The role of dissolved organic matter in fuelling coastal hypoxia: Identifying key molecules and drivers

The escalating prevalence and impact of coastal hypoxia on global aquatic ecosystems necessitate a deeper understanding of the underlying mechanisms. However, the concept of oxygen-consuming organic matter (OxyCOM) driving hypoxia formation remains unclear. Using the Pearl River Estuary as a case study, this study investigates whether particulate (POM) or dissolved organic matter (DOM) plays a larger role in oxygen depletion and hypoxia formation. We characterize and compare water-extractable organic matter (WE-OM, representing the labile POM fraction) with porewater DOM (PW-OM) from hypoxic and non-hypoxic zones using three-dimensional excitation-emission matrix fluorescence spectroscopy and Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS). Both WE-OM and PW-OM shows comparable concentrations, but DOM has smaller size and lower humification indicating higher lability of DOM than WE-OM. Hence, DOM is suggested the primary form of OxyCOM. The protein-like component (C3) was identified as a major constituent of OxyCOM. Furthermore, we introduce a novel method using a DOM molecular database for source resolution with FT-ICR MS, revealing that OxyCOM primarily originates from algal intracellular organic matter. We also propose an indirect role for OM degradation products in hypoxia formation, supported by photochemical and biological reactivity predictions of OxyCOM derived from a machine learning approach.