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Abstract Lakes play a significant role in the global carbon cycle, acting as sources and sinks of carbon dioxide (CO₂). In situ measurements of CO₂flux (FCO₂) from lakes have generally been collected during daylight, despite indications of significant diel variability. This introduces bias when scaling up to whole‐lake annual aquatic carbon budgets. We conducted an international sampling program to ascertain the extent of diel variation in FCO₂across lakes. We sampled 21 lakes over 41 campaigns and measured FCO₂at 4‐h intervals over a full diel cycle. Rates of FCO₂ranged from −3.16 to 4.39 mmol m⁻² h⁻¹. Integrated over a day, FCO₂ranged from −381.68 to 878.49 mg C m⁻²d⁻¹(mean = 76.54) across campaigns. We identified three characteristic diel patterns in FCO₂related to trophic status and show that for half of the campaigns, daily flux estimates were biased by > 50% if based on a single (daytime) measurement. 

Abstract Stream dissolved oxygen (DO) dynamics are an outcome of metabolic activity and subsequently regulate ecosystem functions such as in‐stream solute and sediment reactions. The synchronization of DO signals in and across stream networks is both a cause and effect of the mode and timing of these functions, but there is limited empirical evidence for network patterns of DO synchrony. We used high frequency DO measurements at 42 sites spanning five catchments and stream orders to evaluate DO signal synchrony in response to variation in light (a driver of photosynthesis) and discharge (a control on DO signal spatial extent). We hypothesized that stream network DO synchrony arises when regional controls dominate: when light inputs are synchronous and when longitudinal hydrologic connectivity is high. By complement, we predicted that DO signal synchrony decreases as light becomes more asynchronous and stream flows decline or become discontinuous. Our results supported this hypothesis: greater DO signal synchrony arose with increasing light synchrony and flow connectivity. A model including these two controls explained 70% of variation in DO synchrony. We conclude that DO synchrony patterns within‐ and across‐networks support the current paradigm of discharge and light control on stream metabolic activity. Finally, we propose that DO synchrony patterns are likely a useful prerequisite for scaling subdaily metabolism estimates to network and regional scales.