The main objective of the observing system MOOSE certified as « SNO INSU », integrated in « IR ILICO » and recently re-certified as SNO for 2020-2024 is to monitor the long-term evolution of the north-western Mediterranean Sea (over more than 10 years) in the context of climate change and anthropogenic pressure in order to detect and identify the trend and environmental anomalies of the marine ecosystem. The MOOSE network aims to establish an integrated and multidisciplinary system in the Mediterranean Sea in accordance with the objectives of the national MISTRALS program (HyMeX, MeRMEX and ChARMeX). The MOOSE system is supported by national institutes (CNRS-INSU, French Ministry of Higher Education and Research) and involved different partners (Universities, IFREMER, Meteo France).

The MOOSE network includes "multi-scale" measurement capabilities to accurately document the broad spectrum of hydrodynamic processes already identified (large scale circulation, (sub)mesoscale eddies, biogeochemical provinces). High temporal resolution measurements are obtained from fixed observatories (moorings, buoys) but their spatial distribution remains insufficient. Spatial variability is of the same order as temporal variability and understanding the evolution of this basin as a whole implies being able to dissociate both. Synergy with other strategies (ships, floats, gliders) is essential for the establishment of an optimized observation network in such a system. To address the issues identified by MOOSE, two key areas of the north-western basin have been identified:

• The central and western part of the Ligurian Sea, which constitutes a homogeneous system isolated from direct coastal inputs by rivers and where atmospheric inputs are predominant (DYFAMED and ANTARES). It is also one of the entrance passages of the Intermediate Levantine Water (LIW) in the north-western Mediterranean basin.
• The central area of the Gulf of Lion where winter cooling leads to vertical mixing over 2000 m and sometimes to the bottom. The LION site (42°N 5°E) is ideal for studying the variability of winter convection to better understand mixing processes and dense water formation. It also characterizes the variability of the deep particle flow.

Currently, fixed observation at these sites is carried out by 5 moorings:

• The Planier and Lacaze-Duthiers moorings composed of sediment traps and T/S sensors and current meters, for dense water cascading and particle export studies. These moorings have been set up since 1994 and managed by CEFREM.
• The LION mooring, consisting of a large number of T/S sensors, current meters, and two oxygen sensors, is in the Gulf of Lion convection zone. It has been deployed since 2007, and is managed by CEFREM and LOCEAN. The LIONCEAU mooring that was positionned nearby with a sediment trap near the bottom has been integrated into the LION mooring in 2019.
• The ANTARES mooring is located in the North Current off Toulon and equipped with T/S sensors, current meters and oxygen sensors to quantify the bacteria activity and organic matter remineralization process in a deep marine environment. It exists since 2004, it is managed by the M.I.O. and the CPPM (Marseille). This mooring is part of the ERIC EMSO since 2017.
• The DYFAMED mooring, in the Ligurian Sea, equipped with sediment traps, T/S sensors, current meters and oxygen sensors to monitor the evolution of the water column, the impact of atmospheric dust deposition and marine particles export to deep waters. It exists since 1988, it is currently managed by the Oceanological Observatory of Villefranche-sur-Mer. This mooring is part of the ERIC EMSO since 2017.

This network of mooring lines is one of the "bases" of the MOOSE observation network. The maintenance of these moorings must be carried out every year and forms the basis of the annual MOOSE-GE campaigns. The maintenance of these equipment and the calibration of all the sensors require important operations which are carried out in close collaboration by the various teams of the laboratories involved in the MOOSE network. Additional hydrological measurements are obtained in subsurface (0-250 m) from sensors installed under the Météo France buoys (ODAS Cote d'Azur and Lion buoys). Their maintenance was carried out during this cruise.

During the MOOSE_GE cruises, the transits between sites are used to make hydrological stations sections covering the whole area (42N5E - Balearic Islands - Ligurian Sea - North Corsica). These hydrological stations are used: 1) to calibrate the mooring sensors, 2) to complete their measurements on a larger spatial scale and 3) to provide additional data, only available from seawater sampling (nutrients, dissolved inorganic carbon, phytoplanktonic pigments, plankton species...). This network of stations, carried out in a "quasi-synoptic" manner (in less than 3 weeks), providing large-scale annual mapping of the water masses with their hydrological, chemical and biological characteristics. It is a snapshot of the physical and biogeochemical structure of the NW basin and an essential complement to the higher temporal frequency monitoring carried out at a few key points in the north-western Mediterranean.

The MOOSE-GE campaigns aim: 1) to ensure coordinated and shared maintenance of the French Mediterranean deep moorings and 2) to map the hydrological, biogeochemical and biological characteristics of the water masses representative of the entire north-western Mediterranean basin.