Table 1 Results of the linear mixed-effect models (LME).

Response variable	Fixed effect	χ² (1)		p	Sign
(A) Testing spatial and temporal hypotheses
CO₂ flux difference from day to night	Latitude	7.4207		0.006	−
	Water temperature (day)	0.0168		0.897
	Water temperature (day)*latitude	4.9594		0.026	+
(B) Testing physical and biogeochemical drivers of CO₂ flux changes
CO₂ flux difference from day to night	Δ Water pCO₂	4.9497	0.026		+
CO₂ flux difference from day to night	Δ Gas transfer velocity k	0.5613	0.454
(C) Testing biogeochemical drivers of pCO₂ changes
pCO₂ difference from day to night	Δ Water O₂ concentration	7.9879		0.005	−
	ΔpH	0.0345		0.853
	Δ Conductivity	0.0293		0.864
	ΔT_w−T_a ^a (proxy for heat flux)	1.6720		0.196
	ΔWater temperature	0.8731		0.350
(D) Testing catchment and hydromorphological drivers of pCO₂ changes
pCO₂ difference from day to night	Day length	1.7244		0.189
	Stream wetted width	0.3748		0.540
	Discharge	3.4458		0.063
	% forest	0.0950		0.758
	Catchment area	2.3656		0.124

^aHeat flux calculated as water temperature (T_w) minus air temperature (T_a).
(A) Marginal R² = 0.12, conditional R² = 0.18, sample size = 107.
(B) Marginal R² = 0.08, conditional R² = 0.10, sample size = 77.
(C) Marginal R² = 0.13, conditional R² = 0.33, sample size = 78.
(D) Marginal R² = 0.11, conditional R² = 0.13, sample size = 68.
The effects of latitude and water temperature during the day (A) and the effect of day-to-night differences of pCO₂ and the gas transfer velocity (Δ = night minus day values) (B) on the day-to-night difference of CO₂ fluxes were tested. Furthermore, the effect of day-to-night differences of physical and biogeochemical parameters (C) and the effect of catchment and hydromorphological related parameters (D) on the day-to-night differences of pCO₂ were evaluated. Stream ID was included as a random effect on the intercept. Significances of fixed effects were assessed with likelihood ratio tests with degrees of freedom = 1. The slope direction (sign) of the effect is indicated with – or + when significant. Significant p values < 0.05 are in bold.

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