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This study used a long-term monitoring data to analyse the abundance, seasonal dynamics, spatial distribution and catchment export of total organic carbon (TOC) in one of the largest undisturbed peatland river catchment in Central Europe. Mean annual TOC concentrations in the Narew River basin were higher than 10-11 mg C L-1 which is similar to nemoral or Baltic rivers. The maximal monthly TOC concentrations occurred in May with a one-month delay from the spring maximum of water outflow. High TOC concentrations were accompanied by naturally increased organic nitrogen and phosphorous concentrations resulting in decreased water quality class despite the anthropogenic pressure is low. Mean annual export of TOC from the Narew River catchment amounted to 2.15 g C m(-2), which is nearly 25% higher than for the Vistula River, the recipient. TOC export form the analysed river basin in dry years was 30% lower and 50% higher in wet years than the multiannual average. The primary cause of high TOC concentrations in the Narew River catchment and high TOC export is the occurrence of biogeochemically active wetlands, functioning not only thanks to spring flooding of the valleys, but also based on permanent intensive groundwaters supply from surrounding plateaus. This emphasises the importance of basin hydrology in the regulation of carbon cycling in the temperate lowland catchments. Our results also indicate that in environmental zonation of Europe Narew River catchment should be classified as belonging to the Baltic Province (Nemoral environmental zone) rather then to Continental zone. This is because it is characterized with high riverine TOC concentrations and natural water oxygen depletion in summers. Thus, Environmental Stratification of Europe developed for environmental statistics and monitoring should be re-evaluated. Climate change is likely to affect the organic carbon circulation in peatland catchments and reduce TOC fluxes into river network. (C) 2018 European Regional Centre for Ecohydrology of the Polish Academy of Sciences. Published by Elsevier B.V. All rights reserved.