Posidonia oceanica in the Mediterranean

In the Mediterranean Sea, P. oceanica forms monospecific meadows with different types of coverage pattern (continuous to patchy with leopard-skin, in row distributions) (Molinier and Picard, 1952; Borg et al., 2005) with shoot densities ranging from meadows where densities range between 150 and 300 shoots m-2(very sparse bed) to more than 700 shoots m-2(very dense bed) (Giraud, 1977a). The shoots (Fig. 3) are borne by rhizomes growing either vertically (orthotropic rhizome), avoiding burial, or horizontally (plagiotropic rhizome), enabling colonization. The leaves act as sediment-traps accumulating inorganic and organic particulate matter (Dauby et al., 1995; Gacia et al., 1999; Gacia and Duarte, 2001). The progressive silting and the two types of rhizome growth result in a typical terraced formation called 'matte' consisting of the intertwining of various strata of rhizomes, roots, and sediment. In shallow waters, such an accumulation of material raises the bed to the surface. Refractory organic material remains with little alteration for millennia, with an average accretion rate of 0.175 cm year-1, the age of the debris accumulated in this matte ranges between 0 and more than 3,000 years (Mateo et al., 1997; Mateo et al., Chapter 7). This phenomenon has been used as a record of bed history (Calmet et al., 1988; Pergent-Martini and Pergent, 1995; Hemminga and Mateo, 1996; Mateo et al., 1997).

The rhizomes grow horizontally until space has been completely colonized. The beds have wide spacing between many vertical shoots with few horizontal apices (Boudouresque and Meinesz, 1982).

The P. oceanica bed is the climax community of a successional process (Molinier and Picard, 1952; den Hartog, 1977). The development of the bed seems to need a substratum rich in organic material. Pioneer species such as Caulerpa prolifera set tle, together with small phanerogams of the genera Cymodocea and Zostera can produce suitable sediments for seed germination or shoot growth. Settlement, growth, and multiplication of P. oceanica shoots reduce the light intensity at the seafloor which causes the mortality and the disappearance of the pioneer species (Boudouresque and Meinesz, 1982).

P. oceanica requires stable environmental conditions, preferring a coarse-grained sandy substratum but ranging from soft substrata (from fine sand to pebbly, but not muddy sediments) to rock (Mazzella et al., 1993; Bellan-Santini et al., 1994). It is steno-haline, and is not present when salinity is below 33 ppt. In the Eastern basin, P. oceanica can endure salinities over 40, the hypersaline environment does seem to provide optimum growth conditions (Pergent and Zaouali, 1992) but experimental work indicated significant deleterious effects over 39 (unpublished data). The tidal range is very small (20 cm as a mean) and water levels are often more influenced by barometric pressure than by tides, as is also true of southern Australia. P. oceanica, like its Australian relatives, is not an intertidal species and is not resistant to desiccation, but does tolerate a wide range of temperatures (9-29°C) (Boudouresque and Meinesz, 1982).

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