Nonlinear Responses of Chemodiversity to Total Organic Carbon Content in Lake Sediments

Abstract Lake littoral sediments are critical carbon cycle hotspots that bridge terrestrial and aquatic ecosystems. The chemodiversity of dissolved organic matter (DOM) in these sediments reflects ecosystem multifunctionality, particularly regarding carbon storage. However, the relationship between DOM chemodiversity and sediment total organic carbon (TOC) content, a critical quality‐quantity nexus, remains poorly understood. Here, we evaluated both linear and threshold models to characterize this relationship across 63 freshwater ecosystems spanning a 31° latitudinal gradient. DOM chemodiversity showed nonlinear responses to sediment TOC content, with critical thresholds identified between 1.0% and 1.8% for alpha diversity, compound abundance, functional traits, and functional diversity, and 1.7% for DOM composition. DOM diversity metrics increased with TOC content before these thresholds and plateaued thereafter, indicating post‐threshold saturation of chemodiversity. Across these transition points, the predominant influence of sediment TOC on DOM chemodiversity declined, while the importance of bacterial compositional dissimilarity increased. Structural equation modeling further suggested that external organic carbon inputs dominated before the thresholds, while in‐lake processes became more important thereafter. Overall, this study challenges the conventional assumption of a positive correlation between chemodiversity and carbon quantity, highlighting the need to incorporate nonlinear dynamics into future carbon cycle models for freshwater ecosystems.