Katrin Attermeyer, Joan Pere Casas-Ruiz , Thomas Fuss , Ada Pastor , Sophie Cauvy-Fraunié , Danny Sheath, Anna C Nydahl , Alberto Doretto , Ana Paula Portela, Brian C Doyle, Nikolay Simov, Catherine Gutmann Roberts , Georg H Niedrist , Xisca Timoner , Vesela Evtimova , Laura Barral-Fraga , Tea Bašić, Joachim Audet , Anne Deininger, Georgina Busst, Stefano Fenoglio , Núria Catalán , Elvira de Eyto , Francesca Pilotto , Jordi-René Mor , Juliana Monteiro, David Fletcher, Christian Noss, Miriam Colls , Magdalena Nagler , Liu Liu, Clara Romero González-Quijano , Ferran Romero, Nina Pansch, José L J Ledesma , Josephine Pegg , Marcus Klaus, Anna Freixa , Sonia Herrero Ortega , Clara Mendoza-Lera, Adam Bednařík, Jérémy A Fonvielle , Peter J Gilbert , Lyubomir A Kenderov , Martin Rulík , Pascal Bodmer 

Globally, inland waters emit over 2 Pg of carbon (C) per year as carbon dioxide (CO2), of which the majority originates from streams and rivers. Despite the global significance of fluvial CO2 emissions, little is known about their diel dynamics. We present the first large-scale assessment of day- and night-time CO2 fluxes at the water-air interface across European streams. Fluxes were directly measured four times throughout one year using drifting chambers. Median CO2 fluxes amounted to 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%. Diel CO2 flux variability was mainly attributed to changes in the water partial pressure of CO2 (pCO2) but no consistent drivers could be identified across sites. Our results highlight widespread day-night changes in fluvial CO2 fluxes and that the time of day greatly influences measured CO2 fluxes across European streams.