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.