Profiling float observation of thermohaline staircases in the western Mediterranean Sea and impact on nutrient fluxes

Characterizing the spatio-temporal arrangements of inorganic nutrients is critical to improve our understanding of the marine biological primary production. Among the processes contributing to nutrient distributions, diapycnal diffusion plays a crucial role for the supply of nutrients to the surface productive zone, and for the equilibration of vertical differences in nutrient concentrations induced by large scale thermohaline circulation. This is the case in the western Mediterranean Sea, where Levantine intermediate waters (LIW), that circulate below the surface layer, regionally distribute the nutrient stocks conveyed from the eastern basin or provided by terrestrial inputs, atmospheric deposition, and remineralization of organic matter. In the present study, we focus on the role played by diffusive processes in the LIW fertilization, considering long-term observations of thermohaline staircases. In association with the unprecedented contribution of profiling floats to explore their structural changes, the fine characterization of western Mediterranean thermohaline staircases sampled during the cruise PEACETIME can be carried out from a different perspective. Observations revealed that thermohaline staircases develop over epicentral regions confined inside large scale circulation features and sustained by saltier LIW inflows on the periphery. As observed in the Algerian Basin, these epicentral regions are thought to be site of active mixing, with changes of seawater properties by about + 0.06 °C in temperature and + 0.02 in salinity during the four years of observation. In-situ lateral density ratios are analysed in the view of theoretical predictions to identify and untangle (i) salt fingering as driver of water mass conversion, with (ii) isopycnal diffusion as spreader of heat and salt from the surrounding sources. In the Tyrrhenian Sea, the resulting nutrient fluxes bring upward from deep waters 5 μmol/m2/d in nitrate, which represents one fourth of LIW fertilization by diapycnal diffusion, but remains a secondary contributor to the enrichment of Ionian water inflows.