Decadal shift in lacustrine state inferred from molecular chemodiversity of sedimentary organic matter.

Under disturbances from climate change and anthropogenic activities, lacustrine ecosystems may have experienced transitions in their nutrient statuses, e.g. from eutrophic to mesotrophic state. However, the temporal patterns of lacustrine nutrient state transitions and their underlying biochemical mechanisms remain largely unexplored, mainly limited by the scarcity of historical records. Here, we developed a novel molecular chemodiversity fingerprinting (MCF) tool, integrating chronometry with multivariate metabolomics, to track the sedimentary biochemistry state for reflecting lacustrine nutrient state temporally. Taking Baiyangdian Lake in Northern China as an example, we successfully identified a sedimentary biochemical state shift from allochthonous to autochthonous dominance since 2016 based on MCF, which concurrently propelled the lacustrine nutrient transition from eutrophic to mildly mesotrophic state. Such biochemical state shift was evidenced by a significant 7.43% (p < 0.05) increase of microbial-derived components in sedimentary organic matter compositions since 2016, indicating reinforced internal nutrient cycling. Consistently, allochthonous anthropogenic carbonaceous inputs declined significantly by 18.53% (p < 0.05) since 2016, mainly due to benzene polycarboxylic acid accumulation. Moreover, a carbon-oxygen synergy was revealed in the new nutrient state after 2016 to foster lacustrine health. Within this synergy, a reduction in microbially recalcitrant carbon lowered the theoretical oxygen demand from 2.24 to 2.15 and alleviated the benthic oxygen stress, thereby facilitating colonization of clean-water bioindicators (e.g., Baetis majus naiads). Overall, MCF-captured sedimentary biochemistry shift infers lacustrine nutrient state transition, offering a transferable tool for tracking their temporal patterns and inspiring future biochemistry-monitoring paradigms and management strategies.