Our study focused on three estuaries in the Gulf of Cadiz: Guadalquivir, Guadiana, and Tinto-Odiel. These estuaries, influenced by semi-arid Mediterranean conditions and dam regulations, each have distinct characteristics. The Guadalquivir River has the highest discharge and significant anthropogenic impacts. The Guadiana River features a narrow channel and an extensive wetland. The Tinto-Odiel estuary, known for severe pollution due to mining, has a highly acidic environment and unique water chemistry. We collected surface water samples in July 2017 from coastal and tidal sites in each estuary, capturing data along a salinity gradient. Samples were analyzed for dissolved free amino acids (DFAA) using high-performance liquid chromatography (HPLC) with fluorescence detection. Amino acids were derivatized with o-phthalaldehyde (OPA) and analyzed for concentrations. Our study focused on three Iberian Atlantic estuaries -Guadalquivir, Guadiana, and Tinto-Odiel - that differ in hydrology, land use and DOM sources. We studied the longitudinal distribution of Dissolved Free Amino Acids (DFAA) and their response to tidal cycles across these estuaries. The study was performed during summer 2017.

• Key FindingsDOM Characteristics: Each estuary showed distinct DOM profiles: humic-like materials in Guadalquivir and Guadiana, and protein-like materials in Tinto-Odiel.Amino Acid Analysis: Variations in DFAA concentrations across estuaries reflect differences in environmental and anthropogenic influences.