Source-specific insights into photochemical and microbial degradation of dissolved organic matter in coastal...

Coastal zones are critical for the biogeochemical cycling of dissolved organic matter (DOM) in marine ecosystems, yet the relative importance of photochemical and microbial degradation in DOM transformation remains poorly understood due to complex hydrodynamics, diverse sources, and human activities. Through 14-day laboratory incubations, we investigated DOM transformation mechanisms from three common marine coastal space uses: port, mariculture and inshore areas adjacent to Yantai City. DOM characterization was performed using fluorescence excitation-emission matrix parallel factor (EEM-PARAFAC) and UV-Vis spectroscopic indices. Initial characterization revealed port-area DOM exhibited mixed terrestrial-autochthonous sources; mariculture-area DOM was dominated by autochthonous signals; and inshore-area DOM showed terrestrial signatures with anthropogenic disturbance. DOM degradation dynamics varied significantly across sources: inshore-area DOM was highly photolabile, with humic-like substances degrading by 31.0 ± 2.0 %; mariculture-area DOM preferentially underwent microbial degradation, particularly the tryptophan-like component, which degraded by ∼50.0 %; and port-area DOM showed degradation behavior comparable to marine-dominated DOM. At the cultivation endpoint, port-area DOM demonstrated significant humification; in the mariculture area, microorganisms preferentially degraded protein-like substances, while photochemical processes facilitated the formation of humic-like compounds. Inshore-area DOM enriched with high molecular weight compounds showed significant molecular weight reduction and compositional changes, suggesting synergistic photochemical-microbial processes in DOM transformation under the influence of intensive human activities. Repeated measures ANOVA showed DOM spectral variations significantly correlated with time, system, and region (P < 0.05), indicating DOM transformation is regulated by source and chemical composition, stage-dependent degradation, and environmental drivers (natural processes and human activities). These findings demonstrate that human activities decouple natural DOM degradation processes, generating divergent carbon transformation pathways that restructure coastal biogeochemical functions.