Carbon-nitrogen coupling in plateau lake ecosystems: Dual-isotope fingerprints of climate warming and anthropogenic...

As primary producers in lake ecosystems, phytoplankton mediate aquatic carbon and nitrogen cycling, with stable isotope signatures archived in their biomass‌ serving as sensitive tracers of climatic fluctuations and anthropogenic impacts on biogeochemical processes. Through comprehensive analysis of phytoplankton carbon and nitrogen isotope signatures across nine Yunnan plateau lakes (2017-2022), we reveal divergent regulatory mechanisms under coupled environmental stressors. The isotopic discrimination of phytoplankton carbon isotope, driven by both growth rate and inorganic carbon pool changes, exhibits temperature sensitivity (+0.43 ‰/ °C), showing monsoon-modulated seasonal patterns where warming enhances ¹³C enrichment in lacustrine organic matter, potentially reshaping ‌elemental carbon partitioning under climate change. Phytoplankton nitrogen isotope signatures reveal nonlinear responses to watershed urbanization (population density, built-up area), with isotopic deviations from watershed soil nitrogen isotope baselines tracking anthropogenic nitrogen sources (e.g., ¹⁵N-depleted agricultural fertilizers vs. ¹⁵N-enriched sewage inputs). Projected intensification under climate warming and accelerated urbanization may further amplify dual-isotope (¹³C and ¹⁵N) imprints within lacustrine carbon-nitrogen coupling.‌ Crucially, the dual enrichment of ¹³C and ¹⁵N isotopes elevates baseline signatures at primary trophic levels while reducing energy transfer efficiency via isotope-specific zero-point vibrational effects. These isotope-mediated dynamics decode synergistic interactions between climate variability and human-driven perturbations, establishing an isotopic toolkit for diagnosing carbon-nitrogen coupling and guiding adaptive watershed governance in fragile plateau ecosystems.