Headwater Catchments Govern Biogeochemistry in America's Largest Free‐Flowing River Network

Abstract

Riverine chemistry reflects both watershed conditions and instream processing, both of which vary across river networks, yet little is known about the scales at which watershed attributes regulate biogeochemical constituents. We used spatial stream network (SSN) models to quantify both watershed and instream effects on streamwater constituents in the Kuskokwim River (western Alaska), the largest free-flowing river in the United States. We assessed chemical constituents spanning from labile nutrients (nitrate [NO 3 − ] and orthophosphate [PO 4 3− ]) to biologically and chemically conservative tracers (calcium [Ca 2+ ], strontium [Sr 2+ ], and Sr isotopes [ 87 Sr/ 86 Sr]). We also examined the behavior of dissolved organic carbon (DOC) relative to these constituents to understand whether bulk DOC behaved conservatively in a large boreal river network, where future changes in DOC and nutrient loading are expected under a shifting climate. We derived watershed spatial covariates comprising land cover, geology, and geomorphic characteristics at 14 different spatial extents based on stream order and relative to position in the river network to understand the effect of distant and proximal watershed attributes on streamwater constituents. For all constituents but 87 Sr/ 86 Sr, watershed attributes in low-order headwater catchments yielded the best predictive ability for streamwater constituent concentrations across the entire network. Spatial patterning for conservative tracers and bulk DOC showed strong spatial autocorrelation, whereas PO 4 3− and NO 3 − exhibited spatial patchiness indicative of instream processing. Our work shows that headwater streams are disproportionately important contributors to network-wide biogeochemical constituents supporting aquatic food webs and that conservation and management of aquatic systems should include these small and often remote watershed areas. Plain Language Summary Streamwater chemistry is influenced by watershed attributes like geology, gradient, and land cover and biochemical changes occurring within the stream channel, but how the relative importance of these factors changes as water moves through a river network is not known. We used a statistical model that accounts for water flows and the spatial layout of the river to study how stream chemistry is influenced by both watershed attributes and instream conditions at local to watershed-wide scales. We studied the Kuskokwim River in Alaska, America's largest free-flowing river, and found that conditions in small, headwater streams play a disproportionately important role in shaping streamwater chemistry throughout the river network. We also found that chemicals that are rapidly used by algae and microbes are spatially more variable in the river network when compared to passively transported chemicals that have lower biological demand. This work is important because streamwater chemical patterns can influence how different organisms are distributed in rivers, and certain watershed attributes will influence how changing clima...

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