Compositional divergence of sediment dissolved organic matter across a littoral exposure-inundation gradient

Littoral sediments serve as dynamic biogeochemical interfaces, yet how exposure-inundation gradients dictate the compositional divergence of sediment dissolved organic matter (DOM) through nested physical-chemical pathways remains poorly understood. In this study, sediment DOM dynamics across a littoral exposure-inundation gradient (upper, middle, and permanently inundated zones) were examined by integrating micro-hydrological zonation, depth-resolved physicochemical properties, and PARAFAC-derived fluorescence components. The results demonstrated a pronounced horizontal and vertical divergence in DOM composition along the gradient. Intermittently exposed upper zones (U) exhibited higher relative contributions of humic-like (45.2%) and fulvic-like (40.4%) components with lower DOC concentrations (0.18-0.81 mg g^-1), indicating selective preservation of aromatic structures during oxic-suboxic exposure. Conversely, the permanently inundated zone (D) was dominated by protein-like DOM (43.4%) and elevated DOC (0.25-0.96 mg g^-1), coupled with higher spectral slope ratios (S_R), reflecting enhanced preservation of lower-molecular-weight, labile organic matter under prolonged anaerobic conditions. Piecewise structural equation modeling (SEM) successfully disentangled the hierarchical environmental controls, explaining 47%, 22%, and 19% of the variances in protein-like (C3), fulvic-like (C2), and humic-like (C1) fractions, respectively. Sediment redox potential exerted a significant negative effect on protein-like DOM (standardized coefficient = -0.35), whereas bulk density imposed strong constraints on both humic- and protein-like fractions. These findings underscore that exposure-inundation gradients govern littoral DOM architecture via divergent redox gating and physical matrix filtering. Our results imply that adaptive lake water-level regulations must account for localized sediment physical-chemical coupling to optimize carbon retention and manage internal nutrient loading under fluctuating hydrological regimes.