Fig. 1: Day-to-night changes of CO₂ fluxes at the water–air interface of the sampled European streams.

Stream CO₂ fluxes (in mmol CO₂ m⁻² h⁻¹) at day-time (yellow) and night-time (blue) (a) and the calculated changes from night minus day (ΔCO₂ flux) (b) for all data and separately for each sampling period. In the sampling periods comparisons in (a), CO₂ fluxes for individual stream sites are indicated by red (day) and light blue (night) dots. The boxplots visualize the median of all stream sites (line), the first and third quartiles (hinges), the 1.5*inter-quartile ranges (whiskers), and the outliers outside the range of 1.5*inter-quartile ranges (black dots). On top of (a) are p values retrieved from paired comparisons of median CO₂ fluxes tested by Wilcoxon signed-rank tests and the sample size (n). Significant p values with p < 0.05 are in bold with an asterisk. The differences in the CO₂ fluxes (b) in mmol CO₂ m⁻² h⁻¹ from day to night are for October: 0.5 [0.1, 1.2]; January: 0.5 [0.3, 0.9]; April: 1.1 [0.1, 2.3]; July: 0.3 [−0.2, 1.1] (median [IQR]).