Kenneth Thorø Martinsen

Streams in cultivated lowlands are commonly supersaturated with CO 2 and are a source of CO 2 to the atmosphere. Great uncertainties exist regarding the spatiotemporal variations of CO 2 concentrations and emission rates in stream‐lake fluvial networks and small streams with variable plant cover. We studied this variability and the underlying mechanisms in 40 small, high‐alkalinity Danish streams, including 5 catchments with lakes. Generally CO 2 concentrations were, on average, 9.2 times those of the atmosphere, declining downstream with rising water temperature, chlorophyll a concentration, and decreasing groundwater inputs. We furthermore observed that the concentrations of CO 2 in stream waters declined at the outlet of lakes to values close to or below air saturation due to phytoplankton uptake and atmospheric loss during the long water retention time in the lakes. Downstream, CO 2 concentrations were observed to decrease in summer and in the afternoons, which indicate plant uptake of CO 2 . Sites with deeper water and few plants and low gas transfer velocity retained high CO 2 concentrations. Among 38 fluvial networks where emission could be calculated, it varied 10‐fold (0.41–4.06 g C m −2 d −1 ), but the overall mean was constrained to a narrow confidence interval (1.75–2.50 g C m −2 d −1 ). Our results highlight that a complex of physical, chemical and biological processes cause highly variable carbon dynamics and CO 2 emissions in fluvial networks at local and catchment scales making upscaling challenging.