The EMSO-Azores deep-sea observatory: 15 years of multidisciplinary studies of the lucky strike hydrothermal system, from sub-seafloor to the water column

Faced with climate change and the increasing interest for the deep ocean and ocean resources exploitation in particular, there is an urgent need for more comprehensive studies of deep-sea ecosystems. Hydrothermal vents are key areas for thermo-chemical exchanges between the lithosphere and hydrosphere and host unique biodiversity. Since 2010, the EMSO-Azores observatory has maintained arrays of multidisciplinary sensors at the Lucky Strike Hydrothermal Field (LSHF; 1700 m water depth), on the slow spreading mid-Atlantic ridge. This infrastructure, combined with repeated yearly sampling during the MoMARSAT maintenance cruises, aims at understanding the feedbacks between tectonics, volcanism and hydrothermal circulation, the coupling between these processes and the hydrothermal ecosystem, and their impacts on the water column. In this review, we summarize 15 years of integrated and multidisciplinary study at LSHF and discuss the societal relevance of this observatory. Hydrothermal circulation is strongly constrained by crust permeability and the availability of magmatic heat. Its variability accounts for the spatial heterogeneity of chemical fluxes and biological communities observed. Tidal modulation of seafloor pressure and near seafloor currents impact venting temperatures, species behaviour, and their physiology. Tidal currents also influence plume dynamics and facilitate particle export to the ocean. At pluri-annual scales, geological events (faulting, seismicity, and magmatic replenishment) have led to changes in fluid chemistry, impacting local microbial communities. However, results obtained over the 15 years of monitoring point to a relative stability of the overall vent system, challenging the idea that hydrothermal vents communities are highly dynamic and ephemeral habitats at decadal timescales. • Longest integrated time-series at hydrothermal vents along a slow-spreading center. • Tidal modulation drives vent flow variability, species behaviour and physiology. • Remarkable pluri-annual stability of the vent system. • 15-year experience can inform international advancement in observing systems. • Observatories provide the means to strengthen the link between science and society.