The site where the mooring is deployed and where monthly cruises are carried out is located in the Ligurian sea, one of the sub-basins of the Western Mediterranean sea (Figure below). The water depth is varying between 2350 and 2500 m in this area, and it is exactly 2440 m at the mooring point, which is by 7°54’E,43°22’N.

A precise knowledge of the water depth was critical in order to deploy the taught mooring that have been developed in this project. This depth was determined by performing several deep CTD casts, from which the measurement of the pressure, once translated into a depth thanks to the simultaneous measurement of the temperature and salinity, was added to the measurement of an altimeter (the CTD being stopped at about 10 meters before the bottom is reached). The water-column thickness was therefore determined with a <1 meter accuracy. These measurements were used to calibrate a two-dimensional mapping of the sea bottom that was obtained from the ship echo sounder. This mapping revealed a flat bottom characterized by a weak NW to SE slope (see bathymetry map below).

The tide amplitude, the dynamic height changes and the water level variations due to atmospheric pressure changes are all less than about 50 cm, which is not significant in the frame of this project. They might become problematic in case a similar mooring would be deployed in another sea where such changes would be larger.

Left map : The Northwestern Mediterranean sea, with indication (black star) of the location of our work area. Right map : a zoom from the general map, showing the position of the BOUSSOLE site (black star), the DYFAMED site (circle), and of the meteo buoy (triangle) maintained by the French weather forecast Agency (“METEO France”). The positions of the 6 stations that are sampled once a month from Nice to the BOUSSOLE site are also displayed (open stars).

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Bathymetry of the area including the BOUSSOLE site (at the crossing of the horizontal and vertical lines). The open stars indicate the location of the individual deep CTD casts that allowed the echo-sounder survey (grey dots) to be calibrated in terms of water depth.   

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This site has been selected in particular because currents are extremely low. This peculiarity is due to the position close to the center of the cyclonic circulation that characterizes the Ligurian sea. The Northern branch of this circulation is the Ligurian current, forming a jet flowing close to the shore in the NE to SW direction and creating a front whose position is seasonally varying, closer to shore in winter than in summer. The southern branch is a SW to NE current flowing north of Corsica and the eastern part of the circulation is simply imposed by the geometry of the basin.

Dominant winds are from the west to Southwest and from the Northeast sectors (wind rose below), and are channeled into these two main directions on the one hand by the general atmospheric circulation of the region, and, on the other hand, by the topography formed by the Alps and Corsica. Over the past 4 years (February 1999 to July 2003) only 5 days were recorded with an established wind speed above 40 knots, then respectively 34 and 100 days with wind speeds between 35 and 40 knots, and between 30 and 35 knots. These high wind speeds, and the associated large swells are concentrated from November to March.

This is as well illustrated on the multi-panel figure below, where physical conditions are displayed as a 4-year record of the wind speed, significant wave height and sea surface temperature (SST). These data are obtained from a weather mooring, which is distant by two nautical miles from our station, and which is maintained by Meteo France, the French weather forecast Agency.

The minimum SST is about 12.7°C (associated with a salinity of 38.4 psu), which is a constant value reached in winter when the water mass is fully mixed down to the bottom. This deep mixing contributes to the formation of the bottom waters of the Western Mediterranean sea.

Wind rose at the BOUSSOLE site (data from the meteo buoy maintained by the French weather forecast Agency).

Time series of the sea-surface temperature (top), significant wave height (middle; the H1/3 parameter is the mean wave height computed over the highest third of the wave height distribution), and mean wind speed (bottom), as recorded by the meteo buoy maintained by the French weather forecast Agency in the vicinity of the BOUSSOLE site. The bold curves are running averages computed from hourly measurements (dotted curve).

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Oligotrophic conditions prevail at the BOUSSOLE site in summer with chlorophyll a concentrations below 0.1 mg m^-3, with minima around 0.05 mg m^-3. The higher concentrations are up to about 2 mg m^-3, mostly during the early spring bloom (February to March). Sporadic and intermittent enhancements occur in winter when sunny weather occasionally and temporarily stabilizes the nutrient-rich waters. Moderate concentrations, between 0.1 and 0. 2 mg m^-3, characterize most of the other periods of the year. This is illustrated below.

Time-series of the chlorophyll-a concentration at the DyFAMED site, in the vicinity of the BOUSSOLE site (figure reproduced from Marty et al., 2002, Deep-Sea Res.).