems. An overview of the various energy exchanges is provided, including (1) advection associated with hydrologic processes, and energy exchanges at (2) the stream–atmosphere interface and (3) stream–bed interface. The interaction and spatiotemporal variability of these energy exchange processes are discussed using a water and energy balance framework. A sound physical understanding and appreciation of the complex controls governing stream thermal regimes will help inform effective management strategies to sustain healthy aquatic ecosystems in a changing world.This article is categorized under: Science of Water > Science of Water Science of Water > Hydrological Processes Science of Water > Water Quality Water and Life > Nature of Freshwater Ecosystems

Stream temperature has been increasing in tandem with air temperature, with potentially negative impacts on cold‐water fish such as salmon. Assessing future stream temperature change is critical for developing effective management responses. Empirical models of stream thermal sensitivity generally predict less future warming compared to physically based models. Here we reconcile these discrepancies by using a process‐based hydrology and temperature model to simulate daily flow and water temperature for forested headwater catchments in a maritime region under both historic and projected future climatic conditions. The primary reason that the empirical approach underestimates thermal response to climate change is that it does not account for thermal memory in the catchment, especially related to the effect of snow cover. Empirical thermal sensitivities thus may underestimate stream temperature response to future climate warming. In addition, groundwater‐fed streams may only resist warming in the short‐medium term, due to lagged response of groundwater temperature. More process‐based understanding and modeling of stream thermal regimes is needed to effectively manage aquatic ecosystems.

Understanding stream carbon export dynamics is needed to accurately predict how the carbon balance of peatland catchments will respond to climatic and environmental change. We used a 12 year record (2003–2014) of continuous streamflow and manual spot measurements of total organic carbon (TOC), dissolved inorganic carbon (DIC), methane (CH₄), and organic carbon quality (carbon‐specific ultraviolet absorbance at 254 nm per dissolved organic carbon) to assess interannual and seasonal variability in stream carbon export for a peatland catchment (70% mire and 30% forest cover) in northern Sweden. Mean annual total carbon export for the 12 year period was 12.2 gCm⁻² yr⁻¹, but individual years ranged between 6 and 18 gCm⁻² yr⁻¹. TOC, which was primarily composed of dissolved organic carbon (>99%), was the dominant form of carbon being exported, comprising 63% to 79% of total annual exports, and DIC contributed between 19% and 33%. CH₄ made up less than 5% of total export. When compared to previously published annual net ecosystem exchange (NEE) for the studied peatland system, stream carbon export typically accounted for 12 to 50% of NEE for most years. However, in 2006 stream carbon export accounted for 63 to 90% (estimated uncertainty range) of NEE due to a dry summer which suppressed NEE, followed by a wet autumn that resulted in considerable stream export. Runoff exerted a primary control on stream carbon export from this catchment; however, our findings suggest that seasonal variations in biologic and hydrologic processes responsible for production and transport of carbon within the peatland were secondary influences on stream carbon export. Consideration of these seasonal dynamics is needed when predicting stream carbon export response to environmental change.