Multi-isotope tracing nitrate dynamics and sources during thermal stratification in a deep reservoir

Excessive nitrate (NO₃^-) input to reservoirs is a global concern. However, the dynamics and sources of NO₃^- under thermal stratification in deep reservoirs were rarely explored. In this study, multi-stable isotopes (δ¹⁵N/δ¹⁸O-NO₃^-, δ¹⁵N-particulate nitrogen (PN), δ¹⁵N-dissolved total nitrogen (DTN), and δ²H/δ¹⁸O-H₂O) and a Bayesian mixing model were applied to reveal the biogeochemical processes and sources of NO₃^- in a deep reservoir with obvious nitrogen pollution. The results showed that the reservoir was thermally stratified in July while vertically mixed in October. The distribution of δ²H-H₂O suggested that riverine nitrogen migrated to the epilimnion and metalimnion during stratification in the reservoir. In the epilimnion and metalimnion, the significant reduction in NO₃^- concentration was related to the enhancement of assimilation by thermal stratification. Meanwhile, the positive linear correlations between δ¹⁸O-NO₃^- and δ¹⁸O-H₂O suggested that in-reservoir nitrification occurred, with its depth confined above the hypolimnion. In the hypolimnion, denitrification processes were absent due to the aerobic environment. Overall, NO₃^- dynamics were mainly controlled by nitrogen inflow, in-reservoir nitrification, and assimilation during thermal stratification. The results of the Bayesian mixing model showed that manure and sewage, and soil nitrogen were the dominant NO₃^- sources of the reservoir. This study provides new insights and data to help manage and restore deep waters worldwide in tackling a similar situation of nitrogen contamination.