The site of the "Grand Herbier de la Côte Orientale" (sector 2 of the coast of Corsica, Meinesz et al., 1990) covers more than 70,733 ha. It is divided into five sub-sectors (sectors 2A, 2B, 2C, 2D and 2E). The POSIDCORSE campaign made it possible to finalize its mapping compared to the previous campaign (CORALCORSE) (Figure 2).

Figure 2: Acoustic image mosaics produced during the POSIDCORSE campaign.

The "Grand Herbier de la Côte Orientale" sector is characterized by the presence of a Posidonia oceanica meadow which has a particularly large extension of 20,425 ha (38% of Corsican meadows or 64% of the seabed between 0 and -40m) but it was also possible to identify a significant presence of another marine magnoliophyte, also protected, Cymodocea nodosa which constitutes large meadows (797.6 ha) along the coast (-10 to -15 m) near the coastal rivers or the outlet of coastal lagoons (Figure3).

Figure 3 - Mapping of biocoenoses and bottom types of the "Grand Herbier de la Côte Orientale"

The Posidonia oceanica meadow is characterized by the presence of numerous intermattes and natural erosive structures which are very interesting for understanding the dynamics of this herbarium, but also by numerous anthropogenic traces (trawling, anchorages, dredged material, etc.). At its lower limit, generally located between -34 m and -40 m, this meadow presents very different structures (clear, sparse or regressive limits) and several spots of Cymodocea nodosa are observed in front of the limit. Rhodolith associations are also present but their extension seems to be much more limited than at the level of Cap Corse (Bonacorsi, 2012), in particular due to a fine fraction content more marked in the sediments of the eastern coast and undoubtedly linked to fluvial inputs. The presence of beach rocks, unique on the scale of Corsica, is clearly identified in the sonograms; these formations were verified with the ROV during the campaign and similar, shallower structures were investigated while diving. Figure 2: Mapping of the biocenoses and types of seabed of the "Grand Herbier de la Côte Orientale". The characterization of the matte and in particular its thickness could be successfully tested by seismic reflection during the POSIDCORSE 2015 campaign thanks to the use of three acquisition systems: the Sparker (1 kHz), the sediment sounder (SDS) EDO  (2.5 kHz) and SDS PeskAvel (3.5 kHz). The application of this non-destructive geophysical method has made it possible to obtain a detailed representation of the sedimentary layers along the eastern coast of Corsica. To our knowledge, this acquisition (~ 1,380 km of seismic profiles) constitutes one of the most important investigative efforts carried out within the framework of an inventory of Blue Carbon stocks on a Mediterranean scale (Lo Iacono et al. al., 2008; Tomasello et al., 2009). In view of the results obtained, the velocities of the acoustic wave in the mat (n = 367) is estimated between 1631.9 and 1696.8 m.s-1 (average: 1664.4 m.s-1). The results showed that the thicknesses of mattes, estimated on average at 251.9 cm, were greater in shallow areas (-10 m to -20 m), near the mouths of coastal rivers (eg Golo, Tavignano and Travo-Solenzara) and graus from the lagoons (eg Diana and Urbino), where mats up to 867 cm thick have been found, and locally in the deep part (between -30 m and -40 m). Based on the area occupied by Posidonia oceanica meadows and the thicknesses recorded on the study site (Figure 3), the volume of matte was estimated at 403.5 ± 49.4 million m3 (ratio: 2.2 m3 / m2).

 Figure 4 - Matte thicknesses along the eastern coast of Corsica (A) and uncertainty on the measurement by kriging (B)

 Finally, several anthropogenic impacts likely to degrade Posidonia matte have been identified. These pressures are particularly worrying because they are likely to cause not only a regression of the p.o. meadow, and therefore a decrease in carbon fixation, but also a dismantling of the matte and a release of the carbon stored there. This is the anchoring of large units (tankers, cruise ships, pleasure craft) at the origin of the opening of real trenches in the p.o. meadow, and trawling which erodes the top of the mattes (Figure 5).

Figure 5: Detail of a sonograph showing traces of anchoring in the p.o. meadow (in dark gray) leading to the formation of dead mattes (in light gray)  and diving photography of a trace.