Multiple linear regression models to predict the formation efficiency of triplet excited states of dissolved organic matter in temperate wetlands

The source and composition of dissolved organic matter (DOM) dictates light absorption in surface waters. Sunlight absorption by chromophoric dissolved organic matter (CDOM) forms reactive intermediates and drives global organic carbon processing. Triplet excited states of CDOM (³CDOM*) are primary reactive intermediates formed by sunlight absorption by CDOM. ³CDOM* also produce secondary reactive intermediates, including radicals and reactive oxygen species, which are active in biogeochemical pathways. The efficiency of ³CDOM* formation (apparent quantum yield, AQY_T) depends on DOM composition, especially DOM molecular weight. This dependence may arise from the greater probability of forming intra-molecular charge-transfer (CT) complexes in high-molecular weight DOM that inhibit ³CDOM* formation. There are few examples that demonstrate this in field samples. In this report, vegetation, general hydrology, and watershed characteristics for 39 temperate wetlands, which are critical sources of high-molecular weight DOM, from the United States were defined and related to DOM composition and AQY_T. The DOM bulk composition was assessed using absorbance and fluorescence spectroscopies. AQY_T was estimated under simulated sunlight using the probe 2,4,6-trimethylphenol. Relatively high AQY_T values (7%) were observed in wetlands with long hydroperiods and > 50% cropland watershed land cover compared to wetlands with >50% forest watershed land cover (< 1–4%). Low molecular weight DOM (E2/E3 > 7 and SUVA₂₅₄ < 3 L mg-C⁻¹m⁻¹) and autochthonous DOM (β/α > 0.7) had relatively high AQY_T estimates (∼ 10%), indicating that allocthonous, high-molecular weight compounds produce ³CDOM* less efficiently than autochthonous DOM. The CT theory of DOM light absorption and internal light-screening offer mechanistic explanations for these trends.