Chapter 17 Biology of Posidonia
391
vertical growth occurs if sand accretion is rapid or
space is restricted. There appears to be a distinct differentiation in growth rate depending on whether the
axis is formed from a branch or is a continuation of
the main axis.
IV. Development of Meadows and Patches
A. Posidonia oceanica in the
Mediterranean Sea
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
(Mat´ eo 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; Mat´ eo 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 settle, 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 stenohaline, 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).
B. Posidonia Species in Australia
Along Australian coasts, the P. australis group (P.
angustifolia, P. australis, P. sinuosa) are horizontal spreaders with little vertical shoot production.
P. angustifolia is a fast spreading species, whereas
P. sinuosa forms dense meadows of vertically growing shoots, sometimes reaching 1,000 shoots m
−2
(Kendrick et al., 2002). The P. ostenfeldii group
(P. coriacea, P. denhartogii, P. kirkmanii, P. ostenfeldii, P. robertsoniae) grow vertically only and form
clumps not meadows.
V. Biomass Dynamics and Production
of the Plant
Posidonia is a long lived seagrass (4–30 years) with
long leaf life-span (70–350 days), high biomass,
and productivity (Pergent-Martini et al., 1994; Cambridge and Hocking, 1997; Duarte and Chiscano,
1999; Hemminga et al., 1999; Marb` a and Walker,
1999).
391
vertical growth occurs if sand accretion is rapid or
space is restricted. There appears to be a distinct differentiation in growth rate depending on whether the
axis is formed from a branch or is a continuation of
the main axis.
IV. Development of Meadows and Patches
A. Posidonia oceanica in the
Mediterranean Sea
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
(Mat´ eo 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; Mat´ eo 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 settle, 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 stenohaline, 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).
B. Posidonia Species in Australia
Along Australian coasts, the P. australis group (P.
angustifolia, P. australis, P. sinuosa) are horizontal spreaders with little vertical shoot production.
P. angustifolia is a fast spreading species, whereas
P. sinuosa forms dense meadows of vertically growing shoots, sometimes reaching 1,000 shoots m
−2
(Kendrick et al., 2002). The P. ostenfeldii group
(P. coriacea, P. denhartogii, P. kirkmanii, P. ostenfeldii, P. robertsoniae) grow vertically only and form
clumps not meadows.
V. Biomass Dynamics and Production
of the Plant
Posidonia is a long lived seagrass (4–30 years) with
long leaf life-span (70–350 days), high biomass,
and productivity (Pergent-Martini et al., 1994; Cambridge and Hocking, 1997; Duarte and Chiscano,
1999; Hemminga et al., 1999; Marb` a and Walker,
1999).