Fluorescence Monitoring and Modeling for Understanding Organic Matter Dynamics in European Rivers

This study focuses on exploring the dynamics of dissolved organic matter (DOM) using high-frequency, continuous monitoring coupled with advanced fluorescence spectroscopy and statistical modeling. By combining continuous fluorescence sondes with spot sampling, we show temporal and longitudinal dynamics of DOM over a 14-month period in two UK rivers. The integration of fluorescence excitation-emission matrix (EEM) spectroscopy and Parallel Factor Analysis (PARAFAC) enabled the identification of key fluorescent components, including humic and protein-like substances. Real-time monitoring of these two DOM components reveals significant diel and seasonal variations in both the quantities and characteristics of DOM. External carbon sources (treatment works, agricultural land use) showed increased protein-like DOM, particularly during summer, indicating the influence of labile organic matter. A new fluorescence ratio (humic DOM/protein-like DOM) proved to be a robust indicator for differentiating between microbial-derived labile DOM and more refractory humic substances, offering new insights into organic matter processing and nutrient cycling in the studied ecosystems.

Modeling approaches, based on ANCOVA and logistic regression, demonstrated that allochthonous sources, precipitation, and seasonal temperature variations were key drivers of DOM dynamics. Periods of low temperature and high precipitation were characterized by a notable increase in humic-like DOM concentrations, primarily due to enhanced runoff of terrestrial organic matter into the river system. In contrast, as temperature increased, tryptophan-like DOM concentrations rose, reflecting heightened microbial activity driven by warmer conditions. The elevated temperature not only stimulated microbial metabolism but also accelerated the decomposition of organic matter, leading to the production of more labile, protein-like substances. These contrasting seasonal trends highlight the dual influence of hydrological inputs and temperature-driven biological processes on DOM patterns.