Abstract

Abstract. Dissolved organic matter (DOM) connects aquatic and terrestrial ecosystems, plays an important role in carbon (C) and nitrogen (N) cycles, and supports aquatic food webs. Understanding DOM chemical composition and reactivity is key for predicting its ecological role, but characterization is difficult as natural DOM is comprised of a large but unknown number of distinct molecules. Photochemistry is one of the environmental processes responsible for changing the molecular composition of DOM, and DOM composition also defines its susceptibility to photochemical alteration. Reliably differentiating the photosensitivity of DOM from different sources can improve our knowledge of how DOM composition is shaped by photochemical alteration and aid research into photochemistry's role in various DOM transformation processes. Here we describe an approach for measuring and comparing DOM photosensitivity consistently, based on the kinetics of changes in DOM fluorescence during 20 h photodegradation experiments. We identify several methodological choices that affect photosensitivity measurements and offer guidelines for adopting our methods, including the use of reference material, precise control of conditions affecting photon dose, leveraging actinometry to estimate photon dose instead of expressing results as a function of exposure time, and frequent (every 20 min) fluorescence and absorbance measurements during exposure to artificial sunlight. We then show that our approach can generate photosensitivity metrics across several sources of DOM, including freshwater wetlands, a stream, an estuary, and Sargassum sp. leachate and observed differences in these metrics that may help identify or explain differences in their composition. Finally, we offer an example of applying our approach to compare DOM photosensitivity in two adjacent freshwater wetlands as seasonal hydrologic changes alter their DOM sources.

Highlights

  • The photochemical reactivity of dissolved organic matter (DOM) is inherently linked to its composition, and its photochemical behavior reflects compositional differences between samples

  • The goals of this study are to (1) identify methodological barriers to reproducible determination of DOM photosensitivity and offer experimental guidelines to improve the studies of DOM photodegradation kinetics, (2) test our approach on samples from various environmental settings to see if our derived metrics of photosensitivity might respond to variability in DOM composition, and (3) analyze photosensitivity differences between different DOM sources in detail to better understand the links between DOM composition, environmental setting, and photochemical degradation processes

  • We stated three goals of this study, claiming we would (1) identify methodological barriers to the reproducible determination of DOM photosensitivity and offer experimental guidelines to improve studies of DOM photodegradation kinetics, (2) test our approach on samples from various environmental settings to see if our derived metrics of photosensitivity might respond to variability in DOM composition, and (3) analyze photosensitivity differences between different DOM sources in detail to better understand the links between DOM composition, environmental setting, and photochemical degradation processes

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Summary

Introduction

The photochemical reactivity of dissolved organic matter (DOM) is inherently linked to its composition, and its photochemical behavior reflects compositional differences between samples. To the extent that photochemical reactivity is a property of DOM chemical composition (Boyle et al, 2009; Cory et al, 2014; Del Vecchio and Blough, 2004; Gonsior et al, 2013, 2009; Wünsch et al, 2017), comparing the potential for photochemical transformation of different DOM sources or treatments (hereafter called photosensitivity) may be a useful tool in the continuing effort to characterize DOM composition. Armstrong et al.: Reproducible determination of dissolved organic matter photosensitivity and to describe its susceptibility to sunlight-induced degradation. Such comparisons require robust methods that are sensitive enough to discern ecologically and chemically relevant differences between distinct DOM sources

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