Proceedings of the Second Mediterranean Symposium on Marine

At <strong>the</strong> end <strong>of</strong> <strong>the</strong> summer 1999, an unprecedented mass mortality <strong>of</strong> benthic
invertebrates (mainly gorgonians, sponges, ascidians and bryozoans) occurred along <strong>the</strong>
coasts <strong>of</strong> Provence (France), French Riviera and Ligury (Italy). Exceptionally high
temperatures throughout <strong>the</strong> water column (23-24°C) for over one month, down to
40 m depth, could have created an environmental context favourable to <strong>the</strong> mass
mortality event (Cerrano et al., 2000; Coma et al., 2000; Pérez et al., 2000; Romano et
al., 2000). This episode is <strong>of</strong> particular interest since, should <strong>the</strong> present day warming
trend continue, it could be <strong>the</strong> harbinger <strong>of</strong> even more drastic events.
Unfortunately, as far as macrophytes are concerned, very little information is available
about <strong>the</strong> possible effect <strong>of</strong> <strong>the</strong> warming up <strong>of</strong> <strong>the</strong> <strong>Mediterranean</strong> water: (i) Some
encrusting corallines (Rhodobionta, Plantae) were said to have been affected by <strong>the</strong>
1999 summer warm episode (Cerrano et al., 2000), but nothing is known about <strong>the</strong>
extent <strong>of</strong> <strong>the</strong> phenomenon and <strong>the</strong> subsequent recovery. (ii) The native Caulerpa
prolifera (Chlorobionta, Plantae) is extending its range area northwards, from Corsica, <strong>the</strong>
Tyrrhenian Sea and <strong>the</strong> French Riviera to Ligury and Provence (Bianchi and Morri, 1994
and unpublished data). Outside <strong>the</strong> <strong>Mediterranean</strong>, in <strong>the</strong> Gulf <strong>of</strong> Maine (USA), a
synergy between climate change and introduced species has been observed (Harris and
Tyrrell, 2001).
A CASE STUDY: THE DECLINE OF CYSTOSEIRA FORESTS
More than 50 taxa belonging to <strong>the</strong> genus Cystoseira (Fucophyceae, Chromobionta,
Stramenopiles) thrive in <strong>the</strong> <strong>Mediterranean</strong> Sea (Ribera et al., 1992). Most <strong>of</strong> <strong>the</strong>m are keyspecies
in a variety <strong>of</strong> forest communities which are encountered on subtidal and
circalittoral hard bott-oms (Giaccone and Bruni, 1972-1973). In addition, <strong>the</strong>y are usually K
strategists and long-lived (at least 45 years for C. spinosa; Ballesteros et al., 1995, 1998).
Since <strong>the</strong> 1960s, Cystoseira forests have undergone a severe decline in extensive areas
<strong>of</strong> <strong>the</strong> <strong>Mediterranean</strong>. Several species <strong>of</strong> Cystoseira and <strong>the</strong> communities <strong>the</strong>y dominate
are considered as threatened (Boudouresque et al., 1990). The decline <strong>of</strong> C.
amentacea, which thrives in <strong>the</strong> subtidal fringe under strong surf exposure, has been
related to pollution (Bellan-Santini, 1966). The regression <strong>of</strong> deeper Cystoseira forests
(e.g. C. brachycarpa, C. spinosa and C. zosteroides) may be due to a cascade effect
(sensu Steneck, 1998), though o<strong>the</strong>r causes may be involved (Sala et al., 1998): overfishing
<strong>of</strong> sea-urchin predator fish (e.g. Sparus aurata) leads to a surge in numbers <strong>of</strong>
<strong>the</strong> browser Paracentrotus lividus, which in turn overgrazes benthic macrophytes,
especially palatable and strongly preferred (Verlaque and Nédélec, 1983; Lemée et al.,
1996) Cystoseira species, which results in "barren grounds" with only encrusting
corallines, ephemeral macrophytes and sea-urchins (Verlaque, 1987a; Sala et al., 1998;
Boudouresque and Verlaque, 2001). O<strong>the</strong>r causes <strong>of</strong> more local significance can be put
forward to explain <strong>the</strong> Cystoseira forest's decline, e.g. turbidity (which diminishes
irradiance and <strong>the</strong>refore <strong>the</strong> compensation depth), silting, gill nets and trawling.
ACTES DU DEUXIEME SYMPOSIUM MEDITERRANEEN SUR LA VEGETATION MARINE (ATHENES, 12-13 DECEMBRE 2003)
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