Hydrological drivers govern the trade-off between organic carbon stability and turnover in inland aquatic ecosystems

Inland aquatic ecosystems play a critical but heterogeneous role in the global carbon (C) cycle, yet mechanisms controlling organic carbon (OC) dynamics across aquatic ecosystems remain poorly understood. This study examines the spatial variation and regulatory mechanisms of OC composition, stability, and turnover in ditches, ponds, rivers, and reservoirs across the Danjiangkou Reservoir watershed. Our results establish hydrological conditions as the principal driver of ecosystem-specific OC pathways. We observed significantly higher sediment OC content in ditches (18.23 g kg–1) and ponds (17.77 g kg–1) than in rivers (12.35 g kg–1) and reservoirs (15.39 g kg–1). However, pond sediments exhibited the lowest C stability (stability coefficient = 0.48), whereas river sediments, despite lower OC content, contained a greater proportion of mineral-associated OC, resulting in higher stability. Isotopic evidence (δ13C) indicated that OC sources shift from predominantly a mix of autochthonous and allochthonous in ponds and ditches to largely allochthonous inputs (C3/C4 plants) in rivers. We further identified a significant negative correlation between C flowability (reflecting transformation intensity) and stability (R2 = 0.10, p < 0.05), underscoring a key trade-off wherein high turnover limits long-term sequestration. Random forest and structural equation models corroborated that hydrological factors directly and indirectly modulate these processes by governing water–sediment interactions and sediment environmental conditions. These findings reveal that the C cycling function of inland waters spans a hydrologically driven continuum, necessitating ecosystem-specific frameworks for accurate C accounting and optimized sequestration strategies.