The MoMARSAT cruise series ensures the annual maintenance of the EMSO-Azores observatory on the Lucky Strike vent field. This seabed observatory has been operating since 2010 and 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 - http://emso.eu/), supported in France by the Research Infrastructure (MESR) EMSO-FR whose management is ensured by a collaboration Ifremer-CNRS.

The array includes an observatory infrastructure that comprises a surface buoy (BOREL) ensuring the transfer of data by satellite to a server on land. Two junction boxes (SEAMON) deployed on the bottom communicate acoustically with BOREL and through a cable to the connected instruments. In its current configuration (after MOMARSAT 2020 maintenance), the "connected" part of the observatory includes a surface weather station, a seismometer (OBS), a bottom pressure gauge, a biological observation module (TEMPO- with an HDTV camera and 4 projectors, temperature sensors, oxygen, dissolved iron), an autonomous temperature sensor, two oxygen sensor and a turbidity sensor, a thermistor chain and three hydrophones. During the next cruise, two additional sensors will be deployed: the autonomous sequential hydrothermal fluid sampler (DEAFS) and a generic environmental measurement module (EGIM) that will replace the COSTOF currently servicing SEAMON West.

The infrastructure also includes autonomous instruments which store their data internally: 4 OBS, 2 pressure gauges placed on the bottom, 40 autonomous temperature probes deployed within smokers and on diffusion zones, 7 autonomous current meters placed on the bottom, 4 microbiological colonizers and an oceanographic mooring. Five additional small autonomous cameras will be deployed as part of an experiment to study the restoration of the Bathymodiolus azoricus mussel assemblages after a small-scale disturbance initiated in 2017 on the Montségur edifice. These "unconnected" elements contribute to extend the spatial coverage of the area studied during each maintenance cruise.

Maintenance operations include the replacement of the BOREL-SEAMON infrastructure and the connected instruments, their reconditioning on board and their redeployment. In situ measurements as well as fluids, rocks and biological samples have been taken to complete the set of parameters and calibrate / validate the measurements carried out by the various instruments. A series of chemical characterizations of habitats have also been carried out.

This year, bottom operations have been handled by the manned submersible Nautile. In order to optimize vessel time, a physical oceanography program to study the hydrodynamic circulation on this part of the ridge have been implemented during the available nights.

Associated projects

In 2019, a specific acquisition program (TUSIG, IS BLUE funding) was initiated to characterize small-scale turbulence, map the internal tide and begin geochemical mapping of the hydrothermal plume. This program was enriched in 2020 by the first deployment of the new MicroRiYo @ Sea 3D turbulence observation platform.

The MoMARSAT cruises contribute to WP1, WP2 and WP3 of the European H2020 iAtlantic project. Repeated field visits at Lucky Strike allow the acquisition of a large dataset of underwater images and environmental measurements used for spatio-temporal mapping of habitats and faunal communities at the edifice and field scales (WP2, WP3). Sampling will allow the collection of small B. azoricus mussels in order to carry out an experiment on land to determine the impact of deoxygenation on the behavior of larvae or juveniles (A. Colaço, IMAR, WP4). The data acquired by the observatory will help identify the environmental factors structuring biodiversity and connectivity along the ridge, but also to calibrate high resolution hydrodynamic models (WP1).

Specimens of the Segonzacia mesatlantica crab and Mirocaris fortunata shrimp will be collected, kept alive and transferred to Brest at the center for the oceans "Océanopolis" for the "Abyssbox" general public exhibition started in 2012 in collaboration between Océanopolis, Paris Sorbonne University (UPMC) and Ifremer.

The MoMARSAT cruise and EMSO-Azores observatory are therefore part of the following projects:

• EMSO ERIC (European Multidisciplinary Seafloor and water column Observatory, DG J. Danobeitia)
• EU H2020 iAtlantic (Integrated assessment of the Atlantic Marine Ecosystems in space and time, coordinator M. Roberts University of Edinburgh, grant agreement No 818123).
• Abyssbox: Pressurized aquarium presenting to the general public crabs and shrimp taken from the Lucky Strike hydrothermal field in Océanopolis Brest since 2012. PI D. Barthélémy.

The coordination of maintenance operations and the initial data exploitation are handled by Mathilde Cannat (EMSO France manager) and Pierre Marie Sarradin (Azores node manager). The management of on-board operations has been coordinated by Marjolaine Matabos, Jozée Sarrazin. The data acquired during the Momarsat cruises are available on the portal http://www.emso-fr.org/fr/EMSO-Azores. The study area is part of Portugal's EEZ and is also a "Marine Protected Area" (OSPAR).