A comprehensive conceptual framework for signaling in-lake CO 2 through dissolved organic matter.

Organic carbon (C) and CO ₂ pools are closely interactive in aquatic environments. While there are strong indications linking freshwater CO ₂ 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 CO ₂ drivers and retrieving CO ₂ magnitude through co-trajectories of DOM evolution. Based on this framework, we show that lake CO ₂ during wet period is constrained by a combination of biogeochemical processes, while photo-mineralization of activated aromatic compounds fuels CO ₂ during dry period. We clearly determine that biological degradation of DOM governs temporal variations in CO ₂ 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 CO ₂ by lakes. Using machine learning, in-lake CO ₂ levels are well modelled through DOM signaling regardless of varying CO ₂ mechanisms. This study unravels the mechanistic underpinnings of causal links between lake CO ₂ and DOM, with important implications for understanding obscure aquatic CO ₂ drivers amidst the ongoing impacts of global climate change.
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