Dissolved organic matter (DOM) plays an important role in carbon cycling within inland surface waters. However, a mechanistic understanding of the relationship between DOM composition and its susceptibility to both partial and complete photooxidation in surface waters, such as lakes, is currently lacking. This work combines light exposure experiments with high-resolution mass spectrometry to investigate the relationship between DOM composition and the extent of both complete and partial photooxidation using 17 surface waters in the state of Wisconsin. Sampling sites include core lakes in the Northern Temperate Lakes-Long Term Ecological Research Program, Lake Michigan, and Lake Superior. These waters exhibit a wide range in concentration and composition of dissolved organic carbon due to differences in trophic status and size. The extent of both complete and partial photooxidation are quantified through the measurement of carbon dioxide production and dissolved oxygen consumption, respectively, and vary widely among the sampled freshwaters. Similarly, the formation of photochemically produced reactive intermediates (PPRI) quantified using chemical probes depends on initial DOM composition, with higher yields of hydroxyl radical in bogs and higher yields of singlet oxygen in eutrophic lakes. Bulk organic matter properties are assessed using ultraviolet-visible spectroscopy, fluorescence spectroscopy, and electron donating capacity and can be related to the molecular composition of DOM characterized using Fourier transform-ion cyclotron resonance mass spectrometry (FT-ICR MS). Importantly, the formulas most susceptible to photooxidation are determined by comparing FT-ICR MS spectra before and after irradiation. Collectively, this work provides insight into the relationship between DOM composition and photooxidation, which has important implications for carbon cycling in diverse surface waters.

Dissolved organic matter composition influences its susceptibility to complete and partial photooxidation within lakes

ACS Spring 2022

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