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

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. delta N-15 values of DIN in tributary water differed seasonally and were greatest during snowmelt Incorporation of ammonium delta N-15 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 delta N-15 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 delta N-15 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. Crown Copyright (C) 2020 Published by Elsevier B.V. All rights reserved.

Automated summary

The study found that intensive agriculture and growing human populations contribute to nitrogen sources in streams, with seasonal variations in nitrogen delivery. Nitrogen sources from fertilizers and animal waste were more prominent during spring and summer, while human and livestock waste were the dominant contributors to algae. The study also highlighted the importance of managing nitrogen sources to control downstream nitrogen transfer and protect stream function.