Drivers of dissolved organic matter transport in the Guadalquivir estuary (SW, Spain)

Abstract

The Guadalquivir Estuary is the largest in the southwest Iberian Peninsula and is influenced by various anthropic activities, such as damming and rice farming, which regulate river flow and provide lateral inputs to the system. Despite its role as a major source of freshwater and dissolved organic matter (DOM) to the Gulf of Cadiz, there has been limited research on the seasonal distribution of DOM and dissolved organic carbon (DOC) export. This study chemically characterizes DOM using its optical properties along the salinity gradient and during tidal sampling cycles conducted from 2018 to 2022, improving the understanding of DOM dynamics in the estuary. Results show that the DOM in the estuary is predominantly allochthonous, with humic-like components accounting for 85.8 ± 5.8 % of fluorescent DOM (FDOM), while protein-like components, associated with autochthonous processes, account for only 14.2 ± 5.9 %. Mixing and biogeochemical reactivity are the main processes controlling DOM distribution, with mixing being the most significant. Lateral inputs from marshes and rice crops play an important role, particularly in the intermediate zone. Autochthonous DOM is more prominent during the dry season, especially in Summer. The estuary exports 5.8 g C m⁻² yr⁻¹ of DOC, along with substantial amounts of humic-like and protein-like FDOM, especially during extreme rainfall events that increase river flow, and therefore, discharge. DOC transport from the Guadalquivir Estuary is low if we consider the dimensions of its watershed area, highlighting the impact of damming on DOC transport.