The Mediterranean Sea is a mosaic of geographical, physical and ecological domains with very much diversity and contrasts. The major issue is to develop an in situ observing system, capable of capturing all scales of variability, while avoiding any aliasing effects caused by sub-sampling. This observing system needs to be able to address scales of variability ranging from the very small scale (1km horizontally, few days) to the basin scale (500km, months/years), passing by the mesoscale (15km, weeks/months).
Moreover, in order to build an observation network that would be homogeneous and efficient, it is crucial to better structure the relation between the littoral system, ideal to observe anthropogenic impact, and the open ocean system, a perfect area to study climate change issues. However the range of the space and time scales of the relevant processes is wide. In addition to seasonal or inter-annual variability, the impact of extreme events that control fluxes and budgets in marine ecosystems must be characterized.
This is a very challenging goal, as in situ observing systems have different components complementing information on continental/atmosphere /sea interactions and on the whole water. The field strategy of the MOOSE Network is based on a multisite system of continental/shelf and deep-sea fixed stations as well as Lagrangian platform network to observe the spatio-temporal variability of processes interacting between the coastal-open ocean and the ocean-atmosphere components. (see map figure 2).:
- 2 rivers observatories
- 3 observatories for atmospheric deposition:
- 6 offshore moorings
- 2 HF radars
- Monthly and annual ship transects
- Lagrangian and mobile platforms: 2 glider endurance lines and profiling floats
The use of gliders significantly increases the spatio-temporal coverage for a number of parameters, and provides with a better view of the oceanic variability when deployed simultaneously along the two endurance lines. Gliders are a relatively new technology and autonomously profile from the surface to large depths along sections across basins. Gliders can be considered as ‘steerable’ profiling floats and so, as an active tool able to cover a variety of scales and complement the other components of the observing system. They actually fill the gaps between observations at fixed points, repeating sections in an autonomous way between moorings.
In the MOOSE concept, gliders follow a priori fixed transects (called the “endurance lines”) to observe on the long term key areas as the continental slope, the convection zone and the coastal-open ocean exchange. Two endurance lines are currently operated: Villefranche—Dyfamed-Calvi and Marseille-42N5E-Menorca.
In MOOSE, two river systems have been selected for high frequency monitoring of river loads. These are the Rhône River, nowadays the largest freshwater source for the Mediterranean Sea, and the Têt River, targeted as representative for the functioning of the numerous coastal rivers which discharge to the North-Western basin. Both rivers are equipped with automatic sampling stations close to the river mouths (station SORA at Arles on the Rhone, station POEM-River at Villelongue on the Tet), allowing them to be sampled in daily time steps during low water stages, and in hourly time steps during floods.
The MOOSE atmospheric stations are located all around the French NW Mediterranean coast (Cap Ferrat, Frioul, Cap Bear) providing an east-west gradient, ideal to study anthropogenic impacts and Saharan dust events. Consequently, the MOOSE project aims to maintain such atmospheric survey for the atmospheric inputs study in collaboration with the Working Group of ChArMeX “Deposition of nutrients and Hg”.
all year round are accomplished by combining observations from moorings, lagrangian floats and gliders, ship surveys both in the coastal and open sea regions.
In addition to the Eulerian and Lagrangian observatories, ship transects are essential to acquire more parameters that cannot be obtained by autonomous technologies (glider, float, buoy, satellite) but still necessary for a complete marine environment observation. These in situ data concern essentially chemical (nutrients, CO2, pH) and biological (pigments, plankton description) variables.
The ship survey strategy is based on monthly cruises at 3 sites (MOLA, ANTARES, DYFAMED) using existing facilities during which CTD casts are carried out to record hydrodynamic characteristics of the entire water column (surface to bottom) using standard protocols and references that can be used to calibrate sensors installed on autonomous platforms (mooring lines, buoys, floats and gliders). Niskin bottles are also used during the CTD deployments to collect samples for discrete parameters acquisition (nutrients, oxygen, TCO2-alkalinity, pigments) as well as net sampling for plankton recording. The ship observations also implement modern technologies for having a 2D vision of biological community (size spectrum) by using autonomous instruments: Underwater Video Profiler, Laser Optical Plankton Counter.
An annual cruise MOOSE-GE covering the whole MOOSE domain is mainly dedicated to moorings maintenance. In addition to this maintenance, deep CTDs transects are also performed during this cruise in order to calibrate mooring sensors, to sample on a larger (sub-basin) scale and to measure variables only accessible with water samples (nutrients, dissolved inorganic carbon, phytoplanktonic pigments, zooplankton nets…). Such a CTD survey with cross-basin transects centered on the deep water formation area in the early summer allows to monitor the variability of the water masses of the basin (surface, intermediate and deep) and in particular the amount of the deep waters formed during the previous winter.
The observation of the coastal to offshore transfers along the shelf (canyon export) is represented by one dual station: MOLA station located 19 nautical miles offshore from Banyuls-sur-Mer on the northern flank of a canyon (600m depth) and Lacaze-Duthiers station located in the canyon axis (1,000m deep) and equipped with an instrumented mooring line (sediment traps, current meters) for the particulate flux transferred to the basin (program BILLION; Heussner et al., 2006). As well as Lacaze-Duthiers site, Planier site is equipped to record the occurrence and effects of dense shelf water cascades from the shelf of the Gulf of Lions.Concerning the deep basin, some key areas have been identified: the central part of the Ligurian Sea where atmospheric inputs are predominant (DYFAMED), the central zone in the Gulf of Lion for deep water formation (LION-42N5E/HC) and the ANTARES station located in the Northern Current pathway.
At present, the Eulerian observation is performed by 6 immerged mooring lines:
- Planier and Lacaze-Duthiers (sediment traps and hydrodynamic sensors) for dense water cascading and particle flux studies (since 1994, operated by CEFREM)
- LION (hydrodynamic sensors near the bottom) for hydrology monitoring of deep waters (since 2010, operated by LOCEAN and CEFREM)
- LIONCEAU (sediment traps and hydrodynamic sensors) in the convective zone of Gulf of Lion (operated since 2012 by LOCEAN, CEFREM)
- ANTARES (hydrodynamic sensors near the bottom) for hydrodynamic and organic matter remineralization in deep water (operated since 2004 by MIO)
- DYFAMED (sediment traps and hydrodynamic sensors) for atmospheric and marine flux transfer to the surface and deep waters (operated since 1988 by OOV)