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

• Temperature significantly affects δ ¹³C phy by altering growth rate and DIC pool. • δ ¹⁵N phy traces N inputs (fertilizer/sewage) vs natural baseline. • Global warming and human activities intensify ¹³C-¹⁵N linkages in lake C-N cycling. 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 13 C 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., 15 N-depleted agricultural fertilizers vs. 15 N-enriched sewage inputs). Projected intensification under climate warming and accelerated urbanization may further amplify dual-isotope ( 13 C and 15 N) imprints within lacustrine carbon-nitrogen coupling.‌ Crucially, the dual enrichment of 13 C and 15 N 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.