Emerging Patterns in the Global Distribution of Fluorescent Dissolved Organic Matter

Urban Johannes Wünsch and Kathleen Murphy, Chalmers University of Technology, Architecture and Civil Engineering, Gothenburg, Sweden
The integration of dissolved organic matter (DOM) in global carbon budgets requires insightful and frequent monitoring of spatial and temporal dynamics. Consequently, many studies measure the optical properties of chromophoric and fluorescent DOM (CDOM and FDOM, respectively). For the past 15 years, fluorescence spectroscopy in conjunction with parallel factor analysis (PARAFAC) has been the focus of an increasing number of studies as it promises a distinction of source materials and chemical fractions. In contrast to this, new studies often show highly similar fluorescence landscapes, suggesting a similar composition in samples of different origin. Here, we present the results of two studies aiming to identify common fluorescence components and evaluate their source specificity. First, the components of 90 peer-reviewed PARAFAC models published in the OpenFluor database were compared. Second, the fluorescence of 20 biogeochemically diverse samples was described by fitting individual PARAFAC models to each sample’s photodegradation behavior. The analysis of PARAFAC models in OpenFluor showed that many of the 478 component spectra clustered in only six wavelength regions. Similarly, the photodegradation of individual samples revealed that most measured fluorescence in environmental samples can be explained by ubiquitous spectra with nearly stable optical properties and photodegradation behaviors. Contrary to common understanding, we did not observe evidence for the existence source-specific fluorescence components. Our study highlights that relying solely on fluorescence spectroscopy bears significant risk of misinterpretation. However, a more constrained biogeochemical interpretation of FDOM components may be attainable by considering supplemental compositional information.