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Figure 2. Distribution of Posidonia oceanica meadows per substratum type in Sicily.

and physiographic factors such as: (1) sea floor morphology features, (2) dominance of muddy
and unstable sediments, and (3) low water transparency [41].

Posidonia oceanica is the most common seagrass along Sicilian coasts, covering ∼76,000 ha
of coastal areas, which corresponds to 18.5% of the sea bottom at depths between 0 and 50 m
[39]. In Sicily, P. oceanica meadows show the most extensive bottom coverage of all the Italian
regions after Sardinia [42]. Settlement and development of Posidonia meadows along Sicilian
coasts has been affected by composition and structure of the substratum. About 54% of meadows
settle on matte, 24% on sand and 20% on rocky bottom and only 2% is dead matte (Figure 2).

The distribution of P. oceanica meadows in term of species coverage shows the presence of
dense, extensive beds growing along the western (55%) and south-eastern (19%) coasts of Sicily,
corresponding to the most important carbonatic and calcarenitic outcrops of the island (Figure 1).
In particular, along the western coast of Sicily favourable ecological conditions and highly pristine
natural conditions have allowed the settlement and development of one of the largest P. oceanica
meadows in the Mediterranean [41]. Moreover, the development of the meadow near the shore-
line, facilitated by extensive shallow bottoms, gave rise to peculiar P. oceanica reef formations
(plateau-récifale and récif-barrière) [43,44] and contributed to building and maintaining natural
(Stagnone di Marsala) and artificial (salt mines) lagoon ecosystems. Because if its peculiar envi-
ronmental and geomorphological characteristics, the Stagnone di Marsala can be considered as
a ‘natural laboratory’, in which remote sensing techniques have been tested to map phytobenthic
communities and evaluate biomass density distribution [45,46].

P. oceanica meadows show a discontinuous distribution along the Tyrrhenian and north-eastern
Ionian coasts, characterised by rugged mountains crossed by torrent-like rivers and metamorphic,
igneous, sedimentary (Peloritani Units) and volcanic rocks (Mount Etna).

Wide and dense P. oceanica meadows are present on the sea floor of the calcareous Egadi
Archipelago (where over 70% of the P. oceanica beds mapped in the small Sicilian islands have
been described) and the island of Lampedusa, while in the volcanic islands (Aeolie, Ustica,
Pantelleria and Linosa) P. oceanica beds settle on rock or volcanic sands mixed with biogenic
calcareous detritus.

On the whole, P. oceanica is commonly found in a good condition along Sicilian coasts, except
for meadows close to the main urban and industrial areas [19,47–49], where signs of evident
regression up to complete disappearance have been found (Gulfs of Palermo and Augusta).

The typology of the lower depth limit of P. oceanica meadows [50,51] is considered an important
descriptor to evaluate extension/regression balancing conditions of the meadows, allowing us
to detect the principal local factors controlling the bathymetric distribution of the plant along
Sicilian coasts.