Proceedings of the Second Mediterranean Symposium on Marine

PROCEEDINGS OF THE SECOND MEDITERRANEAN SYMPOSIUM ON MARINE VEGETATION (ATHENS, 12-13 DECEMBER 2003)
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GLOBAL WARMING
Since <strong>the</strong> 1960s or 1970s, both deep and surface <strong>Mediterranean</strong> waters have been
steadily warming up (Béthoux et al., 1990, 1998; Salat and Pascual, 2002). This
warming results from <strong>the</strong> 1 500-year cycle, <strong>the</strong> AMO (Atlantic Multidecadal Oscillation),
<strong>the</strong> NAO (North Atlantic Oscillation) and possibly from <strong>the</strong> greenhouse effect (Bradley,
2000; Kerr, 2000; deMenocal, 2001; deMenocal et al., 2001; Hurrell et al., 2001;
McDermott et al., 2001; Tourre, 2002). Biological indicators, i.e. warm water species,
react to this warming by extending <strong>the</strong>ir range area northwards (Bianchi and Morri, 1993,
1994; Francour et al., 1994). These observations prompt <strong>the</strong> question: does this
warming threaten <strong>Mediterranean</strong> macrophytes and ecosystems dominated by
macrophytes?
Fig. 11. Invasion <strong>of</strong> some subtidal and deep <strong>Mediterranean</strong> communities by <strong>the</strong>
introduced Caulerpa taxifolia: homogenisation <strong>of</strong> habitats and gamma species diversity
decline.
Some authors have suggested that <strong>the</strong> present day flow <strong>of</strong> introduced species into <strong>the</strong>
Mediter-ra-nean may be a consequence <strong>of</strong> global warming. Although this warming trend
may help tro-pical spe-cies to colonize <strong>the</strong> <strong>Mediterranean</strong>, it is worth emphasizing that
(i) most recently intro-duced macro-phytes originate from temperate areas, mainly NE
Asia (including Japan), via aquaculture, ra<strong>the</strong>r than from tropical areas (Boudouresque
and Verlaque, 2002) and that (ii) Caulerpa taxifolia and C. racemosa var. cylindracea,
two <strong>of</strong> <strong>the</strong> most invasive species in <strong>the</strong> <strong>Mediterranean</strong>, though usually referred to as
"tropical green algae", actually do not originate from tropical waters but from <strong>the</strong>
temperate Sou<strong>the</strong>rn Australia (<strong>the</strong> "<strong>Mediterranean</strong>" Australian province) (Jousson et al.,
2000; Meusnier et al., 2001; Verlaque et al., 2003).