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124 A. Alagna et al. / Estuarine, Coastal and Shelf Science 119 (2013) 119e125
It is still unclear if the presence of algal coverage could exert
a trophic role for seedlings by increasing nutrients availability
respect to bare rock. As P. oceanica seedlings reach trophic inde-
pendence within the first year of life, and since seedlings have
shown some evidence of nutritional constrains (Balestri et al.,
2009), it can be supposed that nutrient availability influences
seedling survival after the first year.
Depth seems to play an additional role in seedling persistence
by modulating wave shear on the bottom. In the present study,
seedlings successfully established on rocky microhabitats one to
three meters depth, unlike those reported by Piazzi et al. (1999)
which did not survived on rock at 2 m depth after a few months.
Such differences could be explained by unequal hydrodynamic
conditions between the study areas.
Our findings also confirm high seedling mortality during the
first year of life, supporting the assumption that first-year survival
represent a critical stage in the life cycle of P. oceanica. Seedlings Acknowledgments
lack a well-developed anchoring system during the first year, being
sensitive to displacement by moving water. We suppose that the
first severe storm occurred in the fall 2004 swept away all seedlings
settled on gravel and sand, as well as young seedlings loosely
anchored on rocks covered by turf, leaving only well fixed ones.
During the second year seedling survival remained almost constant
and it was not recorded any significant difference in seedling
density between rocky microhabitats.
As concerned morphological variables, no differences among
microhabitats were detected in the initial size of seedlings. Thus, it
can be supposed that the availability of parental resources was
similar among microhabitats. Short-termed changes in seedling
morphological variables between June and July 2004 yet reveal best
growth performances on rock covered by an algal turf made of
Halopteris-Dilophus spp. and secondly on rock covered by stands of
the erect algae C. amentacea var. stricta, with respect to bare sand.
This finding can not be related to possible differences in nutrients
availability among microhabitats because seedlings growth is
thought to occur at seed expense during first months after germi-
nation (Caye and Meinesz, 1989; Balestri et al., 1998b, 2009); nei-
ther it can be ascribed to hydrodynamics because seedlings on sand
were sheltered and interspersed within the other microhabitats;
nor to grazing since grazing pressure appeared to be very low and
similar among microhabitats. It can be speculated that the unsuc-
cessful anchoring on sand negatively affected the development of
leaves and roots. Further investigations are needed to elucidate
which physiological mechanisms might underlie this observation.
The best performances of seedling long-termed growth be-
tween July 2004 and July 2006 was recorded on rock covered by
turf, followed by rock covered by erect macroalgal stands. It can be
hypothesized that the structure of the algal assemblage may in-
fluence seedling development leading to competitive interactions
for light and space within the canopy structure or to different levels
of available nutrients. Indeed, canopy height is larger in stands
made of Cystoseira spp. than in turfs made of Halopteris-Dilophus
spp., which may have limited light supply to the P. oceanica seed-
lings. Moreover, anchorage structures of Cystoseira spp. are more
developed than those of Halopteris spp. and Dilophus spp., poten-
tially reducing the available space for rooting and anchoring by
P. oceanica seedlings. We also observed deposits of sediment on
rocks covered by Halopteris-Dilophus turf, which could lead to
greater nutrient availability on this microhabitat respect to that
dominated by C. amentacea var. stricta.
In conclusion the reported results show that microhabitat type
is a relevant factor for seedling persistence and growth during
initial life stages of P. oceanica, influencing settlement and
recruitment of new plants. These findings suggest that specific
features of the microhabitat constitute mayor drivers of early
demographic processes in P. oceanica. Substrate stability appears to
control seedling settlement by providing protection against water
flow. Beyond substrate nature, composition and structure of algal
assemblage can affect the recruitment process, influencing early
seedling retention, settlement and growth. Improving knowledge
of environmental factors that control propagule establishment and
successful initiation of new patches is essential to explain patterns
of species distribution and abundance and has direct and relevant
implications on restoration strategies. Till now P. oceanica restora-
tion initiatives reached highly variable results and meadow resto-
ration is thought to be infeasible at the medium and large scale due
to high costs, labour required and uncertain outcomes (Sánchez-
Lizaso et al., 2009; Cunha et al., 2012). There is therefore a strong
need to reconsider habitat requirements of species, in particular
that of early life phases, as the base to develop rehabilitation
strategies more founded on ecological requirements of species and
on the observation of natural life history processes.
This study has been partially conducted during Adriana Alagna
PhD, developed at University of Salento (Italy) in collaboration with
the CNR-IAMC (Italy).
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