Isotopic characterization of nitrate sources and transformations in Lake Winnipeg and its contributing rivers, Manitoba, Canada

Lake Winnipeg (Manitoba, Canada) is in a eutrophic state from a century of increased riverine loadings from agricultural and urban nitrogen (N) and phosphorus (P) sources. This study investigated seasonal patterns of the isotopic composition of nitrate (NO3−) in Lake Winnipeg and its contributing rivers to gain insight into current N nutrient sources and in-lake N dynamics. Elevated NO3− concentrations in Lake Winnipeg tributaries between 0.36 and 2.44 mg/L NO3−-N were associated with high δ15N values between + 5.0 and + 13.9‰, while δ18ONO3 values were <+ 15.0‰. The three major riverine inputs had distinctive mean δ15NNO3 values of + 8.1‰ for the Red River, -0.6‰ for the Winnipeg River, and + 5.0‰ for the Saskatchewan River. The isotopic composition of NO3− in Lake Winnipeg was partly controlled by the isotopic composition of the riverine nitrate for instance via the predominant nitrate input to the South basin from the Red River. Nitrate assimilation and late season mineralization of phytoplankton and N2 fixing cyanobacteria were identified as important additional processes affecting the isotopic composition of lake NO3− resulting in low δ15NNO3 values, especially in the North basin. In the South basin, elevated δ15NNO3 values in spring that changed to lower values by summer indicated a dynamic N cycle within the lake. Agreement between δ15N values of lake NO3−, PON and fish suggests that dissolved nitrate partially affects the flow of nitrogen in the aquatic food webs of Lake Winnipeg.