Page 2 - Tomasello_Borra_etal2009

Tomasello, Di Maida, Calvo, Pirrotta, Borra & Procaccini
the surface down to a depth of 45 m and covers 2–4% of
the whole benthic substratum in the basin (approximately
38,000 km2; den Hartog 1977; Procaccini et al. 2003).
Sexual reproduction is irregular and is sporadic in some
localities (Diaz-Almela et al. 2006, 2007), whereas clonal
growth by means of stolons seems to be the predominant
way of meadow expansion (Migliaccio et al. 2005;
Arnaud-Haond et al. 2007b; Rozenfeld et al. 2007). Large
clones can extend over more than 100 m (Ruggiero et al.
2002; Migliaccio et al. 2005; Zupo et al. 2006). Neverthe-
less, populations generally show high genetic variability as
a result of sexual reproduction or immigration of foreign
genotypes (Arnaud-Haond et al. 2005, 2007b). Posidonia
oceanica can occur in salinities between 33& and 39&
(Gobert et al. 2006; Sa`nchez-Lizaso et al. 2008) and toler-
ate temperatures between 9 °C and 29 °C (Boudouresque
& Meinesz 1982). Coastal lagoons are stressful environ-
ments for P. oceanica because environmental conditions
can reach the survival limits of this species (La Loggia
et al. 2004). The Stagnone di Marsala, a semi-enclosed
lagoon situated at the western coast of Sicily, is a singular Six Posidonia oceanica meadows were selected for this
environment where P. oceanica forms peculiar reef-like
structures (Calvo & Frada` Orestano 1984). In some areas
of the lagoon, these reefs are atoll-shaped, which is a very
rare feature of Posidonia meadows. This phenomenon is
also observed in small areas along the Tunisian, Turkish
and Corsican coasts (Blanpied et al. 1979; Boudouresque
et al. 1990; Pasqualini et al. 1995). Pergent & Pergent-
Martini (1995) and Boudouresque et al. (2006) hypothe-
sized that atolls derive from nearly circular patches where
plagiotropic (horizontal) shoots only grow outwards,
whereas the shoots on the central portion of the atolls
die. According to this hypothesis, one or a few genotypes
should be the founders of such structures without subse-
quent recruitment within the patch itself. In the Stagnone
di Marsala, clear regression of P. oceanica stands has been
recorded over the last decades, both in the overall distri-
bution of the meadows and in the number and diameter
of atolls (personal observation by the authors).
Our initial observations suggest that P. oceanica atolls
in the Stagnone di Marsala occur at the limits of the spe-
cies’ tolerance, with temperature and salinity values
reaching 30 °C and 48&, respectively (Mazzola & Vizzini
2005). We focused our research on this peculiar habitat
because global change is expected to affect most strongly
the populations occurring near the species’ environmental
tolerance limits (Reusch & Wood 2007). Moreover, com-
bined information on ecology and genetic diversity can
add to our understanding of the effects of genetic isola-
tion in a fragmented landscape (Manel et al. 2003; Pro-
caccini et al. 2007).
The aim of this work was to test whether each atoll of
P. oceanica is composed of a single genotype. If correct,
Marine Ecology 30 (2009) 288–300 ª 2009 Blackwell Verlag GmbH 289
Seagrass meadows at the extreme of environmental tolerance
then those structures are expected to be vulnerable to
even small changes of the extreme environmental condi-
tions to which the single genotypes are exposed, and
could thus explain the observed regression. Moreover, we
aimed to define the level of genetic isolation among
stands growing inside, at the entrance and outside the
lagoon. We used all 13 microsatellite loci available for the
species (Procaccini & Waycott 1998; Alberto et al. 2003).
We interpreted the results in respect to growth perfor-
mances and occurrence of flowering in the last few years,
obtained through reconstructive techniques, in an attempt
to better understand the regression in atoll size and distri-
bution observed in the last few years and provide support
for the management of P. oceanica meadows within this
important coastal lagoon.
Material and methods
Study area and sampling design
study along the western coast of Sicily. Samples were col-
lected in two sites inside the Stagnone di Marsala lagoon
(Atolls and Re´cif) and four sites outside the lagoon (Pla-
teau, Nubia, Marsala and Favignana), where continuous
and extensive meadows are present (Calvo et al. 1995)
(Fig. 1). The Stagnone di Marsala lagoon is sub-divided
into a northern and a southern basin by an emerging
P. oceanica reef barrier (Re´cif-barri`ere) (Calvo & Frada`
Orestano 1984). The southern basin has an average depth
of 2 m and is connected to the open sea through an
opening 3000 m wide, in which it is possible to observe a
vast P. oceanica reef platform (Plateau recifale). The
northern basin, where P. oceanica grows in atoll-shaped
structures, is characterized by an average depth of 1 m
and connected with the open sea northwards through an
opening 400 m wide and 20–30 cm deep. This basin
shows more distinct lagoon features than the southern
one, resulting from the limited water exchange and slow
turnover (about 65 days; Calvo et al. 2005). Geomorpho-
logical conformation, shallow water, presence of isles and
emerging vegetation also hinder water circulation (Maz-
zola & Sara` 1995; La Loggia et al. 2004). Indeed, higher
annual variation in temperature and salinity values were
recorded within the lagoon (10–30 °C and 33–48&,
respectively) with respect to the nearby open coastline
(temperature: 14.1–26.4 °C; salinity close to 37&) (Vizz-
ini et al. 2002; Mazzola & Vizzini 2005).
In February 2006, about 40 individual orthotropic
P. oceanica shoots (ramets growing vertically) were ran-
domly sampled for each site. Shoots were collected at
depths ranging between 1 and 6 m and at a minimum
reciprocal distance of 8–10 m, to reduce the risk of sam-