Abstract:

Dissolved organic matter (DOM) is a dynamic carbon pool in aquatic ecosystems, with photochemical degradation profoundly shaping carbon cycling and biogeochemical processes. While sunlight-driven DOM transformations are well-documented, systematic comparisons of photodegradation pathways across large estuaries remain limited. This study investigates the divergent photochemical behaviors of DOM in China’s two largest estuaries—the Yangtze River Estuary (YRE) and Pearl River Estuary (PRE)—through field observations, controlled irradiation experiments, and molecular-level characterization using Fourier-transform ion cyclotron resonance mass spectrometry. By irradiating freshwater and seawater endmember samples, we operationally identified photo-labile, photo-resistant, and photo-produced molecules. Molecular signatures (O/C, H/C, double bond equivalence, aromaticity) derived from comparative irradiation experiments on freshwater and marine endmembers revealed distinct photodegradation pathways across both estuaries. In the YRE, DOM from freshwater and marine consistently exhibited photo-oxidation dominance (elevated oxygen content, higher carbon oxidation state). Conversely, in the PRE, decarboxylation (oxygen loss, hydrogen saturation) prevailed across all sample types, including seawater-influenced DOM. Salinity-gradient analysis further highlighted that photochemical reactivity of DOM were regulated by terrestrial inputs, anthropogenic impacts, and environmental factors (turbidity, nutrients), leading to divergent pathways toward "marine-like" DOM signatures. This work demonstrates that while photodegradation generally drives terrestrial DOM toward marine characteristics in terms of molecular composition, the dominant transformation processes are source and composition-dependent. Our findings advance mechanistic understanding of how regional environmental conditions and photochemical processes modulate organic carbon cycling at land-ocean interfaces.

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