Dark-ocean water mass boundaries and mixing zones as "hot-spots" of biodiversity and biogeochemical fluxes across the Mediterranean Sea and Eastern North Atlantic (HOTMIX)

This project will study the influence of mixing zones between water masses on the metabolism and biodiversity of microbial communities, and their impact on biogeochemical fluxes, in the deep ocean. The project will be carried out in the Mediterranean Sea as an example of experimental laboratory for its dynamic characteristics, with turnover times an order of magnitude smaller than in the open ocean (11 to 100 years). We will follow the evolution of the Levantine Intermediate Water (LIW) from its generation, in the eastern basin of the Mediterranean Sea until its flow to the North Atlantic through the Strait of Gibraltar. During its westward flow the LIW mixes both with surface water of Atlantic origin and deep Mediterranean waters, formed both in the eastern (Adriatic and Aegean Seas) and western (Gulf of Lions) basins. After outflowing through the Gulf of Cadiz, the LIW sinks down and spills over into the eastern Atlantic Ocean, mixing with different Atlantic waters, like the North Atlantic Central Water (NACW), the Subpolar Modal Water (SPMW) and the Labrador Sea Water (LSW). We will reproduce the "in situ" conditions (temperature, pH and hydrostatic pressure) in the deep ocean to study microbial metabolism using advanced methodologies. We will also examine the relationship between diversity, microbial metabolism and the elemental and molecular composition of the available organic matter at these interface regions. Finally, we will compare estimates of metabolic rates with geochemical approaches based on multiparametric analysis of water masses (OMP) and age estimates from transient tracers, helping to resolve the paradox of the imbalance between sources and sinks of carbon in the deep ocean.