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Chemistry and Ecology 255

and the recent isolation from the adjacent coastal meadows due to the almost complete closure of
the northern entrance of the lagoon [23].

Here we present the most acknowledged descriptors of the meadows, most of which have
been traditionally used to assess the status of seagrass meadows [51]. Because of its recognised
ecological importance and wide distribution, most of the data available in the literature have been
obtained on Posidonia oceanica.

Downloaded By: [Furnari, G.] At: 10:31 24 May 2010 4. Density

P. oceanica density is considered an important descriptor of the vitality and dynamics of a meadow
and it is very sensitive to the human impact [51].

Altogether, 1584 counts (5–12 counts per station) of P. oceanica shoot density were made in
174 stations along Sicilian coasts from 1 to 28 m depth (see Supplementary Table – online only).
Mean density values were from 121.1 to 931.3 shoot·m−2 and the 99% confidence interval (CI)
of the mean was estimated at between 370.6 and 425.3 shoot·m−2. On average, shallow and deep
P. oceanica meadows show the highest and lowest density values respectively. This pattern is well
described by a semi-logarithm regression model in which depth accounts for 30% of total density
variability (p < 0.01) (Figure 4). These results are in agreement with the model proposed by
Pergent et al. [53] for the Mediterranean scale, confirming the value of the link between density
and depth also on a regional scale.

Based on the standardised scale for density classification [53], about two-thirds of the Sicilian
P. oceanica meadows, mainly located along the western and south-eastern Sicilian coasts and
small islands, belong to the best classes (‘Higher Sub-normal Density’ and ‘Normal Density’).
The abnormal class (‘Abnormal Density’) represents 16.5% of cases and is mainly detected close to

Figure 4. Relationship between density and depth; the classification according to Pergent et al. [53] is superimposed.