Unraveling contrasting summer endogenous nutrient release regimes in Two Karst Plateau Lakes: Evidence from...

High-altitude karst plateau lakes are increasingly threatened by eutrophication, yet limited knowledge of microscale sediment-water interface (SWI) biogeochemical processes constrains mechanistic understanding and management of internal nutrient loading. Here, we coupled colorimetric diffusive gradients/equilibrium in thin films (DGT/DET) with microelectrode profiling to co-map labile phosphorus (P) and ammonium (NH₄⁺-N) at sub-millimeter resolution across the SWI of two contrasting karst lakes (deep, oligotrophic Lake Fuxian versus shallow, eutrophic Lake Dianchi) during the severe eutrophication period. 2D images revealed steep near-SWI gradients and discrete hotspots where labile P and NH₄⁺-N were co-enriched within overlapping depth intervals, indicating a close microscale association between P and N mobilization. Generally, Lake Dianchi displayed markedly higher near-SWI labile P and NH₄⁺-N concentrations and diffusive fluxes as compared with Lake Fuxian. Typically, Lake Fuxian maintained a relatively stable weakly alkaline pH and deeper O₂ penetration across the SWI, together with low labile nutrient pools and Ca-P-dominated sediments, suggesting stronger near-interface nutrient retention. While Lake Dianchi appeared to be more strongly influenced by coupled water-column and sediment forcing. High OM loading likely altered the localized geochemical microenvironment through porewater acidification and the concurrent accumulation of Fe(II) and S(-II), processes that may have synergistically facilitated the interfacial co-release of P and N by weakening the oxidative barrier. These findings clarify the distinction between lake-scale ecological vulnerability to nutrient enrichment and local SWI-scale geochemical stabilization and reveal contrasting summer SWI nutrient migration patterns that can inform sediment-derived nutrient risk diagnosis in fragile karst plateau lakes.