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Seagrass meadows at the extreme of environmental tolerance
levels of genetic variability than those from less variable
or more monotonous environments (Valentine 1976). In
our study, this only true for heterozygosity, which is
highest in the Atolls population, whereas the other
genetic diversity estimators show a different trend. Geno-
typic diversity (R = 0.79 for Atolls), although lower than
in the other populations analysed here, is still consistently of lower water exchange. This reduction is consistent
higher than the average values recorded in the whole dis-
tributional range of the species. Without considering the
Adriatic Sea, where monoclonal stands are present (Pro-
caccini et al. 2002; Ruggiero et al. 2002; Arnaud-Haond
et al. 2007b), the average R value calculated for 32 mead-
ows within the whole Mediterranean basin is, in fact,
below 0.64 (Arnaud-Haond et al. 2007b).
The results of the Fst analysis showed active gene flow
among all the areas, excluding the innermost section of
the lagoon, stressing the higher isolation of the Atolls
from all the other sites except the Re´cif. This latter area
seems to represent the only possible source of new geno-
types for the innermost section of the lagoon, as resulting
from the position of the Re´cif genotypes in the CFA.
Indeed, all the Atolls genotypes assigned to different pop-
ulations are exclusively assigned to the Re´cif. Results from 1999).
the spatial autocorrelation analysis showed that the auto-
correlation curve in the two sites within the lagoon is sig-
nificant up to almost 300 m, which represents the
putative maximum distance for local gene flow. The local
gene flow decreases at this limit, suggesting that more dis-
tant atolls show lower genetic affinity. This value is higher
than what is normally found in open-sea meadows (Mig-
liaccio et al. 2005; Procaccini et al. 2007). The high corre- tude generally greater than the average in the Western
lation coefficient recorded in these two sites confirms that
the enclosed system of the Stagnone lagoon is genetically
isolated, with efficient water circulation and dispersal only
at a local scale. In the open sea the more frequent immi-
gration of genotypes from other meadows can reduce the
correlation among spatially close genotypes.
Genetic isolation is likely to occur in populations living observed by Diaz-Almela et al. (2007), which indicates a
in confined environments (Cognetti & Maltagliati 2000).
In seagrasses, studies carried out on Zostera marina (Mu-
n˜iz-Salazar et al. 2006) have shown high genetic isolation
of populations living in lagoons relative to the open sea.
Long-term isolation of populations can enhance selection
of locally adapted genotypes, whose existence can be
detected with ad hoc genetic markers (see Vasema¨gi et al.
2005). The degree to which this will occur depends on
the level of isolation and on the extremes of environmen-
tal conditions experienced (Hoffmann & Parsons 1997).
Biometric data indicate low variability in P. oceanica
growth performance among plants growing in different
areas. Only one significant deviation from the general
intercept was detected in the linear model. In particular,
Atolls plants exhibited a lower vertical growth and leaf
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Tomasello, Di Maida, Calvo, Pirrotta, Borra & Procaccini
formation compared to Favignana (baseline of Linear
Model), whereas no significant variation was observed at
the Re´cif site. Retro-dating analyses to estimate P. ocea-
nica growth performance inside the lagoon were carried
out in a previous study (La Loggia et al. 2004), where a
reduction in vertical growth was detected in the presence
with our study suggesting that growth performances may
be sensitive to site specificities, depending in particular
on local hydrodynamic regimes. In the northern and
most enclosed area of the Stagnone, water exchange is
twofold lower than in the Re´cif, where the maximum
within-lagoon water exchange rate has been estimated
(La Loggia et al. 2004). During the summer season, low
water exchange determines maximum salinity and tem-
perature values of 48& and 30 °C, respectively (Mazzola
& Vizzini 2005), which are above the tolerance range
reported for the species (Boudouresque & Meinesz 1982;
Gobert et al. 2006). As observed for species growing in
locations with temperatures above the optimum for
growth or near the upper limit of thermal tolerance, a
further increase in temperature could determine a decline
in their productivity and distribution (Short & Neckles
The results of this study provide evidence that sexual
reproduction is negatively affected by stressful conditions
inside the lagoon. In the Atolls area no sexual reproduc-
tive events were recorded during 24 years of study,
although in the Western Sicily area, flowering events
occurred almost annually with a flowering index magni-
Mediterranean basin (Diaz-Almela et al. 2006, 2007).
Absence of flowering has also been observed in seagrass
populations living at the geographical range limits of the
species, where environmental conditions become extreme
(Ashton & Mitchell 1989; Eriksson 1996; Reusch et al.
1999; Alberto et al. 2001; Billingham et al. 2003). Our
observation is apparently in contrast to the general trend
positive correlation between P. oceanica flowering and
temperature anomalies. We can hypothesize that the
water temperature experienced by the plant in the Stag-
none lagoon is above the threshold limit for flowering
induction in the species. The water temperature of 30 °C
recorded in summer in our study site, in fact, is consis-
tently higher than values observed during temperature
anomalies in the whole Mediterranean. As an example,
the strongest temperature anomaly of the last 30 years,
which corresponded to a clear increase in flowering, was
recorded in 2003, when sea-surface temperature (SST)
increased by about 2 °C in the Western Mediterranean,
reaching a value of about 28 °C (Diaz-Almela et al.
2007). The very high salinity, characteristic of the
Marine Ecology 30 (2009) 288–300 ª 2009 Blackwell Verlag GmbH