Proceedings of the Second Mediterranean Symposium on Marine

PROCEEDINGS OF THE SECOND MEDITERRANEAN SYMPOSIUM ON MARINE VEGETATION (ATHENS, 12-13 DECEMBER 2003)
26
As a result, <strong>the</strong>re is an urgent need for maps <strong>of</strong> macrophytes o<strong>the</strong>r than seagrasses, such
as Cystoseira zosteroides, C. spinosa, C. brachycarpa, sciaphilous Rhodobionta, barren
ground (due to sea-urchin overgrazing) with incrusting corallines, etc. In addition, maps
<strong>of</strong> Posidonia oceanica meadows should distinguish landscape types.
QUALITATIVE VERSUS QUANTITATIVE MAPS
Most <strong>of</strong> <strong>the</strong> available maps only deal with <strong>the</strong> presence or absence <strong>of</strong> a species (or a
community): <strong>the</strong>y are qualitative. A very few maps take into consideration an
intermediate category, e.g. degraded Posidonia oceanica beds or high versus low
covering or density (e.g. Fredj et al., 1990; Cinelli et al., 1992; Paillard et al., 1993;
Pasqualini, 1997; Bonhomme et al., 2003). As far as shoot density <strong>of</strong> P. oceanica is
concerned, it has been mapped via kriging (e.g. Francour and Marchadour, 1989).
Bonhomme et al. (2003) did not map <strong>the</strong> actual shoot density, which de-creases
naturally with depth (so that it does not constitute a pertinent estimate <strong>of</strong> <strong>the</strong> seagrass
health), but considered (i) values equal to or above <strong>the</strong> normal density at a given depth
(according to Pergent et al., 1995; Pergent-Martini et al., 1999) and (ii) values below
that normal density. In addition, information on <strong>the</strong> seagrass dynamics (regression or
expansion) was superimposed on <strong>the</strong> map.
For <strong>the</strong> mapping <strong>of</strong> Caulerpa taxifolia, three levels <strong>of</strong> colonization (covering) are
distingui-shed: covered area, affected area and concerned area (Meinesz et al., 2001,
2003). For <strong>the</strong> mapping <strong>of</strong> Lithophyllum byssoides, isolated individuals versus a rim, and
<strong>the</strong> width <strong>of</strong> <strong>the</strong> rim, were distinguished (Bianconi et al., 1987; Bernard et al., 2003b).
For <strong>the</strong> mapping <strong>of</strong> <strong>the</strong> Cystoseira amentacea belt in <strong>the</strong> subtidal fringe, Ballesteros (in
Soltan, 2001) utilizes 5 categories, from 1 (isolated individuals) to 5 (dense and
continuous stand). Finally, for Cystoseira brachycarpa, <strong>the</strong> 6-level Braun-Blanquet scale
(Boudouresque, 1971) was used (Clarisse, 1984).
In order to achieve some <strong>of</strong> <strong>the</strong> purposes <strong>of</strong> <strong>the</strong> cartography, namely <strong>the</strong> management
<strong>of</strong> coastal areas and <strong>the</strong> monitoring <strong>of</strong> change over time, <strong>the</strong> changeover from qualitative
to quantitative maps would be particularly welcome.
CAN WE TRUST EARLY MAPS?
In many regions, no or very few ancient maps are available. As a result, it is difficult or
impossible to assess <strong>the</strong> dynamics <strong>of</strong> <strong>the</strong> vegetation. In an area near Marseilles
(Provence, France) where <strong>the</strong>re is a series <strong>of</strong> maps <strong>of</strong> <strong>the</strong> seagrass Posidonia oceanica
beds spanning more than one century (dating from 1883, 1897, 1958, 1975, 1979,
1991, 2000 and 2003, respectively from Marion, 1883; Pruvot, 1897; Picard, 1965;
Blanc, 1975; Cristiani, 1980; Niéri et al., 1991; Bernard et al., 2000; Bonhomme et al.,
2003), <strong>the</strong> last one very accurate (aerial photographs, side scan sonar and validation by
ground truth), Leriche et al. (2004) pointed out dramatic differences between <strong>the</strong>se
maps (Fig. 1). In contrast with some o<strong>the</strong>r benthic communities, specific biological