The current assessments of the carbon turnover in the Arctic tundra are subject to large uncertainties. This problem can (inter alia) be ascribed to both the general shortage of ﬂux data from the vast and sparsely inhabited Arctic region, as well as the typically high spatiotemporal variability of carbon ﬂuxes in tundra ecosystems. Addressing these challenges, carbon dioxide ﬂuxes on an active ﬂood plain situated in the Siberian Lena River Delta were studied during two growing seasons with the eddy covariance method. The footprint exhibited a heterogeneous surface, which generated mixed ﬂux signals that could be partitioned in such a way that both respiratory loss and photosynthetic gain were obtained for each of two vegetation classes. This downscaling of the observed ﬂuxes revealed a differing seasonality in the net uptake of bushes (-0.89 µmol m^-2 s^-1) and sedges (-0.38 µmol m^-2 s^-1) in 2014. That discrepancy, which was concealed in the net signal, resulted from a comparatively warm spring in conjunction with an early snowmelt and a varying canopy structure. Thus, the representativeness of footprints may adversely be affected in response to prolonged unusual weather conditions. In 2015, when air temperatures on average corresponded to climatological means, both vegetation-class-speciﬁc ﬂux rates were of similar magnitude (-0.69 µmol m^-2 s^-1). A comprehensive set of measures (e.g. phenocam) corroborated the reliability of the partitioned ﬂuxes and hence conﬁrmed the utility of ﬂux decomposition for enhanced ﬂux data analysis. This scrutiny encompassed insights into both the phenological dynamic of individual vegetation classes and their respective functional ﬂux to ﬂux driver relationships with the aid of ecophysiologically interpretable parameters. For comparison with other sites, the decomposed ﬂuxes were employed in a vegetation class area-weighted upscaling that was based on a classiﬁed high-resolution orthomosaic of the ﬂood plain. In this way, robust budgets that take the heterogeneous surface characteristics into account were estimated. In relation to the average sink strength of various Arctic ﬂux sites, the ﬂood plain constitutes a distinctly stronger carbon dioxide sink. Roughly 42 % of this net uptake, however, was on average offset by methane emissions lowering the sink strength for greenhouse gases. With growing concern about rising greenhouse gas emissions in high-latitude regions, providing robust carbon budgets from tundra ecosystems is critical in view of accelerating permafrost thaw, which can impact the global climate for centuries.