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Intermittent rivers and ephemeral streams (IRES) may represent over half the global stream network, but their contribution to respiration and carbon dioxide (CO2) emissions is largely undetermined. In particular, little is known about the variability and drivers of respiration in IRES sediments upon rewetting, which could result in large pulses of CO2. We present a global study examining sediments from 200 dry IRES reaches spanning multiple biomes. Results from standardized assays show that mean respiration increased 32‐fold to 66‐fold upon sediment rewetting. Structural equation modeling indicates that this response was driven by sediment texture and organic matter quantity and quality, which, in turn, were influenced by climate, land use, and riparian plant cover. Our estimates suggest that respiration pulses resulting from rewetting of IRES sediments could contribute significantly to annual CO2 emissions from the global stream network, with a single respiration pulse potentially increasing emission by 0.2–0.7%. As the spatial and temporal extent of IRES increases globally, our results highlight the importance of recognizing the influence of wetting‐drying cycles on respiration and CO2 emissions in stream networks. Key Points: Sediment respiration in intermittent rivers and ephemeral streams increases substantially in response to rewettingRespiration pulses are driven by sediment properties, which, in turn, are influenced by climate and catchment characteristicsEffects of wetting‐drying cycles on respiration and CO2 emissions in stream networks need consideration in upscaling and modeling efforts [ABSTRACT FROM AUTHOR]

Novel observation techniques (e.g., smart tracers) for characterizing coupled hydrological and biogeochemical processes are improving understanding of stream network transport and transformation dynamics. In turn, these observations are thought to enable increasingly sophisticated representations within transient storage models (TSMs). However, TSM parameter estimation is prone to issues with insensitivity and equifinality, which grow as parameters are added to model formulations. Currently, it is unclear whether (or not) observations from different tracers may lead to greater process inference and reduced parameter uncertainty in the context of TSM. Herein, we aim to unravel the role of in‐stream processes alongside metabolically active (MATS) and inactive storage zones (MITS) using variable TSM formulations. Models with one (1SZ) and two storage zones (2SZ) and with and without reactivity were applied to simulate conservative and smart tracer observations obtained experimentally for two reaches with differing morphologies. As we show, smart tracers are unsurprisingly superior to conservative tracers when it comes to partitioning MITS and MATS. However, when transient storage is lumped within a 1SZ formulation, little improvement in parameter uncertainty is gained by using a smart tracer, suggesting the addition of observations should scale with model complexity. Importantly, our work identifies several inconsistencies and open questions related to reconciling time scales of tracer observation with conceptual processes (parameters) estimated within TSM. Approaching TSM with multiple models and tracer observations may be key to gaining improved insight into transient storage simulation as well as advancing feedback loops between models and observations within hydrologic science. Plain Language Summary: Solute experiments and transport models, called commonly tracer experiments, are used to understand the relative importance of different stream processes, especially those that influence water, solutes, and nutrients as they move through a stream network. Within these tracer experiments, there are processes that exchange mass beyond the main stream channel to other parts of the river valley bottom environment. Sometimes, there are single or multiple types of tracers used and modeled to try to understand this exchange. There are also multiple models with different equations and structures to simulate these tracers. This study shows that what you can learn about these stream processes depends on experiment choices and which model you use. Hence, refining future multiple tracer experiments and models is needed to determine how we best obtain consistent measurements of key stream processes. Key Points: TSM interpretation improved with analysis of multiple tracers but results in increased parameter uncertainty for a more complex modelNonconservative tracers enabled interpretation of parameters that were highly uncertain when estimated by conservative tracers aloneAchieving reliable parameter estimates depends on choice of tracers and model framework and should be coupled with uncertainty assessment [ABSTRACT FROM AUTHOR]

Abstract: Stream metabolism is a fundamental, integrative indicator of aquatic ecosystem functioning. However, it is not well understood how heterogeneity in physical channel form, particularly in relation to and caused by in‐stream woody debris, regulates stream metabolism in lowland streams. We combined conservative and reactive stream tracers to investigate relationships between patterns in stream channel morphology and hydrological transport (form) and metabolic processes as characterized by ecosystem respiration (function) in a forested lowland stream at baseflow. Stream reach‐scale ecosystem respiration was related to locations (“hotspots”) with a high abundance of woody debris. In contrast, nearly all other measured hydrological and geomorphic variables previously documented or hypothesized to influence stream metabolism did not significantly explain ecosystem respiration. Our results suggest the existence of key differences in physical controls on ecosystem respiration between lowland stream systems (this study) and smaller upland streams (most previous studies) under baseflow conditions. As such, these findings have implications for reactive transport models that predict biogeochemical transformation rates from hydraulic transport parameters, for upscaling frameworks that represent biological stream processes at larger network scales, and for the effective management and restoration of aquatic ecosystems. [ABSTRACT FROM AUTHOR]