Water masses and nutrient distribution in the Gulf of Syrte and between Sicily and Libya

Abstract

This paper analyzes for the first time the water masses circulation in the Gulf of Syrte (Libya) and along a Sicily–Libya transect (central Mediterranean Sea) based on a new dataset of hydrological and nutrients data. The collected dataset highlights the presence of three main water masses with different chemical–physical features: Atlantic Water, Levantine Intermediate Water and Deep Water. Atlantic Water shows an intrusive low-salinity water near the Sicilian (≤37.6) and Libyan coasts (≤37.8), linked to the Atlantic Ionian Stream and the Atlantic Libyan Current respectively. The surface circulation evidences meandering structures throughout the area and the presence of an anti-cyclonic vortex in the central part of the Gulf of Syrte. In this latter area no coastal surface current is recognized, suggesting a seasonal character for such coastal circulation. In the Gulf the anti-cyclonic pattern characterizes also the intermediate water circulation. The nutrient distribution confirms the oligotrophic character of the area with a strong reduction in concentration in the surface layer due to the assimilation of phytoplankton in the euphotic zone. Furthermore, there is an evident increase in the deep water caused by the re-mineralization of organisms. The nitrate:phosphate ratio is ~10 and ~30 in the surface waters and deep waters, respectively, the latter being far in excess of the Redfield ratio (16:1) found in the oceans' deep waters. Nutrients data close to the Libyan coast do not show any enrichment pattern as a potential effect of the input of Saharan dust.