Application of Fluorescence Spectroscopy in Environmental Sciences: a review of field and experimental assessments in temperate aquatic systems of the Southern Andes (Patagonia, Argentina)

Abstract

During the last decades, fluorescence spectroscopy has evolved into a powerful technique for environmental studies. Its application in the analysis of the fluorescent fraction of the dissolved organic matter (DOM) has been improved with the development of different proxies that characterize its origin and diagenetic state. As a whole, DOM plays a key role in the global C cycle, particularly in the mobility of C among environmental compartments. It regulates the structure and functioning of aquatic ecosystems as it influences on several biogeochemical processes interacting with metals in solution, enhancing nutrient availability, regulating the underwater light climate and thermal structure, and controlling the autotrophic and heterotrophic production. DOM is a complex and heterogeneous mixture of substances of low to high molecular weight that exhibits different solubilities, reactivities and optical properties depending on the molecular structure. The chromophoric DOM fraction absorbs light and contains the fluorescent subfraction (FDOM). FDOM can be accurately characterized through fluorescence spectroscopy by means of emission and synchronous spectra and Excitation-Emission matrices (EEMs), which can be decomposed using parallel factor analysis (PARAFAC). The EEMs reveal FDOM characteristics (“fingerprint”), providing information on its origin (autochthonous or allochthonous), diagenetic state, reactivity, enabling the identification of humic and non-humic substances. In this chapter, we present different field and experimental studies based on fluorescence spectroscopy addressing the spatio-temporal dynamics of DOM in headwaters of North Patagonia. The PARAFAC analysis of EEM spectra identified three fluorescent components, C1, C2 and C3: the humic components C1 and C2 are a combination of components of peaks A+M and A+C, respectively; and the component C3 (peak T) is associated with non-humic and aliphatic compounds. The main results of the FDOM analysis evidenced the prevalence of allochthonous DOM in mountain streams, whereas in lakes, allochthonous and autochthonous sources as well as processing signatures were fingerprinted. Also, FDOM analysis revealed the influence of climate patterns. The marked seasonality of precipitation in North Patagonia influenced the quantity and quality of lake DOM, with terrestrial inputs concentrating in the cold wet season and internal DOM transformation processes favored in dry and warmer periods. This pattern was reflected by the increasing trend in the C3/C2 ratio (non-humic to humic relationship) between the wet and dry seasons. Moreover, the concentration of Hg2+ in lakes reflected the precipitation regime and related to allochthonous DOM inputs. The higher the concentration and the larger the DOM molecular weight, the complexation of Hg with organic molecules was more important, reducing its bioavailability. On the contrary, environments with low DOM concentrations provided favorable conditions for Hg bioaccumulation, due to lower DOM-Hg complexation, resulting in greater Hg availability. Other studies reviewed here addressed the occurrence of the reactive oxygen species hydrogen peroxide (H2O2) in different aquatic systems of Andean Patagonia in relation to DOM properties, both assessed through fluorescence-based techniques. Overall, these studies underscored the effectiveness of DOM fluorescent properties to characterize the state and functioning of temperate freshwater systems of the Southern Hemisphere, and to assess spatio-temporal environmental changes, DOM-metal interactions as well as production of reactive oxygen species.