The study investigates the properties of fluorescent dissolved organic matter (DOM) in the Gironde Estuary, a transitional zone between the continent and the Atlantic Ocean. The estuary is characterized by high turbidity and significant exchange between freshwater and marine waters. Samples were collected during three cruises in 2002 and 2006, analyzed using excitation-emission matrix (EEM) spectroscopy, and characterized by fluorescence indices such as the humification index (HIX) and the index of recent autochthonous contribution (BIX). The results show that fluorescent DOM and dissolved organic carbon (DOC) do not behave conservatively in the estuary, with DOC content varying seasonally and DOM fluorescence showing less seasonal variation. The estuary is divided into three zones based on salinity: a turbid zone with low salinity and high suspended particulate matter (SPM), a mid-estuarine zone with low autotrophic productivity and strongly degraded organic material, and a higher salinity area with increased autotrophic productivity and marine influence. The HIX and BIX indices are useful tools for defining and classifying DOM characteristics in estuarine waters. The study highlights the complex dynamics of DOM in estuarine environments, influenced by factors such as mixing of freshwater and marine waters, primary production, and photo-degradation.The study investigates the properties of fluorescent dissolved organic matter (DOM) in the Gironde Estuary, a transitional zone between the continent and the Atlantic Ocean. The estuary is characterized by high turbidity and significant exchange between freshwater and marine waters. Samples were collected during three cruises in 2002 and 2006, analyzed using excitation-emission matrix (EEM) spectroscopy, and characterized by fluorescence indices such as the humification index (HIX) and the index of recent autochthonous contribution (BIX). The results show that fluorescent DOM and dissolved organic carbon (DOC) do not behave conservatively in the estuary, with DOC content varying seasonally and DOM fluorescence showing less seasonal variation. The estuary is divided into three zones based on salinity: a turbid zone with low salinity and high suspended particulate matter (SPM), a mid-estuarine zone with low autotrophic productivity and strongly degraded organic material, and a higher salinity area with increased autotrophic productivity and marine influence. The HIX and BIX indices are useful tools for defining and classifying DOM characteristics in estuarine waters. The study highlights the complex dynamics of DOM in estuarine environments, influenced by factors such as mixing of freshwater and marine waters, primary production, and photo-degradation.