Seasonal distribution of dissolved organic matter in the Gulf of Cadiz

Abstract

The Gulf of Cádiz (GC) is a semi-enclosed basin that connects the North Atlantic Ocean and the Mediterranean Sea playing a key role in the exchange of organic matter between the two basins through the Strait of Gibraltar. Moreover, four rivers, with different organic matter quality, flow into this basin discharging large amounts of freshwater to the Atlantic Ocean (González-Ortegón et al. 2018). On the other hand, chlorophyll levels followed a seasonal pattern with highest concentrations at coastal zone during spring (González-Garcia et al. 2018). In the GC, vertical and seasonal dynamics of dissolved organic matter (DOM) were assessed using optical properties during four oceanographic cruises. Fluorescence DOM (FDOM) was analyzed combining emission-excitation matrixes and multivariate parallel factor analysis (PARAFAC, Murphy et al. 2013, 2014). Five widely distributed PARAFAC components/fluorophores were identified; a marine humic-like, two terrestrial humic-like and two protein-like that resembles the amino acids tryptophan and tyrosine. Seasonal differences were observed in FDOM components indicating changes in the DOM quality and source. During summer, the fluorophore tryptophan-like accounted for almost 80% of the total FDOM pool. On the other hand, humic-like compounds dominated the FDOM pool in autumn and winter. Marine humic-like substances made 30-40% of total fluorescence and terrestrial humic-like compounds accounted for ~50%. Source and sinks of FDOM giving rise to the observed seasonal distribution will be discussed