Methodology for identifying hydrologic landscapes:

Hydrodynamic indices, developed in previous studies, selected here are:

- Sea surface salinity (SS): "river plumes" index.

- Surface temperature (SST, °C).

- Index of stratification of the water column : potential energy deficit which represents the energy required to homogenize (in temperature, salinity or density) the water column with the average density of the water column, the density at depth z, the maximum depth (set here at 100m), and the height of the free surface (Huret M., Struski C, Léger F, Petitgas P., Lazure P. and Sourisseau M. 2009. Modélisation couplée physique-biogéochimie du golfe de Gascogne sur la période 1971-2007. R.INT.DOP/EMH/ 2009-01.)

Turbidity may also play a role in the spatial distribution of pelagic populations. Regarding fish species, it may be advisable to extend the notion of biotope by taking into account the plankton, forming the food supply for certain fish populations, and thus influencing their distribution. This is why the addition of the Chlorophyll-a parameter was considered in this study, in the absence of sufficiently synoptic data for the zooplankton

Sources of data:

To describe the physical properties influencing the spatial distribution of populations, in situ measurements of salinity and temperature throughout the water column are used.

In the Mediterranean, the MEDAR / MEDATLAS (2004) climatology brings together in-situ measurements collected during oceanographic campaigns for over a century. Estimates by vertical interpolation making possible to obtain monthly averages over the entire water column (at irregular intervals). The spatial resolution for the entire Mediterranean is 0.2 ° but a finer resolution of 0.05 ° is available in a smaller area around the Gulf of Lion, used for thsi analysis. This area does not cover the entire sub-region of the western Mediterranean marine MSFD but offers more relevant results.

For the other three parameters, estimates of monthly averages (2003 to 2010) of the surface temperature and concentrations of inorganic suspended solids and for chlorophyll a in the surface layer are made possible through the use of satellite images. The spatial resolution of the data is 0.044 ° in longitude and latitude (cf. thematic contribution "chlorophyll") [10]. The monthly climatic data type used here can not make it possible to capture the spatial and temporal variability associated with hydrodynamic structures at mesoscale, which characterize the hydrodynamics of the Mediterranean. These transient structures, such as eddies, frontal zones and upwellings, can explain the spatio-temporal distribution of some populations.

In addition, other indicators suggested in previous studies, contributing to potential energy deficit and describing the characteristics of the water column in an integrated manner were not included in this analysis (depth of the mixed layer and maximum vertical temperature gradient ). These indices have proven relevant for the identification of hydrological landscapes in other marine areas under the MSFD but calculated from MEDATLAS data, they failed to identify relevant hydrological structures regarding the understanding of the hydrodynamics of this region. In contrast, the surface temperature, which was set aside for the combined analysis of the Channel, Celtic Sea and the Bay of Biscay, in order to avoid a too strong latitudinal stratification that induces the classification of habitats, has been taken into account in the Mediterranean area, which is for more spatially limited. Similarly, chlorophyll a, which does not meet the objective of this study of identifying "natural landscapes", participated to the analysis in the same way as other physical indexes. The addition of these two variables has allowed to discriminate certain hydrological structures which play an important role in the hydrodynamics and the distribution of pelagic populations in this region.