Carbon Biogeochemistry and Export Governed by Flow in a Non‐Perennial Stream.

Non‐perennial headwaters experience extremes in flow conditions that likely influence carbon fate. As surface waters contract through dry periods, reconnect during storms, and re‐expand or dry again, there is a great deal of variability in carbon emissions and export. We measured discharge, dissolved oxygen, carbon dioxide (CO2), and dissolved organic carbon (DOC) continuously in a persistent pool at the base of a non‐perennial, forested headwater stream in the southeastern United States to characterize how flow changes affect carbon emissions and export as the stream expands and shrinks. We also compared carbon concentrations and export during different stream flow categories before and after fall wet‐up. CO2 concentrations were high when discharge was lowest (median = 10.2 mg L−1) and low during high flows (3.2 mg L−1) and storms (1.1 mg L−1). High CO2 concentrations led to high emissions on a per area basis during low flow times, but whole‐channel stream CO2 emissions were limited by the small surface area of the stream during periods of surface water disconnection. DOC concentration varied by season (range = 0.1–16.2 mg L−1) with large pulses during smaller summer storms. We found that CO2 and DOC concentrations differed among binned stages of stream flow. As non‐perennial streams become more prevalent across the southeastern United States due to shifts in climate, the relationships between flow and carbon movement into and out of stream networks will become increasingly critical to understanding stream carbon biogeochemistry. Plain Language Summary: Non‐perennial streams lack continuously flowing surface water. We studied the movement of carbon through a forested non‐perennial headwater stream in the mountains of the southeastern United States. The stream channel was nearly empty at certain times in summer and early fall, when surface water was reduced to small pools, but storms could briefly cause the stream to flow again. These cycles of drying and rewetting influence concentrations of carbon dioxide gas and organic carbon dissolved in the stream water. As the stream dried up, oxygen decreased and carbon dioxide increased. When a storm occurred, much of the carbon dioxide in the stream was emitted into the atmosphere as water turbulence increased. Dissolved organic carbon was almost always very low, but storms sent large amounts of carbon downstream very quickly. Categorizing different levels of stream flow also helped explain how carbon cycles through a non‐perennial stream because distinct low flow phases varied in carbon concentration and transfer to downstream ecosystems. Non‐perennial streams are predicted to become more common as climate change affects precipitation patterns, so tracking how carbon moves into and out of streams that dry up and rewet will improve our understanding of freshwater contributions to the global carbon cycle. Key Points: Surface water disconnection led to high CO2 concentrations and emissionsDissolved organic carbon was low, except when storm pulses first reconnected surface water fragments with C sources within and beyond the stream channelInterim flows generated distinct C dynamics; focusing on only flowing or fragmented states likely misses key biogeochemical transitions [ABSTRACT FROM AUTHOR]