Decadal Changes in Anthropogenic Inputs and Precipitation Influence Riverine Exports of Carbon, Nitrogen, and Phosphorus, and Alter Ecosystem Level Stoichiometry

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 in Quebec, Canada, 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.