Natural and human features on land result in differential loadings of carbon (C), nitrogen (N), and phosphorus (P) to rivers that influence within ecosystem processing. However, little is known about how land use, together with seasonal changes in climate and hydrology, influence the relative proportions of C, N, and P in rivers. To evaluate the spatial and temporal patterns in ecosystem-level C: N: P stoichiometry, we sampled 13 sites once per season for 3 yr along the main stem of a north temperate river with winter ice-cover that flows across a gradient of forested, urban, and agricultural landscapes. We found that C concentrations were rather stable along the continuum, whereas N and P rapidly increased downstream due to urban and agricultural land uses. The flow-weighted C: N: P ecosystem stoichiometry ranged from 2319: 119: 1 in the most upstream site to 368: 60: 1 at the outlet. The dominant form of N generally shifted from dissolved organic nitrogen in upstream forested reaches to nitrate in more impacted, downstream reaches, and winter stoichiometry was enriched in inorganic N and dissolved P forms. Concentrations of all three elements were generally lower in spring during year-high flow due to dilution. The spatial and temporal variation in stoichiometry in this north temperate river covered much of the range previously observed between litter ratios and the Redfield ratio. This suggests that even moderate human impacts can have profound effects on riverine ecosystem stoichiometry, and that these effects are modulated by seasonal trends in temperature and hydrology.

AcknowledgementsWe wish to thank L. Galantini for continued fieldwork and analysis support, R. LaBrie for help with PARAFAC modelling, and M. Talluto for help with the Bayesian model. We also thank M. Botrel for her curiosity and expertise in data visualisation and D. Bélanger for laboratory analyses support. Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery grants to J.-F.L. and R.M. funded this work, as well as partial student support to S.S. via her NSERC collaborative Research and Training Experience Program (CREATE) EcoLac scholarship. We carried out the work for this article on the traditional lands of the Kanien'kehà:ka (Mohawk), Omàmiwinini (Algonquin) and Anishinabewaki.

Changes in precipitation and land use influence carbon (C), nitrogen (N) and phosphorus (P) exports from land to receiving waters. However, how these drivers differentially alter elemental inputs and impact subsequent ecosystem stoichiometry over time remains poorly understood. Here we quantified long-term (1979-2020) trends in C, N, and P exports at three sites along the mainstem of a north temperate river, that successively drains forested, urban, and more agriculturally impacted land-use areas. Riverine N and to a lesser degree C exports tended to increase over time, with major inter-annual variation largely resolved by changes in precipitation. Historical increases in net anthropogenic N inputs on land (NANI) also explained increases in riverine N exports, with about 35% of NANI reaching the river annually. Despite higher Net anthropogenic P inputs, NAPI, over time, P exports tended to decrease at all riverine sites. This decrease in P at the forested site was more gradual, whereas a precipitous drop was observed at the downstream urban site, following legislated P removal in municipal wastewater. Changes in historical ecosystem stoichiometry reflected the differential elemental exports due to natural and anthropogenic drivers and ranged from 174: 23: 1 to 547: 76: 1 over the years. Our work shows how C, N, and P have responded to different drivers in the same catchment over the last four decades, and how their differential riverine exports have influenced ecosystem stoichiometry.

Different sources and fates control riverine dissolved organic matter (DOM) composition in catchments of contrasting land use, and climate. However, assessing the changes in DOM composition together with nutrient forms along rivers exposed to these gradients remains rare. Here we quantified the spatial and temporal patterns in DOM components and nutrient forms along the mainstem of a 5th order river through sequential forested, urban, and agricultural reaches during low flow moments in summer and winter, and two contrasting springs, one with a historically rare flooding event. There were widespread abrupt shifts in the composition of DOM in low flow seasons that coincided with changes in land use whose sources could be inferred by endmember samples and nutrient changes. Compared to summer, DOM pools considered bio- (microbial-like) and photo-labile (Peak C) tended to dominate during the winter, along with reactive nutrient forms (ammonium, dissolved phosphorus). This implied higher processing potentials during summer, where microbial-humic-like DOM and nitrate dominated, and accumulation of reactive forms during winter. DOM composition remained relatively stable under typical flow conditions with reduced retention time and processing, but major shifts were observed during an extreme flood year, pointing to unusual loadings of highly labile sources of DOM. Overall we found that, despite relatively small changes in the quantity of DOM flowing in this north temperate river, there were major spatial and temporal shifts in its composition and associated nutrients that reflected contrasting loading and processing potentials depending on land use and seasonal patterns in temperature and hydrology.