Reproducible determination of dissolved organic matter photosensitivity

Abstract

Dissolved organic matter (DOM) connects aquatic and terrestrial ecosystems, plays an important role in C and N cycles, and supports aquatic food webs. Understanding DOM chemical composition and reactivity is key to predict 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 to measure and compare DOM photosensitivity consistently based on the kinetics of changes in DOM fluorescence during 20h photodegradation experiments. We assess the influence of experimental conditions that might affect reproducibility, discuss our modelling approach, offer guidelines for adopting our methods, and illustrate possible applications for ecological inferences. Central to our approach is the use of a reference material, precise control of conditions, leveraging actinometry to estimate photon dose, and frequent (every 20 minutes) fluorescence and absorbance measurements during exposure to artificial sunlight. We compared DOM from freshwater wetlands, a stream, an estuary, and Sargassum sp. leachate and observed differences in sensitivity that could help identify or explain differences in their composition. Finally, we offer an example applying our approach to compare DOM photosensitivity in two adjacent wetlands as seasonal hydrologic changes alter their DOM sources. Our approach may improve reproducibility when compared to other methods and captures time-resolved changes in optical properties that may have been missed previously.