The MoMARSAT cruises serie ensures the annual maintenance of the EMSO-Azores observatory on the Lucky Strike vent field. This seabed observatory aims to acquire long time-series data (>10 years) on hydrothermal, tectonic, volcanic processes and the associated ecosystems of an active hydrothermal field located on the Mid-Atlantic Ridge. EMSO-Azores is part of the European network EMSO ERIC (European Multidisciplinary Seafloor and water column Observatory), supported in France by the Research Infrastructure (MESR) EMSO-FR whose management is ensured by a collaboration Ifremer-CNRS.

The EMSO-Azores observatory at Lucky Strike has first been deployed in 2010. It is primarily directed at understanding a mid-ocean ridge black smoker hydrothermal system, from the magma chamber that supplies the heat, to the microhabitats that host the hydrothermal fauna, and to the hydrothermal plume in the water column. The maintenance procedure is performed every year and requires the use of a deep-sea submersible. It includes recovery of seafloor instruments and moorings, full onboard checkup and repairs of each component, and redeployment after reconditioning on board. In addition, we perform yearly sampling of fluids, rocks and fauna, and capture videos of vents and habitats that provide critical ground-truth information to instrumental time-series data. The Lucky Strike hydrothermal field is located in the center of one of the most volcanically active segment of the MAR.

In its actual configuration after the 2024 maintenance cruise, the observatory infrastructure is composed of two Sea Monitoring Nodes (SEAMON). The first node dedicated to large scale geophysical studies is moored in the center of the fossil lava lake characteristic of the Lucky Strike vent field. This node hosts an Ocean Bottom Seismometer (OBS), a pressure gauge (JPP) and a generic environmental measurement module (EGIM). The EGIM is equipped with a CTD, an optode, a turbidimeter, a hydrophone and an ADCP. The second node, SEAMON East, is deployed at the base of the Tour Eiffel active edifice to study the links between faunal dynamics and physico-chemical factors. This node is composed of an Aanderaa optode and a Wetlab turbidimeter. An HD video camera equipped with 2 LED lights is located 10 meters away on the edifice. An array of 4 Ocean Bottom Hydrophones (Hydroctopus) is deployed south of the active edifice.

The MoMARSAT experimental design also includes autonomous instruments : 4 OBS, 7 autonomous current meters and a temperature probe array (29 autonomous probes deployed in smokers and diffusing areas), a sequential fluid sampler (DEAFS), 4 autonomous video cameras, 11 biological and microbiological colonization devices. Site studies including video surveys, sampling (rocks, water and organisms), and monitoring of long-term colonization/perturbation experiments are also performed during the cruises.The 2 pressure gauges (JPP) were retrieved this year for maintenance and replacement of their stand that appeared to be corroded. In situ sampling of rocks, fluids, fauna, microorganisms and the acquisition of imaging transects on targeted sites allow the multi-year monitoring of the system and complete the infrastructure data. These measurements are also used to calibrate/validate the measurements made by the instrumental array.Imagery surveys were conducted to map geological and biological features at the vent field scale as well as scientific gear and dead weights. These maps will be shared with the Azoreans government to inform marine protected area management (PROTECT project).

The MoMARSAT 2024 campaign and the EMSO-Azores observatory are part of the following projects:

• EMSO ERIC (European Multidisciplinary Seafloor and water column Observatory, www.emso-eu.org/, DG J. Danobeitia).
• Collaboration with Telmo Morato (Univ. Azores) will help to complete the mapping of the area as part of Seabed 2030.
• Deep-Rest (European BIODIVERSA project, coordinator J. Sarrazin, Ifremer). This project aims to develop new approaches to inform environmental managers. The mission will contribute to the implementation of experiments to assess the growth rate of dominant species and the response of communities to a plume of hydrothermal sediment.
• Lifedeeper (PPR project, coordination MA Cambon, Ifremer) also aims to inform environmental conservation strategies in the context of deep-sea mining. The acquisition of images and the sampling of individuals will contribute to a better understanding of species distribution and connectivity along the ridge.
• Abyssbox: Pressurised aquarium displaying crabs and prawns collected from the Lucky Strike hydrothermal vent. PI D. Barthélémy (Océanopolis). Segonzacia mesatlantica crabs and Mirocaris fortunata shrimp are collected, kept alive and transferred to Brest at Océanopolis for the public exhibition "Abyssbox" started in 2012 in collaboration with the University of Paris Sorbonne (UPMC) and Ifremer.
• ANR IRONWOMAN (ANR-21-CE02-0012, PI C. Rommevaux 2021-2025): the sampling of iron-rich microbial mats will validate the hypothesis of the key role of marine iron-oxidising bacteria (FeOB) in the development of iron-rich mats, and the impact on the biogeochemical cycle of iron and primary production in the deep oceans, depending on variations in the environmental conditions involved.
• ANR HotHg: fluid samples will help to assess the flux, speciation and transformation of mercury in hydrothermal springs.
• SMASH: an experiment has been deployed in 2021 to study the degradation of biodegradable polymers. Some of these samples will be recovered this year.
• VentRythm :Biological rhythms are essential to life, but the deep ocean is often considered an acyclic exception. However, tidal reversals of local currents, which cause extreme but predictable changes in physico-chemical parameters at vents, can explain the appearance of biological rhythms whose period corresponds to that of the tides. In this project, funded by the Human Frontier Science International Program (HFSP), we aim to understand the evolution and collective emergence of biological rhythms in the deep environment and associated sensory systems (i.e. chemicals, biotic or abiotic light), from the molecular level to that of the organism and the multi-species community. This approach is the first to integrate temporal organisation into the study of hydrothermal ecosystems.
• The PROTECT project aims to assess the impact of research activities on the Lucky Strike hydrothermal vent where 36 scientific missions have taken place since 1992. After compiling and mapping all research activities and pressures, including ROV trajectories (light, vibration and noise) and sampling, mooring and waste locations, cumulative human pressure indices will be calculated to assess the potential impact of human activities. A collaboration has recently been initiated with the government of the Azores to use the infrastructure as a tool for managing marine protected areas. The government is currently redefining the spatial extent and regulations of MPAs. The EMSO-Açores team will act as a consultant, drawing on the results of the observatory and providing maps.
• BioBlue: Coordinator Pierre-Pol LIEBGOTT (MIO), partner C. Rommevaux, Fondation AMIDEX Aix-Marseille funding: prospecting for biohydrogen production by coupling obscure fermentations and bioelectrochemical methods producing energy vectors in thermophilic and halophilic conditions.

Bottom operations were carried out by the Remotely Operated Vehicle VICTOR6000 onboard the Research Vessel L'Atalante. Maintenance operations and initial data processing are coordinated by Pierre Marie Sarradin (Azores Regional Team leader). Onboard operations were managed by Pierre-Marie Sarradin, Marjolaine Matabos and Laurent Gautier. The data acquired during the cruises are available at https://www.emso-fr.org/fr/EMSO-Azores according to European directives.. Sample transfers to the various teams and projects involved are formalised by MTAs.  The study area is part of Portugal's EEZ and an OSPAR Marine Protected Area.

We took  advantage of the ship transit from Horta to Lucky Strike and return to acquire MultiBeam Sonar data for the Instituto Hidrográfico (IH) in Portugal and contribute to the mapping effort around the Azores. The two mapping areas were defined in collaboration between Ifremer (PI Benoît Loubrieu), the IH Portugal mapping division and colleagues from the University of the Azores (contact Telmo Morato).

The EMSO-Azores observatory is part of the One Ocean Network for Deep Observation action of Ifremer endorsed by the UN Ocean Decade program (https://www.oceandecade.org/actions/one-ocean-network-for-deep-observation/).