Unraveling the mechanism of persistent lake eutrophication: Organic phosphorus molecular composition mediates internal phosphorus feedback.

Sediment organic phosphorus (OP) is a key internal source sustaining lake eutrophication after external P reduction, yet its molecular-level response to trophic gradients and regulatory mechanisms remain unclear. In this study, we characterized OP fractions, bioavailability, and molecular composition in sediments from 11 lakes across trophic states in China using chemical extraction, enzymatic hydrolysis, and Fourier transform ion cyclotron resonance mass spectrometry. The results showed that sediment OP content was positively correlated with trophic states. Bioavailable OP, including labile OP (LOP) and moderately labile OP (MLOP), accounting for 28.8%-76.5% of sediment OP. With increasing trophic level, LOP and MLOP contents increased significantly; LOP hydrolysis shifted from labile monoester P to diester P dominance, whereas MLOP remained labile monoester P dominated. The P‑containing molecules showed higher molecular weight, enhanced aromaticity, lower oxidation state, and greater labile molecules abundance with increasing trophic states. Relative abundances of lipid‑, protein‑, and lignin‑like compounds of P‑containing molecules were positively correlated with trophic level, indicating algal production and external inputs synergistically drove sediment OP accumulation. Partial least squares path modeling showed that OP bioavailability had a negative direct effect on trophic level, but a positive indirect effect via molecular composition, suggesting molecular composition may act as a important mediator converting potentially available OP into long‑term ecological effects. Our findings indicate that enrichment of labile molecules (lipids and proteins) in sediment OP enhances bioavailability, exacerbates eutrophication and promotes algal sedimentation, forming a self‑reinforcing positive feedback. This study reveals the molecular mechanisms by which sediment OP drives persistent eutrophication, providing a basis for internal P risk assessment and targeted eutrophication control.