title: Agriculture causes nitrate fertilization of remote alpine lakes.
abstract: Humans have altered Earth's nitrogen cycle so dramatically that reactive nitrogen (Nr) has doubled. This has increased Nr in aquatic ecosystems, which can lead to reduced water quality and ecosystem health. Apportioning sources of Nr to specific ecosystems, however, continues to be challenging, despite this knowledge being critical for mitigation and protection of water resources. Here we use Δ(17)O, δ(18)O and δ(15)N from Uinta Mountain (Utah, USA) snow, inflow and lake nitrate in combination with a Bayesian-based stable isotope mixing model, to show that at least 70% of nitrates in aquatic systems are anthropogenic and arrive via the atmosphere. Moreover, agricultural activities, specifically nitrate- and ammonium-based fertilizer use, are contributing most (∼60%) Nr, and data from other North American alpine lakes suggest this is a widespread phenomenon. Our findings offer a pathway towards more effective mitigation, but point to challenges in balancing food production with protection of important water resources.
authors: Department of Geography, The University of Western Ontario, Social Science Centre Room 2322, 1151 Richmond Street, London, Ontario, Canada N6A 5C2.
authors: E J
authors: EJ
authors: Hundey
authors: Department of Earth Sciences, The University of Western Ontario, Biological and Geological Sciences Building, 1151 Richmond Street, London, Ontario, Canada N6A 5B7.
authors: S D
authors: SD
authors: Russell
authors: Department of Earth Sciences, The University of Western Ontario, Biological and Geological Sciences Building, 1151 Richmond Street, London, Ontario, Canada N6A 5B7.
authors: F J
authors: FJ
authors: Longstaffe
authors: Department of Geography, The University of Western Ontario, Social Science Centre Room 2322, 1151 Richmond Street, London, Ontario, Canada N6A 5C2.
authors: K A
authors: KA
authors: Moser