Proceedings of the Second Mediterranean Symposium on Marine

PROCEEDINGS OF THE SECOND MEDITERRANEAN SYMPOSIUM ON MARINE VEGETATION (ATHENS, 12-13 DECEMBER 2003)
48
The recovery <strong>of</strong> Cystoseira forests, after <strong>the</strong> causes <strong>of</strong> <strong>the</strong>ir decline have ceased to
operate, generally takes a very long time. Eight years after <strong>the</strong> setting up <strong>of</strong> <strong>the</strong> Marseilles
(Provence, France) sewage treatment plant, <strong>the</strong> recovery <strong>of</strong> Cystoseira amentacea was
hardly discernible (Soltan, 2001; Soltan et al., 2001). The reason may be that Cystoseira
eggs are ra<strong>the</strong>r large, sink rapidly and <strong>the</strong>refore are probably not disseminated beyond a
few meters from parent individuals (Guern, 1963, 1964; Lüning, 1990), as occurs in
Sargassum species (Chapman, 1986). In addition, <strong>the</strong> growth <strong>of</strong> most Cystoseira
species is quite slow (Verlaque, 1987b). As a result, where C. amentacea has been
extirpated from large areas, step-by-step recovery can take decades (Soltan et al., 2001).
The absence <strong>of</strong> colonization by Cystoseira species <strong>of</strong> some breakwaters constructed
decades ago (and apparently suitable) highlights <strong>the</strong> difficulty <strong>of</strong> <strong>the</strong>se species in
colonizing new or lost habitats.
A CASE STUDY: THE FATE OF LITHOPHYLLUM BYSSOIDES TROTTOIRS
The incrusting coralline Lithophyllum byssoides (Rhodobionta, Plantae), previously
known as L. lichenoides or L. tortuosum, lives in <strong>the</strong> lower midlittoral zone, i.e. slightly
above mean sea level. Under conditions <strong>of</strong> dim light and strong surf exposure, e.g. in
small coves, corridors, fends, crannies and along cliffs, it builds up rims; wide rims are
usually named "trottoirs". They consist <strong>of</strong> a wide overhanging cornice with a flat or slightly
depressed upper surface, ending in a salient rim with a vertical face (Delamare-
Debouteville and Bougis, 1951; Blanc and Molinier, 1955; Pérès and Picard, 1964;
Laborel, 1987; Laborel et al., 1994a).
The Lithophyllum byssoides rim is a common feature in <strong>the</strong> nor<strong>the</strong>rn and central parts
<strong>of</strong> <strong>the</strong> western <strong>Mediterranean</strong> basin and <strong>the</strong> Adriatic Sea. The most spectacular trottoirs
are those <strong>of</strong> Grand Langoustier on Porquerolles Island (Provence, France), Cala Litizia
(Punta Palazzu, Scandola nature Reserve, Corsica) and Kvarner Gulf (Croatia). They are
2 m wide in places. Lithophyllum byssoides rims are less common in <strong>the</strong> south <strong>of</strong> <strong>the</strong>
western basin and very rare in <strong>the</strong> eastern basin (Huvé, 1963; Lovric, 1971; Zimmermann,
1982; Bianconi et al., 1987; Harmelin et al., 1987; Laborel, 1987; Laborel et al.,
1994a).
Datings by 14 C have shown that <strong>the</strong> building up <strong>of</strong> large Lithophyllum byssoides trottoirs
takes several centuries, even more than a thousand years, and a relatively stable (or just
very slowly rising) sea level, which has rarely been <strong>the</strong> case over <strong>the</strong> last 30 000 years
(Laborel et al., 1983, 1994b).
The Lithophyllum byssoides trottoir is sensitive to pollution (especially hydrocarbons).
The trottoirs have died in French Catalonia, in <strong>the</strong> Marseilles area (France) and in <strong>the</strong> Gulf
<strong>of</strong> Palermo (Sicily): bio-erosion (perforating organisms) no longer being compensated for
by bio-construction, <strong>the</strong>y are progressively eroded and end up disappearing (Laborel
et al., 1994a; Riggio et al., 1994). Bearing in mind <strong>the</strong> length <strong>of</strong> time it takes for <strong>the</strong>m