10/31/2023 0 Comments Diurnal vs nocturnal fluctuationWe hypothesized that CO 2 fluxes would differ between day and night due to diel variations in terrestrial inorganic carbon inputs, in situ metabolism, and temperature. The aim of this study was to assess the magnitude and drivers of stream CO 2 flux variations between day and night across European streams. However, besides mostly local studies that indirectly infer CO 2 fluxes from pCO 2 and k 11, 12, 17, 18, no direct measurements exist that compare day-time and night-time CO 2 fluxes from streams on a larger spatial scale. Direct field observations provide the means to improve estimates and understanding of the drivers behind spatiotemporal variability, and thus the dynamics of CO 2 outgassing from running waters. This approach fails to capture the high spatiotemporal variability observed for k and pCO 2 and therefore can provide imprecise estimates of CO 2 fluxes 15, 16. ![]() Presently, most fluvial CO 2 emission values are derived from k estimates based on water velocity and stream channel slope and on water pCO 2 values indirectly calculated from alkalinity, pH, and temperature 3. However, questions remain regarding the magnitude and relative drivers of seasonal and diel fluctuations of CO 2 fluxes in streams. Temperature and solar radiation also directly influence water pCO 2, the former by changing the solubility of the gas and the latter due to photomineralization 14. Ecosystem respiration, a source of CO 2 in the stream, takes place throughout the whole day, and gross primary production, a sink of CO 2, occurs only during daylight. Furthermore, seasonal and diel changes in stream pCO 2 are attributed to stream metabolism driven by temperature and solar radiation 10, 11, 12, 13. The supply of CO 2 from external sources, such as soil water or groundwater, into streams, varies with reach and season 5, 9. While k in streams is mostly driven by water turbulence created by variations in flow and stream morphology 7, the water pCO 2 is influenced by the degree of hydrological connectivity between the stream and the adjacent riparian soils 8 as well as by in-stream processes (e.g., stream metabolism). The high spatiotemporal variability of k and water pCO 2 can be attributed to a complex interplay of underlying controls. Both parameters are highly variable in space and time 5, 6, causing uncertainty in the magnitude of regional and global fluvial CO 2 emissions 2. Fluvial CO 2 fluxes are primarily controlled by the gas exchange velocity at the water-air interface ( k) and the gradient between the water and atmospheric partial pressures of CO 2 ( pCO 2) 4. Streams and rivers therein represent major CO 2 emitters 3. Inland waters are important sources of atmospheric carbon dioxide (CO 2) partially offsetting the terrestrial carbon sink 1, 2. Our findings highlight widespread day-night changes in fluvial carbon dioxide fluxes and suggest that the time of day greatly influences measured carbon dioxide fluxes across European streams. However, no consistent drivers could be identified across sites. We attribute diel carbon dioxide flux variability mainly to changes in the water partial pressure of carbon dioxide. Median fluxes are 1.4 and 2.1 mmol m −2 h −1 at midday and midnight, respectively, with night fluxes exceeding those during the day by 39%. ![]() We directly measured fluxes four times between October 2016 and July 2017 using drifting chambers. Here we present a large-scale assessment of day- and night-time carbon dioxide fluxes at the water-air interface across 34 European streams. ![]() Despite the global significance of fluvial carbon dioxide emissions, little is known about their diel dynamics. Globally, inland waters emit over 2 Pg of carbon per year as carbon dioxide, of which the majority originates from streams and rivers. Carbon dioxide fluxes increase from day to night across European streams
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