Abstract
Riverine fluxes of carbon and inorganic nutrients are increasing in virtually all large permafrost-affected rivers, indicating major shifts in Arctic landscapes. However, it is currently difficult to identify what is causing these changes in nutrient processing and flux because most long-term records of Arctic river chemistry are from small, headwater catchments draining <200 km2 or from large rivers draining >100,000 km2. The interactions of nutrient sources and sinks across these scales are what ultimately control solute flux to the Arctic Ocean. In this context, we performed spatially-distributed sampling of 120 subcatchments nested within three Arctic watersheds spanning alpine, tundra, and glacial-lake landscapes in Alaska. We found that the dominant spatial scales controlling organic carbon and major nutrient concentrations was 3–30 km2, indicating a continuum of diffuse and discrete sourcing and processing dynamics. These patterns were consistent seasonally, suggesting that relatively fine-scale landscape patches drive solute generation in this region of the Arctic. These network-scale empirical frameworks could guide and benchmark future Earth system models seeking to represent lateral and longitudinal solute transport in rapidly changing Arctic landscapes.
Highlights
The fate of carbon and nutrients liberated from rapidly changing Arctic landscapes is a factor of critical concern, affecting both local habitat and global climate[1,2,3,4,5]
To explain how, where, and why changes in lateral solute flux are occurring in Arctic landscapes, we must identify the drivers of this signature, and quantify the patch size or spatial scale of sources and sinks of dissolved
Most measurements of Arctic river chemistry are from the outlets of large rivers (>100,000 km2), where observations cannot distinguish the relative importance of diffuse and discrete solute release mechanisms[47]
Summary
We assessed the spatial and temporal patterns in solute processing within each watershed using a synoptic sampling approach, which allowed us to quantify variance collapse, subcatchment leverage, and spatial stability (Fig. 2). Variance collapse thresholds for NO3− in the Lake and Alpine watersheds occurred at small to intermediate scales (3–15 km2), reflecting finer-grained heterogeneity of nutrient sources and sinks[46] Stated differently, both Lake and Alpine watersheds had high variability in DOC and NO3− concentrations in the smaller headwater catchments, with signals quickly reduced as surface-water networks mixed these small patches as a result of stream-lake interactions[61,62] or network topography[54,63]. The DOC and NO3− variance collapse scales confirm that the watersheds themselves are distinct, both in terms of the patch scale of apparent drivers that contribute to solute and carbon export, and landscape-driven network constraints Together, these results reveal the importance of intermediate landscape scales between 3–30 km[2] as regulators of Arctic carbon and nutrient sources and sinks, and the utility of synoptic campaigns for identifying emergent watershed patterns. Phosphorus is highly limiting in stream and lake ecosystems on the North Slope (64–66), meaning that phosphorus concentration at a particular moment in time in a stream network could be primarily a consequence of immobilization and mineralization in the aquatic environment[67]
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