Deciphering anthropogenic nutrient impacts on aquatic eutrophication through coupled nitrate isotope tracers and...

Excess nutrient inputs have emerged as a critical environmental concern due to their direct role in triggering algal blooms and accelerating the eutrophication of water ecosystems. Here, the hydrogeochemical fingerprints associated with the nitrate (NO₃^-) isotope abundances of δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^- were used to clarify the eutrophic level and pollution source of the typical macrophyte-dominated and algal-turbid lake areas in the plateau. Our results show high concentrations of nitrogen (N), phosphorus (P), and organic matter, dominated by the dissolved N forms and particle P. The higher N-to-P ratio promoted submerged plant growth. However, a significant contribution of dissolved N and particle P to the nutrient load of water in the algal-turbid lake region. Inorganic N prevailed in the N content, with a predominance of NO₃^--N over ammonium nitrogen (NH₄⁺-N) in the overlying water. Organic N contributed to the N content in the pore water. Both NO₃^--N and NH₄⁺-N were diffused to pore water in the macrophyte-dominated clear-water lake region, while NO₃^--N was released in the algal-turbid region. The ion ratio and NO₃^- isotopes revealed that sewage/manure discharge and chemical N fertilizer contributed to water pollution. The soil N loss exacerbated NO₃^--N loading. Rainfall and fertilization have led to fluctuations in the N and P loads of water bodies, and these variations exist regionally, which are driven by land use patterns and the intensity of agricultural activities. This resulted in the shift of nutrient restriction for algae growth from N to P over the past decades. This study applied hydrochemical fingerprints with isotopes to reveal water nutrient migration and trace anthropogenic effects, which is valuable for the control of the N and P cascade and the restoration of the aquatic environment.