Estuarine plumes: Modulators of dissolved organic matter molecular signatures and biogeochemical fate in coastal...

Estuarine plumes (EPs) are recognized as critical drivers of dissolved organic matter (DOM) heterogeneity in coastal zones, primarily by inducing phytoplankton blooms and subsequent bottom-water dissolved oxygen (DO) depletion. However, the specific mechanisms governing the EP-driven transformations of DOM molecular composition and biogeochemical fate remain elusive. Here, we integrated optical spectroscopy and Fourier transform ion cyclotron resonance mass spectrometry to characterize the molecular signatures of DOM and their biogeochemical transformations within EP-influenced bottom waters of the Pearl River Estuary. Results showed that despite bulk dissolved organic carbon concentrations being ∼14 % lower in the low-DO zones (LDOZ; DO < 5 mg/L), these zones exhibited significantly higher levels of chromophoric DOM as well as terrestrial and microbial humic-like components compared to the non-LDOZ (NLDOZ; DO ≥ 5 mg/L). At the molecular level, DOM in the LDOZ exhibited greater molecular richness and was characterized by significantly higher intensity-weighted average hydrogen-to-carbon and sulfur-to-carbon ratios, and a lower oxygen-to-carbon ratio relative to the NLDOZ, indicating a compositional shift towards less oxidized, more saturated, and sulfur-rich compounds. Furthermore, low DO conditions likely inhibited the complete microbial mineralization of labile compounds, leading to their accumulation and a concurrent reduction in the formation of nitrogen-containing metabolic by-products. Riverine inputs, concentrated by the EP, were identified as a significant source of the increased relative abundance of sulfur-containing and biorefractory compounds in the LDOZ. These findings reveal that EPs function as transient biogeochemical reactors that actively reshape DOM composition, profoundly impacting carbon sequestration and contaminant cycling in coastal ecosystems.