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Northern rivers connect a land area of approximately 20.5 million km 2 to the Arctic Ocean and surrounding seas. These rivers account for~10% of global river discharge and transport massive quantities of dissolved and particulate materials that reflect watershed sources and impact biogeochemical cycling in the ocean. In this paper, multiyear data sets from a coordinated sampling program are used to characterize particulate organic carbon (POC) and particulate nitrogen (PN) export from the six largest rivers within the pan-Arctic watershed (Yenisey, Lena, Ob', Mackenzie, Yukon, Kolyma). Together, these rivers export an average of 3055 × 10 9 g of POC and 368 × 10 9 g of PN each year. Scaled up to the pan-Arctic watershed as a whole, fluvial export estimates increase to 5767 × 10 9 g and 695 × 10 9 g of POC and PN per year, respectively.POC export is substantially lower than dissolved organic carbon export by these rivers, whereas PN export is roughly equal to dissolved nitrogen export. Seasonal patterns in concentrations and source/composition indicators (C:N, δ 13 C, Δ 14 C, δ 15 N) are broadly similar among rivers, but distinct regional differences are also evident. For example, average radiocarbon ages of POC range from~2000 (Ob') to~5500 (Mackenzie) years before present. Rapid changes within the Arctic system as a consequence of global warming make it challenging to establish a contemporary baseline of fluvial export, but the results presented in this paper capture variability and quantify average conditions for nearly a decade at the beginning of the 21st century.

As surface temperatures are expected to rise in the future, ice-rich permafrost may thaw, altering soil topography and hydrology and creating a mosaic of wet and dry soil surfaces in the Arctic. Arctic wetlands are large sources of CH , and investigating effects of soil hydrology on CH fluxes is of great importance for predicting ecosystem feedback in response to climate change. In this study, we investigate how a decade-long drying manipulation on an Arctic floodplain influences CH -associated microorganisms, soil thermal regimes, and plant communities. Moreover, we examine how these drainage-induced changes may then modify CH fluxes in the growing and nongrowing seasons. This study shows that drainage substantially lowered the abundance of methanogens along with methanotrophic bacteria, which may have reduced CH cycling. Soil temperatures of the drained areas were lower in deep, anoxic soil layers (below 30 cm), but higher in oxic topsoil layers (0-15 cm) compared to the control wet areas. This pattern of soil temperatures may have reduced the rates of methanogenesis while elevating those of CH oxidation, thereby decreasing net CH fluxes. The abundance of Eriophorum angustifolium, an aerenchymatous plant species, diminished significantly in the drained areas. Due to this decrease, a higher fraction of CH was alternatively emitted to the atmosphere by diffusion, possibly increasing the potential for CH oxidation and leading to a decrease in net CH fluxes compared to a control site. Drainage lowered CH fluxes by a factor of 20 during the growing season, with postdrainage changes in microbial communities, soil temperatures, and plant communities also contributing to this reduction. In contrast, we observed CH emissions increased by 10% in the drained areas during the nongrowing season, although this difference was insignificant given the small magnitudes of fluxes. This study showed that long-term drainage considerably reduced CH fluxes through modified ecosystem properties.

Land−ocean linkages are strong across the circumpolar north, where the Arctic Ocean accounts for 1% of the global ocean volume and receives more than 10% of the global river discharge. Yet estimates of Arctic riverine mercury (Hg) export constrained from direct Hg measurements remain sparse. Here, we report results from a coordinated, year-round sampling program that focused on the six major Arctic rivers to establish a contemporary (2012−2017) benchmark of riverine Hg export. We determine that the six major Arctic rivers exported an average of 20 000 kg y −1 of total Hg (THg, all forms of Hg). Upscaled to the pan-Arctic, we estimate THg flux of 37 000 kg y −1 . More than 90% of THg flux occurred during peak river discharge in spring and summer. Normalizing fluxes to watershed area (yield) reveals higher THg yields in regions where greater denudation likely enhances Hg mobilization. River discharge, suspended sediment, and dissolved organic carbon predicted THg concentration with moderate fidelity, while suspended sediment and water yields predicted THg yield with high fidelity. These findings establish a benchmark in the face of rapid Arctic warming and an intensifying hydrologic cycle, which will likely accelerate Hg cycling in tandem with changing inputs from thawing permafrost and industrial activity.

G lobal climate change is visibly and tangibly manifested through the Arctic cryospheric system: sea ice loss, earlier spring snowmelts, thawing permafrost, retreating glaciers, and coastal erosion. While not as visibly manifest, the role of the atmosphere is also a critical component in determining the trajectory of the Arctic system. The atmosphere not only drives change, but is reciprocally being modified through a complex web of feedbacks, and is the fast-track mechanism for the transport of energy and moisture through the global system that links climate and weather. For decades, it has been recognized that fundamental components of the atmospheric system such as clouds, atmospheric trace gases, aerosols, and atmosphere-surface exchange processes compose some of the major uncertainties that limit the diagnostic or predictive skill of coupled atmosphere-ice-ocean-terrestrial models (IPCC 2013, chapter 9). Arctic nations have responded in recent decades by establishing  A micrometeorological tower in Tiksi, Russia is used to determine the atmospheric-surface energy balance. (Photo credit: Vasily Kustov)