Photo-reactivity of natural dissolved organic matter from fresh to marine waters in the Florida Everglades, USA

Abstract

Natural dissolved organic matter (DOM) is the major absorber of sunlight in most natural waters and a critical component of carbon cycling in aquatic systems. The combined effect of light absorbance properties and related photo-production of reactive species are essential in determining the reactivity of DOM. Optical properties and in particular excitation-emission matrix fluorescence spectroscopy combined with parallel factor analysis (EEM-PARAFAC) have been used increasingly to track sources and fate of DOM. Here we describe studies conducted in water from two estuarine systems in the Florida Everglades, with a salinity gradient of 2 to 37 and dissolved organic carbon concentrations from 19.3 to 5.74 mg C L(-1), aimed at assessing how the quantity and quality of DOM is coupled to the formation rates and steady-state concentrations of reactive species including singlet oxygen, hydroxyl radical, and the triplet excited state of DOM. These species were related to optical properties and PARAFAC components of the DOM. The formation rate and steady-state concentration of the carbonate radical was calculated in all samples. The data suggests that formation rates, particularly for singlet oxygen and hydroxyl radicals, are strongly coupled to the abundance of terrestrial humic-like substances. A decrease in singlet oxygen, hydroxyl radical, and carbonate radical formation rates and steady-state concentration along the estuarine salinity gradient was observed as the relative concentration of terrestrial humic-like DOM decreased due to mixing with microbial humic-like and protein-like DOM components, while the formation rate of triplet excited-state DOM did not change. Fluorescent DOM was also found to be more tightly coupled to reactive species generation than chromophoric DOM.

Molecular Composition and Photochemical Reactivity of Size-Fractionated Dissolved Organic Matter

“…This has been observed in natural systems as a previous study of DOM photochemistry identified divergent trends in 1 O 2 and 3 DOM production as DOM moved through an estuary. 90 Our observation that 3 DOM formation rates are constant with molecular weight suggests that environmental or engineered processes that modify molecular weight may not alter the carbon-normalized 3 DOM formation rates. However, the decrease in light absorbance that accompanies decreasing DOM molecular weight may allow indirect photodegradation to occur deeper in the water column and increase the overall contaminant loss rate.…”

Influence of dissolved organic matter on the photodegradation and volatilization kinetics of chlorpyrifos in coastal waters

“…However, at high concentration of SRNOM and in salt waters indirect photolysis can have a significant effect on chlorpyrifos degradation. The importance of the amount and the type of dissolved organic matter (DOM) on the steady-state concentration of intermediate radicals was reported by Timko et al [40]. The authors found out that that the formation rates of singlet oxygen ( 1 O 2 ) radical, hydroxyl radicals ( OH) and carbonate radical and their steady state concentration decreased along the estuarine salinity gradient as a consequence of a decrease of terrestrial humic-like DOM.…”

Characterizing the Molecular Structure and Reactivity of Natural Organic Matter in The Everglades

“…Moreover, studies in fresh-water and estuarine water showed that estuarine samples are more photo-reactive and have higher RS formation rates along with higher organic carbon content than fresh-water samples Housari et al, 2010). Another finding demonstrated that increasing humic concentration could also enhance •OH photoproduction rates (Mopper et al, 1990) and the main source of ¹O₂ are the moieties that are present in the terrestrial humic substances (Timko et al, 2014). Alternatively, halide radicals such as bromide or chloride could contribute to the photoproduction of RS from DOM as well .…”

“…Thus, TS7 November sample showed a low a₂₅₅ and DOC value which is similar to FB samples. On the basis of previous reports, the DOC concentration in the SRS decreased from the upstream to downstream while the TS showed an opposite trend Timko et al, 2014;Romera-Castillo et al, 2015). Peat based fresh-water marsh sub-environment (SRS2) showed a significantly higher concentration of DOC (Avg.…”

“…Maie et al, 2005), it would be of interest to understand how RS production is coupled to such DOM diversity. Timko et al (2014) studied the formation rates and steady-state concentrations of singlet oxygen, hydroxyl radical and triplet excited-state DOM and how these species corresponded to the optical properties of DOM during the late dry season of the Everglades. Their study suggested that DOM quality and quantity is coupled to the production of RS.…”