Page 19 - climate-change2013

Mass mortalities the initial stress, and there may be no significant signs
of recovery.
of macrobenthic Given that the affected species are in general long-lived
communities organisms characterized by slow population growth
and limited larval dispersal, the ability of affected popu-
lations to recover is probably quite limited. Habitat-form-
Unprecedented mass-mortality events and diseases ing species such as these gorgonians provide shade
linked to climate warming have been observed in the and shelter for other species by means of their skeletal
Mediterranean in recent decades. More than 30 species structure. Significant changes in their abundance can
in Mediterranean hard-bottom communities have therefore have a major effect on the organization and
been affected by mass-mortality events associated functioning of the community.
with unusual increases in seawater temperature along
thousands of kilometres of coastline, mainly in the
north-western Mediterranean (Cerrano et al., 2000;
Garrabou et al., 2009).

Coralligenous formations, considered one of the rich-
est habitats in the Mediterranean, have suffered most
severely. These communities, mostly living in what is
essentially a cold-water environment due to the forma-
tion of a seasonal thermocline, are adapted to a chang-
ing environment produced by intermittent and transitory
processes, including upwellings, downwellings, vertical
mixing, horizontal advection and heat waves. Converse-
ly, other affected species that live in shallower coastal
habitats such as caves and rocky, alga-dominated en-
vironments are adapted to a more stable environment
during summer periods.
Photo: J. Garrabou
Mediterranean gorgonia Paramuricea clavata affected by rising temperature.
Seasonal thermocline: Temporary
layers between which the temperature
changes abruptly with depth
Population blooms

Mass-mortality events in these hard-bottom communi-
ties have mainly been observed along the north-west- Changes in temperature and other conditions have also
ern Mediterranean coast, from north-eastern Spain and been linked to increasingly frequent reports of blooms
the Balearic Islands to France and the Ligurian coast of of a variety of organisms.
Italy, and to a lesser extent around Corsica. In 1999 and
2003, these events were the most severe ever record- Several studies have demonstrated a significant in-
ed in the area and affected a wide variety of species crease in jellyfish abundance in different areas of the
and taxa along more than 1,000 km of these coasts northern hemisphere, probably related to climate
down to a depth of 50 m (Garrabou et al., 2009). Other change and food-web modifications (such as overfish-
similar events, although at a smaller scale, have been ing of predators). Blooms of native jellyfish species (Pe-
observed in other Mediterranean areas involving other lagia noctiluca and Aurelia aurita) as well as alien spe-
organisms (e.g. bald sea-urchin disease affecting Para- cies (Rhopilema nomadica) in Mediterranean coastal
centrotus lividus). Many of these mortality events have areas are seriously interfering with human activities,
been linked to a particularly strong summer stratifica- including tourism and fisheries. Analysis of a long-term
tion of the water column and a possible reduction in data series covering more than 200 years has revealed
food resources (Coma et al., 2009). that a combination of certain conditions such as high
seawater temperatures, stable weather conditions dur-
The impact of mortality events on populations has ing reproduction and reduced rainfall seems to favour
been severe, especially on Mediterranean gorgonians the development and increasing occurrence of P. noc-
(Paramuricea clavata, Eunicella singularis, E. cavolinii, tiluca blooms in the Mediterranean (Goy et al., 1989).
Lophogorgia ceratophyta and Corallium rubrum) and Other data series from the Adriatic Sea as well as from
sponges (Ircinia fasciculata, Spongia officinalis and S. experimental settings have yielded similar findings:
agaricina). For example, in some affected areas up to blooms of P. noctiluca and other jellyfish species are
90% of red gorgonian Paramuricea clavata colonies promoted (at least partly) by a warmer sea. Environ-
show total or partial mortality. Other species however, mental changes, whether natural or resulting from hu-
such as the yellow gorgonian Eunicella cavolinii, seem man influence, could lead to changes in the seasonality
to be more resistant to these warming events, although of jellyfish blooms, so that they develop earlier or last
the effects on their reproductive biology, vulnerability to longer, whereas climate change seems to lead to an
disease and growth can still be seen several years after increase in the ability of these organisms to thrive.

MEDITERRANEAN MARINE PROTECTED AREAS AND CLIMATE CHANGE: A GUIDE TO REGIONAL MONITORING AND ADAPTATION OPPORTUNITIES 17