Distribution of dissolved organic matter in estuaries of the southern Iberian Atlantic Basin: Sources, behavior and export to the coastal zone

Abstract

The Gulf of Cádiz, located in the Southern Iberian Atlantic Basin, connects the North Atlantic Ocean and the Mediterranean Sea and receives freshwater input from three main estuaries: Guadalquivir, Guadiana and Tinto-Odiel. These estuaries differ in their hydrology, basin characteristic and land use. However, little is known about the export of DOM from these estuaries to the Gulf of Cádiz. In this work, the estuaries were sampled during the dry season. Dissolved organic matter (DOM) was chemically characterized along a longitudinal gradient to better understand the reactivity and fate of carbon exported to the Gulf of Cádiz. Moreover, a tidal study was also performed at the mouth of each estuary to understand how the tide affects the DOM composition and its export to the coast. Fluorescent DOM (FDOM) modeling employing the multivariate parallel factor analysis, and analytical pyrolysis was used to characterize DOM. DOM from Guadalquivir and Guadiana estuaries presented a predominant allochthonous origin with humic-like compounds making up to ~80% of the total fluorescent DOM. These estuaries received lateral inputs from surrounding watersheds and agricultural practices. Instead, Tinto-Odiel estuary DOM was predominantly autochthonous with a higher content of protein-like material associated to the prevalence of the marine influence over the low water discharges. Tidal cycles affected DOM distribution and its quality with higher humic-like material during low tide and protein-like substances predominating during high tide. During the dry season, the three estuaries represented a source of DOM to the Gulf of Cádiz. Guadalquivir was the main contributor with 104.0 mol C s-1 and 0.22 x 103 m2 s-1, for DOC and CDOM, respectively. Around 70 % of the FDOM exported from the three estuaries was of humic nature. The results obtained in this work highlight the importance of local conditions, such as the basin characteristic and estuarine biogeochemical reactivity in the composition of the DOM exported to the coastal ocean. Moreover, it shows that analytical pyrolysis is a very useful technique to characterize DOM origin and composition.