Contribution of nitrogen sources to streams in mixed-use catchments varies seasonally in a cold temperate region

• Published in: The Science of the total environment 2021-04, Vol.764, p.142824, Article 142824
• Main Authors: Painter, Kristin J., Brua, Robert B., Koehler, Geoff, Spoelstra, John, Yates, Adam G.
• Format: Article
• Language: English
• Subjects: Artificial sweeteners; Canada; Environmental Monitoring; Humans; Land use; Manitoba; Nitrates - analysis; Nitrogen; Nitrogen - analysis; Nitrogen Isotopes - analysis; Stable isotopes; Stream ecology; Water Pollutants, Chemical - analysis
• ISSN: 0048-9697; 1879-1026
• DOI: 10.1016/j.scitotenv.2020.142824

Intensive agriculture and growing human populations are important nitrogen (N) sources thought to be associated with eutrophication. However, the contribution and seasonality of N delivery to streams from human activities is poorly understood and knowledge of the role of stream communities in the assimilation of N from human activities is limited. We used N and oxygen stable isotope ratios of dissolved inorganic N (DIN) and concentrations of artificial sweeteners to identify the relative contribution of key sources of anthropogenic N (i.e., fertilizers, human, and livestock waste) to tributaries of the Red River Valley (RRV), Manitoba, Canada. Water and algae were sampled in 14 RRV tributaries during snowmelt, spring, summer, and autumn; and water was sampled at three locations in the Red River in spring, summer, and autumn. δ15N values of DIN in tributary water differed seasonally and were greatest during snowmelt. Incorporation of ammonium δ15N provided evidence for the importance of manure N to tributaries during snowmelt. Fertilizer and municipal lagoons served as principal sources of N to streams in spring and summer. Human and livestock waste sources of N were the dominant contributor to algae at greater than 90% of sites and algae δ15N was greatest at sites downstream of municipal lagoons. We also showed that the tributaries contribute human and livestock waste N to the Red River, though much of the nitrate in the river originates outside of Manitoba. Overall, our study determined that the anthropogenic sources of N to RRV streams vary seasonally, likely due to regional hydrologic conditions. Our study also showed the potential of artificial sweeteners and ammonium δ15N as tools for identifying N sources to rivers. Moreover, we demonstrate the need for the management of N sources and the protection of stream function to control downstream transfer of N from landscapes to waterbodies. [Display omitted] •Contribution and timing of delivery of nitrogen to streams is poorly understood.•N sources identified using stable isotopes varied seasonally.•Use of artificial sweeteners aided differentiation of human and livestock waste.•Livestock N was dominant in snowmelt and sewage lagoons were important in summer.•Evidence of transport of waste sources of N from tributaries to the Red River