Streams link terrestrial and marine environments by transporting, storing, and transforming terrestrial carbon before reaching the oceans. Terrestrial debris and solutes enter low-order streams, accumulate in high-order rivers, and eventually discharge into coastal marshes and oceans. Flowing waters emit CO₂ passively (as vectors for terrestrial-atmospheric exchange) or actively (via in-situ production), with high-discharge events driving greater total CO₂ fluxes as turbulence accelerates gas emission and limits in-situ reactions. However, the relative contribution of active and passive CO₂ across flow regimes, and their response to spatiotemporal fluctuations in discharge, remains underexplored.
The North Florida flatwoods, a wetland-dense ecosystem with high carbon storage and export, supports blackwater streams receiving carbon primarily from overland flow or a perched water table, making active-to-passive CO₂ proportions dependent on watershed inundation. We hypothesized that passively sourced CO₂ dominates across flow regimes, while actively produced CO₂ becomes more prominent during baseflow. To test this, we used high-frequency sensor observations (CO₂, dissolved oxygen, pH, and depth) and discrete water sampling (CO₂, CH₄, DOC, DIC, and POC) to examine spatiotemporal carbon dynamics across nine tannic, headwater streams. Stream metabolism estimates differentiated active from passive CO₂ orgins, as higher respiration rates indicate greater in-situ CO₂ production. DIC, DOC, and POC concentrations were analyzed for discharge responses and correlations to stream CO₂ emissions.
Total CO₂ fluxes were positively correlated with discharge, with actively produced carbon contributing 53% of total CO₂ fluxes but varying by flow regime and baseline alkalinity. The active-to-total CO₂ ratio was inversely correlated with discharge—passively sourced carbon dominated (70–90%) during high discharge, whereas active carbon peaked (~75%) during baseflow. DOC remained high (~50–110 mg/L) and positively correlated with discharge, suggesting flow state, rather than DOC supply, dictates ecosystem respiration. These findings challenge assumptions concerning DOC-CO2 dynamics and highlight the need for further study of carbon-rich, shallow-water table landscapes in regional and global carbon cycling.