Terrestrial and Aquatic Controls on the Movement of Material Transported Hydrologically Across Vast Arctic River Basins

Substantial increases in solute fluxes (e.g., dissolved organic matter [DOM] and nutrients) have been observed in most major Arctic river networks, but it is unclear why. One hypothesis suggests these patterns are due to in-stream microbial and photochemical processing of DOM, though residence times of soil and stream water are not long enough (given observed biogeochemical rates) to account for all processing that must occur for this to be the only explanation. An alternative hypothesis suggests the landscape and soil water, rather than only the aquatic environments of the mainstem and tributaries, are controlling the processing. Said otherwise, systematic differences in vegetation, soil microbial community, and hydrologic residence time create longitudinal patterns in the source and reactivity of DOM and nutrients. To test the above hypotheses, we employed a novel longitudinal study design over a large spatial area, developing hydrologic, chemical, and microbial profiles in multiple basins of similar size from the headwaters to the coast on the North Slope of Alaska. We collected a range of geogenic, biogenic, and exogenous/novel material in soil and water samples in different hydrologic features (e.g., soil water track, tributary, and mainstem) at different landscape positions (e.g., alpine, foothills, and coastal plain) across a longitudinal gradient of ~300 km and an elevation gradient of ~1400 m. Initial results show that profiles of nutrient availability and primary productivity are distinct between the landscape (water track) and aquatic environments (tributary and mainstem), while also showing additional patterns in the longitudinal direction from mountains to sea. Together, these results suggest that both of the above hypotheses are important. Furthermore, our study also provides some of the first measurements that can relate landscape position to the accumulation of human-introduced exogenous matter, such as mercury and microplastics. Thus, our study improves conceptual understanding of material flow in the cryosphere at a critical time when the fate and transport of this material may impact Arctic and global ecosystems in complex ways, from degrading pristine terrestrial and aquatic environments to releasing greenhouse gases to the atmosphere.