Linking the spatiotemporal dynamics of DOM molecular characteristics to dissolved oxygen level in an urban estuarine water and sediment

Dissolved organic matter (DOM) drives deoxygenation, a critical threat to estuaries globally, yet its characteristics linkages to dissolved oxygen (DO) level remain unclear. This study integrates multiple methods to unravel the spatiotemporal connection between DOM molecular characteristics and DO levels in Shenzhen Estuary, China. Results reveal seasonal DO concentration decline, even hypoxia in the river section during wet periods, while the downstream bay section maintained aerobic year-round. For DOM characteristics, water DOM was richer in humic acids and autochthonous protein-like fluorescence (62.9%), while sediment DOM was dominated by fulvic acids and humic-like components (67.9%). From wet to dry period, river water DOM, CODM, FDOM amount and hydrophobic/hydrophilic components declined but their bioavailability increased since decreased aromaticity, hydrophobicity, humification degree and increased fulvic acid proportion, whereas sediment showed opposite trends. Spatially, DOM amount and components in water and sediments decreased from river to bay in the dry period, with DOM bioavailability declining in water but rising in sediments. During wet periods, water DOM molecular characteristics became homogenized while sediment gradients persisted. Relationship analysis demonstrated that higher DOM amount, bioavailability, and microbial activity in water correlated with lower DO levels, while greater sediment DOM amount, higher humification and aromaticity, as well as microbial activities in sediments correspondent to lower DO level. Enhanced terrestrial DOM inputs, microbial transformations, and sediment DOM releases were important reason for hypoxia in river section during wet periods. This study establishes a framework linking specific DOM compositional shifts to DO level patterns, providing valuable insights for oxygen management in estuaries.