Distinctive response of ROS generation to multi-spectral irradiations during DOM photodegradation from eutrophic and...

Dissolved organic matter (DOM) constitutes the primary carbon pool in lakes, with its photodegradation behavior predominantly influenced by indirect processes mediated by reactive oxygen species (ROS). These processes play a crucial role in regulating the carbon cycle and transforming pollutants; however, the potential mechanisms of ROS generation remain unclear, especially against a backdrop of eutrophication in shallow lakes. In this study, six shallow lakes with different nutrient levels located in the middle and lower reaches of the Yangtze River were selected to investigate the response mechanisms of DOM composition and ROS generation through different spectral irradiation experiments. The results indicated that DOM in eutrophic lakes exhibited higher molecular weight, aromaticity and hydrophobicity. Furthermore, all lakes displayed a mixture of internal and external sources, which intensified with increasing trophic levels. UVB, UVA and blue light irradiation led to a significant decrease in fluorescence intensity, DOC, molecular weight, aromaticity and hydrophobicity of DOM in eutrophic lakes, accompanied by a notable generation of single oxygen (¹O₂) and triplet states of DOM (³DOM*). And Visible light drives efficient photodecarboxylation. Furthermore, the generation rates and steady-state concentrations decreased with increasing spectral wavelength. The ¹O₂ production rates from DOM in eutrophic lakes under UVB, UVA, and blue light irradiation were 6.71 × 10^-9, 4.74 × 10^-9 and 2.17 × 10^-9 mol/(L·s)), respectively, which were higher than those corresponding to DOM in oligotrophic lakes (1.26 × 10^-9, 0.74 × 10^-9 and 0.43 × 10^-9 mol/(L·s)). DOM fluorescence components and UV-Vis characteristics were direct regulators of ROS generation rates. Notably, the a₃₅₀ and DOC of DOM in oligotrophic lakes were significantly and positively correlated with ³DOM* and ¹O₂ generation rates. The linear regression model based on DOC can accurately predict the formation rate of ¹O₂. The E2:E3 values of DOM in eutrophic lakes under UVB and blue light irradiation exhibited a significant negative correlation with both factors. The physicochemical parameters and nutrient levels of these lakes indirectly regulated ROS production by influencing DOM composition. The coupled mechanism model of nutrient level, DOM composition characteristics, spectral response and ROS generation efficiency highlighted in this study provides a novel perspective for simulating carbon balance in shallow lakes.