Significance: Elevated levels of mercury in Arctic marine wildlife have been linked to midlatitude anthropogenic mercury emissions which are transported to the Arctic Ocean by air. Modeling studies, however, suggest that Arctic rivers contribute equal amounts of mercury to the Arctic Ocean. In this study, we provide comprehensive mercury data on large Eurasian rivers. We find that the spring flood mercury flux from Eurasian rivers is indeed large, which confirms a new Arctic mercury cycling paradigm: Mid-latitude anthropogenic emissions reach the terrestrial Arctic by air, whereby vegetation uptake transfers atmospheric mercury to tundra and boreal peat soils. Springtime snowmelt subsequently mobilizes peat soil mercury to the Arctic Ocean, where photochemistry drives net export of mercury back to the atmosphere.

Mountains contain many small and fragmented peatlands within watersheds. As they are difficult to monitor, their role in the water and carbon cycle is often disregarded. This study aims to assess the stream organic carbon exports from a montane peatland and characterizes its contribution to the water chemistry in a headstream watershed. High frequency in situ monitoring of turbidity and fDOM were used to quantify respectively particulate organic carbon (POC) and dissolved organic carbon (DOC) exports at the inlet and outlet of a peatland over three years in a French Pyrenean watershed (1,343 m.a.s.l.). The DOC and POC signals are both highly dynamic, characterized by numerous short peaks lasting from a few hours to a few days. Forty‐six percent of the exports occurred during 9% of the time corresponding to the highest flows monitored at the outlet. Despite its small area (3%) within the watershed, the peatland contributes at least 63% of the DOC export at the outlet. The specific DOC flux ranges from 16.1 ± 0.4 to 34.6 ± 1.5 g m²year⁻¹. POC contributes 17% of the total stream organic carbon exports from the watershed. As the frequency of extreme climatic events is expected to increase in the context of climate change, further studies should be conducted to understand the evolution of underestimated mountainous peatland carbon fluxes and their implication in the carbon cycle of headwaters.

In Arctic regions, a major concern is the release of carbon from melting permafrost that could greatly exceed current human carbon emissions. Arctic rivers drain these organic-rich watersheds (Ob, Lena, Yenisei, Mackenzie, Yukon) but field measurements at the outlets of these great Arctic rivers are constrained by limited accessibility of sampling sites. In particular, the highest dissolved organic carbon (DOC) fluxes are observed throughout the ice breakup period that occurs over a short two to three-week period in late May or early June during the snowmelt-generated peak flow. The colored fraction of dissolved organic carbon (DOC) which absorbs UV and visible light is designed as chromophoric dissolved organic matter (CDOM). It is highly correlated to DOC in large arctic rivers and streams, allowing for remote sensing to monitor DOC concentrations from satellite imagery. High temporal and spatial resolutions remote sensing tools are highly relevant for the study of DOC fluxes in a large Arctic river. The high temporal resolution allows for correctly assessing this highly dynamic process, especially the spring freshet event (a few weeks in May). The high spatial resolution allows for assessing the spatial variability within the stream and quantifying DOC transfer during the ice break period when the access to the river is almost impossible. In this study, we develop a CDOM retrieval algorithm at a high spatial and a high temporal resolution in the Yenisei River. We used extensive DOC and DOM spectral absorbance datasets from 2014 and 2015. Twelve SPOT5 (Take5) and Landsat 8 (OLI) images from 2014 and 2015 were examined for this investigation. Relationships between CDOM and spectral variables were explored using linear models (LM). Results demonstrated the capacity of a CDOM algorithm retrieval to monitor DOC fluxes in the Yenisei River during a whole open water season with a special focus on the peak flow period. Overall, future Sentinel2/Landsat8 synergies are promising to monitor DOC fluxes in Arctic rivers and advance our understanding of the Earth’s carbon cycle.