Abstract
Tropical headwater streams are potentially important sources of greenhouse gases, with seasonal shifts in flow regimes and hydrological connectivity likely influencing their sources and sinks of greenhouse gases. We developed a mass balance for carbon dioxide (CO 2 ) and methane (CH 4 ) over a year in a well‐instrumented headwater stream in the seasonal tropics of northern Australia, quantifying sources and sinks at daily (CO 2 ) and monthly (CH 4 ) timescales. Additionally, we relate changes in CO 2 concentration and fluxes to changes in discharge to understand the drivers of CO 2 fluxes in this system. The stream received 3.2 g C‐CO 2 m −2 y −1 and 0.06 g C‐CH 4 m −2 y −1 (standardized to the catchment area), mostly from surface runoff and shallow groundwater, with disproportional amounts (82% and 72%, respectively) imported during the 6‐month wet season. Less than 10% of CO 2 was produced by internal aquatic metabolism, with the rest being imported from the surrounding terrestrial environment. Over 90% and 77% of the CO 2 and CH 4 received by the stream was emitted to the atmosphere, with emission rates two and 1.5 times higher than global river mean emission rates. CO 2 import and emission to the atmosphere was positively correlated with discharge while CO 2 concentration in the stream was negatively correlated. Collectively, these data demonstrate strong seasonal patterns of carbon sources and sinks driven by the extreme seasonal hydrological cycling.