Contrasting responses of DOM leachates to photodegradation observed in plant species collected along an estuarine salinity gradient

A series of abiotic processes affected by salinity changes involve light-mediated reactions and different degradation pathways of organic compounds, including altered photo-oxidation, photodegradation, and photolysis of organic matter (OM). Sunlight is known to degrade, oxidize, or mineralize dissolved organic matter (DOM) in waterways, creating large changes in compositional structure of DOM near the water surface and ultimately in the mixed layer. DOM derived from various vegetation types has differing levels of susceptibility to photolytic degradation depending on initial chemical composition and in what matrix degradation takes place (e.g., salinity). The effect of sunlight and salinity on degradation of leached DOM derived from three dominant vegetative species, Avicennia germinans, Juncus romerianus, and Taxodium distichum, along a riverine continuum in northeast Florida was determined. Leachates from these three sources in a deionized or seawater matrix were irradiated in a continuous flow-through photolytic system over the course of 20 h. Avicennia germinans and Juncus roemerinaus DOM readily degraded as indicated by decreases in absorbance across all wavelengths during the irradiation period, while Taxodium distichum DOM was found to increase in absorbance across all wavelengths in the freshwater matrix, but not in seawater. PARAFAC analysis indicated differences in photochemical components and % change of absorbance and fluorescence over time indicate the importance and variability of individual contributions to the DOM pool across an estuarine continuum. This work characterizes the photochemical properties of three individual DOM sources, exhibits the need for further research on this topic, and explores the salinity effect on photodegradation of DOM from unique plant-derived DOM.