CHARACTERIZATION OF BENTHIC ASSEMBLAGES IN THE FRAMEWORK OF A NEW MPA PROJECT ALONG THE NORTH COAST OF CORSICA

GIS Posidonie

Pergent-Martini, C.1, Bonacorsi, M. 1, Clabaut, P2., Pergent, G. 1

1 – UMR 6134/FRES 3041, University of Corsica, 20250 Corte, France - pmartini@univ-corse.fr; bonacorsi@univ-corse.fr; pergent@univ-corse.fr; 2 – Clabaut Consultant – 62240 Selles, France – philippe.clabaut@wanadoo.fr

Abstract

In the Mediterranean Sea, seagrass meadows, coralligenous assemblages and rhodolith beds appear as key habitats and form the basis of its specific richness. If the extension of Mediterranean seagrass is relatively well known and often include in Marine Protected Areas, there is a considerable lack of information concerning the distribution of coralligenous and rhodolith beds. The designation of two «  Natura 2000  » sites, along the coastline of Cap Corse (North Corsica, France) gave the opportunity to investigate, during two oceanographic campaigns, the bottom between the surface and 100 m depth, with the support of the “Agence des Aires Marines Protégées”. These investigations, realized with a side-scan sonar and a multibeam echosounder, allow to map an area of 750 km2. Coralligenous and rhodolith beds appear well represented, with more than 112  km2 occupied by rhodolith beds. In addition new coralligenous assemblages (coralligenous “atolls”), never described, were discovered in the northern part of Cape Corse (15 miles from the Corsican coasts). These results demonstrate the interest of this area and confirm its designation as « Natura 2000 » marine sites. In conclusion, it will be interesting, according to the high value of this area in term of biodiversity, to implement a marine park, including the coralligenous “atolls”. This allow to both create a Marine Protected Area with a “high-sea” component and to increase the representativity of these important habitats in the existing MPA.

General context Seagrass meadows and coralligenous are considered as the main Mediterranean benthic ecosystems (Fig. 1). Coralligenous assemblages consist of a hard substratum of biogenic origin that is mainly produced by the accumulation of calcareous encrusting algae growing in dim light conditions and in relatively calms waters [1]. Mediterranean rhodoliths beds should be defined as sedimentary bottoms covered by a carpet of free-living calcareous algae (Corallinales or Peyssonneliaceae) also developing in dim light conditions [2]. These bioconcretions seem common at the global level, but their distribution poorly known [3]. The designation of Cap Corse as Natura 2000 sites give the opportunity to study the distribution of the main benthic habitats.

Bibliography [1] E. Ballesteros, Mediterranean coralligenous assemblages: a synthesis of present knowledge. Oceanogr. and Mar. Biol. - An Ann. Rev. 44 (2006), pp 123-195. [2] C. Barbera, C. Bordehore, J.A. Borg, M. Glemarec, J. Grall, J.M. Hall-Spencer, C.H. De La Huz, E. Lanfranco, M. Lastra, P.J. Moore, J. Mora, M.E. Pita, A.A. Ramos-Espla, M. Rizzo, A. Sanchez-Mata, A. Seva, P.J.Schembri, C.Valle, Conservation and management of northeast Atlantic and Mediterranean maerl beds. Aquat Conserv 13, (2003) pp 65-76. [3] S. Agnesi, A. Annunziatellisi, M.L.Cassese, T.Di Nora, G. La Mesa, G. Mo, C. Pergent-Martini, L. Tunesi,. Analysis on the coralligenous assemblages in the Mediterranean sea: a review of the current state of knowledge in support of future investigations. In: “Proceedings of the 1st Symposium on the Coralligenous and other calcareous bio-concretions of the Mediterranean Sea” C. Pergent-Martini, M.Brichet, edits., CAR/ASP Publ., Tabarka, 15-16 January 2009, (2009) pp 41-46. [4] M. Bonacorsi, N. Bréand, P. Clabaut, B. Daniel, M. Marengo, G. Pergent, C. Pergent-Martini, M. Verlaque, Cartography of main coastal ecosystems in a NATURA 2000 site, In: “Proceedings of the tenth international conference on the Mediterranean Coastal environment” E. Ozhan, edits., Medcoast Publ., Rhodes, Greece 55-29 october 2011, (2011), pp 253-262. [5] V. Pasqualini, Caractérisation des peuplements et types de fonds le long du littoral corse (Méditerranée, France). Thèse Doctorat « Ecologie marine », Université de Corse (1997) pp 1-172.

Material & method The map of main assemblages and bottom-types, between 0 and 100 m depth was performed between July 2010 and September 2011. The shallow water (from 0 to −15 m) was mapped using a complete photographic coverage (146 color aerial photographs at 1/5 000, from the BD ORTHO® 2007 of the French National Geographic Institute) with a resolution of 0.5 m. Remote sensing was applied to each photograph using Envi 4.4® software [4]. The deep zone (15-100 m) was mapped using exhaustive coverage acoustic data (coupling multibeam echosonder EM 1000™ and side scan sonar Klein 3000™). More than 800 field data (bathyscope and scuba-diving observations, Remote Operated Vehicle  images, grabs) allow to validate the interpretation of aerial photographs and the mosaic of the sonograms. All of the data are included in a Geographic Information System (GIS; ArcGis 10® ; projection Mercator-WGS84).

Results The map of the main biocenosis and associations 750 km2 (fig. 1) shows the importance of the “biocenosis of the coastal detritic bottom” (82 %)which is partially covered by free encrusting rhodoliths. This association of rhodoliths forms accumulations in the sandy depressions (13 %) or constitutes “carpets” (2 %), until 90 m depth. The Posidonia oceanica meadow is well represented with 14 % of the bottom. This meadow covers 75 % of the bottoms between the surface and 40  m depth. The coralligenous assemblages cover occupy less than 1 % of the total area but are underestimated according to the method used (side scan sonar) which only take into account horizontal or subhorizontal surfaces (3D assemblage).

In addition in the northern part, new structures have been disco-vered (Fig. 2). These structures, never identified in the Mediterranean Sea, are named "corall igenous atolls" because of their circular shape.

9 970 ha (14 %) Depth 0-40 m

1 663 ha (2%) « carpet » 9 560 ha (13%) depressions

505 ha (0.7 %) Horizontal surfaces

Fig. 1: Map of the biocenosis and associations of Cap Corse (Corsica – France)

Fig. 2: Structure of “coralligenous atolls” in northern Cap Corse

Discussion & conclusion This study underline the high vitality of Posidonia oceanica meadows in this sector with no significant regression since 15 years [5]; however regressions are recorded in some sectors in relation with trawling. The produced map constitutes a significant contribution for the distribution of the rhodolith beds and coralligenous assemblages, according to the existing data [3]. In particular, it demonstrates the importance of the surface occupied by the rhodolits beds in these Natura 2000 sites (15  % of the total mapped surface of the western Mediterranean basin [3] ). Our results are in accordance with the existing data concerning the bathymetric distribution of coralligenous [4] and rhodolits associations [2] even if our observations are among the deepest recorded for the coralligenous in the western basin. The discovery of new coralligenous structures (“coralligenous atolls”) associated to sea-mounts, below the territorial water represents a challenge in term of scientific studies (undestanding the origin and dynamic of these structures) and conservation strategy. The creation of a Marine Protected Area with a “high-sea” component appears as a particularly relevant goal to ensure the preservation of biodiversity and the conservation of these structures. According to the importance of this area, a new oceanographic campaingn, focused on “coralligenous atolls” and deeper sectors (until 200 m depth), is already scheduled for next summer (August 2013).

The origin and growth dynamics of these atolls are still unknown but their form doesn’t appear to result from the action of the hydrodynamic, and an anthropogenic origin also seems unlikely.

Posidonia oceanica

Coralligène

Fig. 1: Key ecosystems of the Mediterranean sea.