Abstract: Stream particles, or seston, represent an understudied flux of carbon (C) and nitrogen (N) in river networks despite their important role as connectors of terrestrial-aquatic and upstream-downstream processes. Here, we summarize stream seston C:N stoichiometry from 20 wadeable streams and 3 rivers sampled across the US by the National Ecological Observatory Network (NEON) from 2014-2022. NEON has sampled seston seasonally three times per year since 2014, and biweekly beginning in 2021. Although seston C and N content each spanned several orders of magnitude respectively, we found stream seston C:N was remarkably consistent across space and time. Mean stream seston C:N (by moles) was 13:1, ~2-fold higher than Redfield C:N, and the interquartile range was 9:1-16:1. Standardized major axis fits corroborated tight coupling between seston C and N, with a slope of 10.2 (95% CI = 9.8-10.6) and intercept through the origin (i.e., not different from zero), suggesting simultaneous depletion of these elements from stream seston. Greater than 92% of the variation in seston C was explained by seston N across the 20 stream sites. While stream seston C:N was relatively consistent among sites, mean site-level seston C:N decreased as a function of watershed size, suggesting either different inputs or differential C vs N processing along river continua from headwaters to coasts. For example, C:N stoichiometry of the seston from three NEON river sites (Flint River [Georgia, USA], Black Warrior River [Alabama, USA], Lower Tombigbee River [Alabama, USA]) were 8.6, 6.6, and 7.3, respectively, which were ~50% lower than the mean C:N from the 20 wadeable stream sites. Taken together, these preliminary patterns highlight that seston transported within streams has consistent C:N stoichiometry, regardless of the macroecological context of the surrounding watershed. However, upstream-to-downstream processes may be an exception, suggesting either that seston C and N are actively processed as they move downstream, or that inputs change in quality longitudinally. We also anticipate that seston C:N behaves differently than seston C:P, which is likely more strongly associated with geologic context and mineral soil transport to streams.
