CO2 time series patterns in contrasting headwater streams of North America

We explored the underlying patterns of temporal stream CO₂ partial pressure (pCO₂) variability using highfrequency sensors in seven disparate headwater streams distributed across the northern hemisphere. We also compared this dataset of [40,000 pCO₂ records with other published records from lotic systems. Individual stream sites exhibited relatively distinct pCO₂ patterns over time with few consistent traits across sites. Some sites showed strong diel variability, some exhibited increasing pCO₂ with increasing discharge, whereas other streams had reduced pCO₂ with increasing discharge or no clear response to changes in flow. The only ‘‘universal’’ signature observed in headwater streams was a late summer pCO₂ maxima that was likely driven by greatest rates of organic matter respiration due to highest annual temperatures. However, we did not observe this seasonal pattern in a southern hardwood forest site, likely because the region was transitioning from a severe drought. This work clearly illustrates the heterogeneous nature of headwater streams, and highlights the idiosyncratic nature of a non-conservative solute that is jointly influenced by physics, hydrology, and biology. We suggest that future researchers carefully select sensor locations (within and among streams) and provide additional contextual information when attempting to explain pCO₂ patterns.