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, AQYT) 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 AQYT. The DOM bulk composition was assessed using absorbance and fluorescence spectroscopies. AQYT was estimated under simulated sunlight using the probe 2,4,6-trimethylphenol. Relatively high AQYT 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 AQYT 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.
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© 2018 The Authors Limnology and Oceanography published by Wiley Periodicals, Inc. on behalf of Association for the Sciences of Limnology and Oceanography