Abstract
<jats:p>Abstract. The Mediterranean Sea is an oligotrophic basin characterised by anti-estuarine circulation and low productivity. In comparison to the global average, nutrients in the upper water column have high nitrate: phosphate ratios (or N*, nitrate minus phosphate x 16), suggested to arise from high external nutrient inputs. Modified Atlantic Water (MAW) enters with a low inorganic/organic nitrogen ratio, becomes saltier and denser moving eastwards, sinks to form Levantine Intermediate Water (LIW) ultimately feeding the Mediterranean Overflow Water (MOW), supplying nutrients to the Atlantic. This study employed stoichiometric and isotopic analyses of nitrate, dissolved organic nitrogen (DON), and particulate nitrogen to assess the roles of atmospheric deposition and nitrogen fixation on nutrient dynamics within MAW and LIW formation regions. In the western basin, atmospheric deposition, primarily from anthropogenic sources, dominates nitrogen inputs, maintaining a high N:P ratio (N*) and low δ15N (~3.5 ‰) in MAW. Moving eastward, anthropogenic nitrogen inputs decline, lowering the atmospheric N:P contribution, allowing dust-derived phosphorus to be increasingly important and potentially stimulating nitrogen fixation. This further decreases δ15N (< 3 ‰) while altering MAW nutrient stoichiometry (lower N* and nutrient concentrations, higher organic contributions), passing these into initial LIW properties. During LIW formation, dissolved organic phosphorus (DOP) concentration decreased, suggesting DOP hydrolysis via high alkaline phosphatase activity could support nitrogen fixation along with a dust-derived nutrient source. This could contribute 54.94 ± 29.03 Gg N of new nitrogen, alongside direct atmospheric nitrogen inputs (7.4–37.2 Gg N). These new nitrogen additions are transferred to depth, forming the source composition of the LIW and influencing the properties of subsequent water masses formed further along the LIW pathway. At the basin scale, the LIW nitrogen content is largely determined by Atlantic DON, modified by changes in nitrogen inputs from atmospheric nitrogen deposition and nitrogen fixation to the surface water, controlled by phosphorus availability.</jats:p>