The study investigates the properties of fluorescent dissolved organic matter (DOM) in the Gironde Estuary, focusing on its sources, processing, and seasonal variations. DOM is of interest due to its interaction with contaminants and its role in environmental processes. The estuary, an exchange zone between the continent and the Atlantic Ocean, is characterized by varying salinity and DOM composition. Samples were collected during three cruises in 2002 and 2006, analyzed using excitation-emission matrix (EEM) spectroscopy, which allows direct analysis of water samples. Fluorescent DOM and dissolved organic carbon (DOC) did not behave conservatively in the estuary, indicating complex interactions and transformations. Seasonal variations in DOC content were observed, while DOM fluorescence showed limited seasonal changes. The study evaluated DOM sources and processing using two fluorescence indices: the humification index (HIX) and the index of recent autochthonous contribution (BIX). HIX reflects the degree of humification, while BIX indicates the presence of autochthonous organic matter. Based on fluorescence and DOC results, the estuary was divided into three zones: (i) a turbid zone with low salinity and high suspended particulate matter, (ii) a mid-estuarine zone with low autotrophic productivity and degraded organic material, and (iii) a high-salinity area with increased autotrophic productivity and marine influence. The HIX and BIX indices were shown to be useful tools for characterizing DOM in estuarine waters. The study also found that fluorescence intensity and DOC content varied with salinity, with a decrease in fluorescence intensity and DOC concentration in the downstream part of the estuary. The results suggest that DOM in the estuary is influenced by a combination of terrestrial and marine sources, with seasonal variations in primary production and organic matter degradation. The study highlights the complexity of DOM dynamics in estuarine environments, influenced by factors such as salinity, turbidity, and biological activity. The findings contribute to understanding the carbon cycle and the role of DOM in environmental processes.The study investigates the properties of fluorescent dissolved organic matter (DOM) in the Gironde Estuary, focusing on its sources, processing, and seasonal variations. DOM is of interest due to its interaction with contaminants and its role in environmental processes. The estuary, an exchange zone between the continent and the Atlantic Ocean, is characterized by varying salinity and DOM composition. Samples were collected during three cruises in 2002 and 2006, analyzed using excitation-emission matrix (EEM) spectroscopy, which allows direct analysis of water samples. Fluorescent DOM and dissolved organic carbon (DOC) did not behave conservatively in the estuary, indicating complex interactions and transformations. Seasonal variations in DOC content were observed, while DOM fluorescence showed limited seasonal changes. The study evaluated DOM sources and processing using two fluorescence indices: the humification index (HIX) and the index of recent autochthonous contribution (BIX). HIX reflects the degree of humification, while BIX indicates the presence of autochthonous organic matter. Based on fluorescence and DOC results, the estuary was divided into three zones: (i) a turbid zone with low salinity and high suspended particulate matter, (ii) a mid-estuarine zone with low autotrophic productivity and degraded organic material, and (iii) a high-salinity area with increased autotrophic productivity and marine influence. The HIX and BIX indices were shown to be useful tools for characterizing DOM in estuarine waters. The study also found that fluorescence intensity and DOC content varied with salinity, with a decrease in fluorescence intensity and DOC concentration in the downstream part of the estuary. The results suggest that DOM in the estuary is influenced by a combination of terrestrial and marine sources, with seasonal variations in primary production and organic matter degradation. The study highlights the complexity of DOM dynamics in estuarine environments, influenced by factors such as salinity, turbidity, and biological activity. The findings contribute to understanding the carbon cycle and the role of DOM in environmental processes.