Spatial distribution of dissolved free amino acids in three Iberian Atlantic estuaries

Abstract

Dissolved organic matter (DOM) constitutes one of the largest reservoirs of reduced carbon on Earth. Within this pool, dissolved free amino acids (DFAA) represent the most labile fraction, offering valuable insights into DOM composition and diagenetic processes. Our study focused on three Iberian Atlantic estuaries—Guadalquivir, Guadiana, and Tinto-Odiel—examining the longitudinal distribution of DFAA and their response to tidal cycles. Despite similar overall DFAA concentrations between estuaries (176.6 nM to 1770 nM), variations in specific amino acids like glutamic acid, taurine, and aspartic acid pointed to an anthropogenic influence in Tinto-Odiel and the input of fresh organic matter in Guadalquivir. Predominant amino acids—serine (Ser), glycine (Gly), ornithine (Orn), and asparagine (Asn)—comprised over more than 50 mol% in of the estuarine DFAA pool. The dominance of Ser, Gly, and Orn indicated substantial DOM degradation, possibly associated with the loss of labile DOM during estuarine transport. Concurrently, Asn prevalence was linked to allochthonous DOM inputs, particularly associated with terrestrial runoff, lateral input, and anthropogenic activities at estuarine margins. Our results underscore the impact of tidal cycles on DFAA distribution and, emphasize the potential of DFAA studies in unraveling estuarine DOM dynamics and their role as indicators of reactivity and composition in estuarine biogeochemistry.