Sediment amplifies organic matter cycling and nutrients feedback in eutrophic lake zones

Eutrophication alters the composition of dissolved organic matter (DOM) in lakes, yet its impact on particulate (POM) and sedimentary organic matter (SOM) remains poorly explored, particularly in large shallow lake systems. Here, we investigated spatial variation in DOM, POM, and SOM across different eutrophic zones in a large shallow lake using optical spectroscopy and ultrahigh-resolution Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS). Compared to the light-eutrophic region, middle-eutrophic region exhibited elevated dissolved organic carbon, chromophoric DOM, and fluorescent DOM concentrations but lower molecular weights, with cyanobacteria abundance strongly correlated with SOM composition. While DOM molecular signatures were relatively consistent across different eutrophic zones, as eutrophication intensifies, SOM showed more pronounced changes, including higher proportions of aliphatic, lipid-like, and sulfur-containing (CHOS) compounds, and reduced levels of phosphorus-containing compounds (CHOP) and carboxyl-rich alicyclic molecules (CRAM). These compositional shifts, along with declining aromaticity and oxidation state, suggest enhanced accumulation of algal-derived labile organic matter and microbially-mediated release of phosphorus from sediments. Our findings reveal that SOM is more responsive to eutrophication than DOM and may act as both a sink and a source of nutrients, contributing to internal feedbacks that sustain algal blooms. This study provides new insights into organic matter dynamics in eutrophic lakes and supports more targeted management strategies for carbon and nutrient regulation.