A comprehensive conceptual framework for signaling in-lake CO₂ through dissolved organic matter

Organic carbon (C) and CO₂ pools are closely interactive in aquatic environments. While there are strong indications linking freshwater CO₂ to dissolved organic matter (DOM), the specific mechanisms underlying their common pathways remain unclear. Here, we present an extensive investigation from 20 subtropical lakes in China, establishing a comprehensive conceptual framework for identifying CO₂ drivers and retrieving CO₂ magnitude through co-trajectories of DOM evolution. Based on this framework, we show that lake CO₂ during wet period is constrained by a combination of biogeochemical processes, while photo-mineralization of activated aromatic compounds fuels CO₂ during dry period. We clearly determine that biological degradation of DOM governs temporal variations in CO₂ rather than terrestrial C inputs within the subtropical lakes. Specifically, our results identify a shared route for the uptake of atmospheric polycyclic aromatic compounds and CO₂ by lakes. Using machine learning, in-lake CO₂ levels are well modelled through DOM signaling regardless of varying CO₂ mechanisms. This study unravels the mechanistic underpinnings of causal links between lake CO₂ and DOM, with important implications for understanding obscure aquatic CO₂ drivers amidst the ongoing impacts of global climate change.