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Abstract Suspended particulate matter, or seston, represents an understudied flux of carbon (C), nitrogen (N), and phosphorus (P) in river networks. Here, we summarize riverine seston C : N : P stoichiometry data from 27 streams and rivers sampled regularly from 2014 to 2022 across the United States by the National Ecological Observatory Network (NEON). We examine relationships among seston C, N, and P content using standardized major‐axis (SMA) and ordinary least squares slopes to test congruence with a constant‐ratio model (scaling coefficient = 1), and hierarchical models to identify watershed‐level covariates of seston C : nutrient stoichiometric allometry. At the continental scale, C and N were tightly coupled and conformed to the constant‐ratio model, while seston C : P and N : P indicated weaker coupling and inconstant ratios across the range of C vs. P and N vs. P values. At the stream‐site scale, C : N, C : P, and N : P often exhibited slopes < 1, indicating that within individual streams seston becomes more nutrient‐rich as seston concentration increases. Watershed forest cover, season, and discharge helped explain stoichiometric allometry across streams, where forested sites in wetter climates had lower scaling slopes, and slopes decreased with low flows. Our study underscores the importance of suspended particles as a material flux in river networks and highlights the interplay between biotic and abiotic factors that drive the relative consistency of its C : nutrient stoichiometry during transport from local to continental scales. 

Abstract While inland freshwater networks cover less than 4% of the Earth's terrestrial surface, these ecosystems play a disproportionately large role in the global cycles of [C]arbon, [N]itrogen, and [P]hosphorus, making streams and rivers critical regulators of nutrient balance at regional and continental scales. Foundational studies have established the relative importance of the hydrologic regime, land cover, and instream removal processes for controlling the transport and processing of C, N, and P in river networks. However, particulate C, N, and P can make up a large proportion of the total material in large rivers and during high flows. To constrain the patterns of the biogeochemistry of riverine particulates, we characterized and modeled dissolved and particulate concentration variability at the continental scale using open‐access data from 27 National Ecological Observatory Network (NEON) sites across the United States. We analyzed these data using Boosted Regression Trees (BRTs) to statistically identify if land cover characteristics could predict nutrient quantity and quality of stream particulates. The BRT models revealed that land cover does not strongly predict particulate dynamics across NEON sites but indicate that instream processes might be more important than catchment characteristics alone. In addition, our study demonstrates the consistent importance of particulates relative to dissolved forms, highlighting their likely significance for biogeochemical processes along the freshwater continuum.