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100 G. Di Maida et al. / Marine Environmental Research 87-88 (2013) 96e102
Fig. 4. Mean values (ÆSE) of shoot surface, leaf length, leaf width and coefficient A estimated for each substratum in three locations.
sand and matte. A number of factors linked to sediment dynamic
may explain this reduction. It is general opinion that variations in
P. oceanica rhizome vertical elongation are strictly dependent on
sedimentation. Plants exposed to different sediment rates are able
to modulate their growth to counteract leaf meristem burial
(Boudouresque et al., 1984; Di Carlo et al., 2011; Manzanera et al.,
2011; Marbà and Duarte, 1997). Sandy substrata are composed of
different textures depending on hydrodynamic regime (De Falco
et al., 2008), but their presence generally derives from accumu-
lating processes that may be enhanced by the P. oceanica canopy
itself (Duarte et al., 1999; Gacia et al., 1999). Also matte edification is
the result of a substantial long-term positive sedimentation balance
in the interstitial within the roots and rhizomes system
(Boudouresque and Meinesz, 1982) which provides mechanical
support for the vertical accretion of plants. Moreover, in both cases
the presence of sediment is indicative of nutrient availability for
seagrass, since the root system plays the dominant role in nutrient
Table 2
Frequencies distributions of leaves with and without their apex among substrata. growth, defined as growth without sedimentation stimulation
Chi-square test is performed in each location.
Location Substratum c2 d.f. P
Rock Sand Matte
Mondello Leaf apex Broken 36 40 45 2.7 2 n.s.
Whole 32 22 24 2.6 2 n.s.
2.5 2 n.s.
Solanto Leaf apex Broken 37 29 50
Whole 26 27 26
Trappeto Leaf apex Broken 26 16 23
Whole 54 59 57
Significance code: n.s. ¼ P > 0.05.
uptake (Hemminga and Duarte, 2000; Touchette, 2007; Touchette
and Burkholder, 2000). Indeed, sediment pore water has a much
higher nutrient concentration than the water column (Romero
et al., 2006).
In contrast, P. oceanica shoots anchored visibly on rock outcrops
are exposed to particular environmental conditions of deposition,
re-suspension and sediment removal, thus preventing its accu-
mulation. Under these circumstances an important abiotic com-
pound where nutrients can be storage is left deficient, with the
consequent limitation of the plant’s nutrient acquisition capacity
(Romero et al., 2006), which may be responsible for the significant
decrease in growth performance.
Although the components of variance suggest spatial hetero-
geneity in how much growth performance differed among sub-
strata, it should be noted that in all locations P. oceanica growth on
rock never exceeded that on sand and matte. Hence, it can be
argued that P. oceanica on rock most likely exhibits endogenous
(sensu Boudouresque and Jeudy de Grissac, 1983). Earlier studies
reported that endogenous growth may range from 5 to
7 mm yearÀ1 (Boudouresque and Jeudy de Grissac, 1983; De Falco
et al., 2008), while here we observe a range of 6e10 mm yearÀ1
(Fig. 3), suggesting that it can vary along latitudinal and longitu-
dinal gradient.
Differences among substrata are also evident for leaf biometric
features. Leaf length is lower on rock than on matte and sand, and
the model estimated that leaf length of shoots on rock is on average
23% less than on sand and matte. This pattern become more evident
for shoot surface, especially between rock and matte, where the
degree of difference is greater than 32% in all locations. This