Large rivers integrate processes occurring throughout their watersheds, and are therefore sentinels of change across broad spatial scales. Riverine chemistry also regulates ecosystem function across Earth's land-ocean continuum, exerting control from the micro-(e.g., food web) to the macro-(e.g., carbon cycle) scale. In the rapidly warming Arctic, a wide range of processes have been hypothesized to alter river water chemistry. However, it is unknown how the land-ocean flux of waterborne constituents is changing at the pan-Arctic scale. Here, we show profound shifts in the concentration and transport of biogeochemical constituents in the six largest Arctic rivers (the Ob', Yenisey, Lena, Kolyma, Yukon, and Mackenzie) since 2003, near river mouths capturing two-thirds of the pan-Arctic watershed. While some constituent fluxes increase substantially at the pan-Arctic scale (alkalinity and associated ions), others decline (nitrate and associated inorganic nutrients) or are overall unchanged (dissolved organics). These clear but divergent trends diagnose a multi-systems perturbation that indicates alteration of processes ranging from chemical weathering on land, to primary production in the coastal ocean. We anticipate these findings will refine models of current and future functioning of the coupled land-ocean Arctic system, and spur research on scale-dependent change across the riverintegrated Arctic domain.
MainLarge rivers are planetary linchpins, connecting vast swaths of terrestrial landmass to the world's coastal oceans. On land, rivers integrate patchy landscapes and the variable biogeochemical processes that these landscapes host, as water moving through watersheds incorporates the chemical signature of its flow path. In the coastal ocean, the chemical signature of water transported by rivers regulates nearshore biogeochemical 1,2 and ecological 3,4 function; over broader scales, river water and its composition modify ocean physics 1 . Nowhere is this more consequential than in the Arctic, where ~11% of Earth's riverine discharge drains into an enclosed basin containing ~1% of global ocean volume 5 . This drainage occurs predominantly via six large rivers (Figure 1, Extended Data Table 1). As a result, quantifying trends in river water chemistry at a constrained series of downstream sites allows us to diagnose change across much of the pan-Arctic watershed, better understand the current functioning of the connected land-ocean Arctic system, and predict what the future may hold for this rapidly changing region 6 .