400
S. Gobert, M. L. Cambridge, B. Velimirov, et al.
oceanica leaves but also eats the associated fauna
(Boudouresque and Meinesz, 1982). The herbivore
production in P. oceanica meadows is much lower
than in tropical seagrass meadows, probably due to
the higher consumption rates and assimilation efficiencies in tropical seagrass systems (Cebri´ an et al.,
1996).
P. oceanica rhizomes represent a suitable habitat
for sciaphilous subtidal phytocoenoses (Panayotidis,
1980), such as epiphytic bryozoans which are qualitatively richer on the rhizomes than on leaves (Kocak
et al., 2002). About 70 species of macroalgae grow
on rhizome (Piazzi et al., 2002). This plant part is a
support providing food and habitat for the associated
fauna.
In summary, where it occurs, Posidonia plays a
major role in shallow water diversity. Some Posidonia ecosystems are important as nursery areas but
others are less important or have not been studied.
While direct grazing of the leaves appears unimportant, food webs based on grazing of the epiphytes
growing on Posidonia leaves are a major contributor to higher trophic levels. Below-ground detritusbased food webs appear to be less important. There
is evidence for wide spatial variation in trophic relations, which is probably linked to local environmental characteristics. Future studies of food web
will require innovative use of techniques such stable
isotope analyses, as well as further improvements in
knowledge of systematics and ecology.
IX. Species Status and
Anthropogenic Impact
Despite the ecological and economical importance
of seagrass beds, an increasing number of reports
document the ongoing loss or regression of seagrasses in all countries (Duarte et al., 2002; Walker
et al., Chapter 23; Kenworthy et al., Chapter 25).
The surface area occupied by Posidonia beds is
limited by environmental factors (light, turbidity,
salinity, temperature), but in some coastal regions
where anthropogenic pressures (trawling, aquaculture, wastewater outfalls) are high, severe damage
is observed. The loss of Posidonia beds is unfortunately not recent; it has been reported since 1930s
(Boudouresque and Meinesz, 1982; Cambridge and
McComb, 1984; Shepherd et al., 1989; Larkum and
West, 1990; Walker and McComb, 1991). Recent
mapping of the historical evolution of Posidonia distributions generally shows a decline in seagrass coverage (e.g. Pasqualini et al., 2000; Kendrick et al.,
2002) reflecting anthropic activities, port developments, grazer impacts, shading of seagrass leaves
by excessive growth of epiphytic algae or phytoplankton following nutrient enrichment of column
water (Cambridge et al., 1986; Walker and McComb,
1992; Short and Wyllie-Echeverria, 1996; Chisholm
et al., 1997). Climate change will affect seagrass distributions (Short and Neckles, 1999; Duarte et al., in
press). The recolonization of altered sites is rare but
does occur (e.g. Kendrick et al., 1999, 2000; Meehan and West, 2000; Pergent-Martini and Pasqualini,
2000; Cambridge et al., 2002)—but is slow because
Posidonia are long lived species with a slow rhizome spread (0.3–21 cm year
−1 ) (Boudouresque et
al., 1984; Meinesz and Lef` evre, 1984; Kirkman and
Kuo, 1990; Marb` a and Duarte, 1998; Marb` a and
Walker, 1999; Paling and McComb, 2000). The loss
of Western Australian Posidonia meadows is caused
by a reduction in light reaching the meadows, either
through decreased water clarity or shading by epiphytic or unattached algae (Cambridge et al., 1986;
Walker and McComb, 1992; Kendrick et al., 2002;
Walker et al., Chapter 23). Some mechanical transplantation of Posidonia species has been conducted
with some success (Paling et al., 2001).
The main strategies for marine environmental
management in Australia (7.7 × 10
6 km
2 and 18 millions permanent residents) include maintenance of
water quality, prohibiting or regulating destructive
and unsustainable activities, zoning for particular
uses to separate and control incompatible uses, protection of vulnerable and threatened species and
regulation of fisheries through licenses, size limits,
quotas, etc. Australia is a world leader in using Marine Protected Areas (Fogarty et al., 2000; Kenworthy et al., Chapter 25) but our understanding of the
marine environment is still patchy.
The Mediterranean Sea (3 × 10
6 km
2 and 44 million permanent residents) is known for the considerable diversity of its fauna and flora as well as for
the high rate of species endemism. But generally,
Mediterranean countries have coastlines that are the
most intensively used in the world for tourism and related recreational activities. The coastline (including
islands) receives some one hundred million visitors
per year from all nationalities. As such, the Mediterranean coast and nearshore waters experience the increasing impact of tourism but also the effects of resident demographic growth. In addition, the coastal
S. Gobert, M. L. Cambridge, B. Velimirov, et al.
oceanica leaves but also eats the associated fauna
(Boudouresque and Meinesz, 1982). The herbivore
production in P. oceanica meadows is much lower
than in tropical seagrass meadows, probably due to
the higher consumption rates and assimilation efficiencies in tropical seagrass systems (Cebri´ an et al.,
1996).
P. oceanica rhizomes represent a suitable habitat
for sciaphilous subtidal phytocoenoses (Panayotidis,
1980), such as epiphytic bryozoans which are qualitatively richer on the rhizomes than on leaves (Kocak
et al., 2002). About 70 species of macroalgae grow
on rhizome (Piazzi et al., 2002). This plant part is a
support providing food and habitat for the associated
fauna.
In summary, where it occurs, Posidonia plays a
major role in shallow water diversity. Some Posidonia ecosystems are important as nursery areas but
others are less important or have not been studied.
While direct grazing of the leaves appears unimportant, food webs based on grazing of the epiphytes
growing on Posidonia leaves are a major contributor to higher trophic levels. Below-ground detritusbased food webs appear to be less important. There
is evidence for wide spatial variation in trophic relations, which is probably linked to local environmental characteristics. Future studies of food web
will require innovative use of techniques such stable
isotope analyses, as well as further improvements in
knowledge of systematics and ecology.
IX. Species Status and
Anthropogenic Impact
Despite the ecological and economical importance
of seagrass beds, an increasing number of reports
document the ongoing loss or regression of seagrasses in all countries (Duarte et al., 2002; Walker
et al., Chapter 23; Kenworthy et al., Chapter 25).
The surface area occupied by Posidonia beds is
limited by environmental factors (light, turbidity,
salinity, temperature), but in some coastal regions
where anthropogenic pressures (trawling, aquaculture, wastewater outfalls) are high, severe damage
is observed. The loss of Posidonia beds is unfortunately not recent; it has been reported since 1930s
(Boudouresque and Meinesz, 1982; Cambridge and
McComb, 1984; Shepherd et al., 1989; Larkum and
West, 1990; Walker and McComb, 1991). Recent
mapping of the historical evolution of Posidonia distributions generally shows a decline in seagrass coverage (e.g. Pasqualini et al., 2000; Kendrick et al.,
2002) reflecting anthropic activities, port developments, grazer impacts, shading of seagrass leaves
by excessive growth of epiphytic algae or phytoplankton following nutrient enrichment of column
water (Cambridge et al., 1986; Walker and McComb,
1992; Short and Wyllie-Echeverria, 1996; Chisholm
et al., 1997). Climate change will affect seagrass distributions (Short and Neckles, 1999; Duarte et al., in
press). The recolonization of altered sites is rare but
does occur (e.g. Kendrick et al., 1999, 2000; Meehan and West, 2000; Pergent-Martini and Pasqualini,
2000; Cambridge et al., 2002)—but is slow because
Posidonia are long lived species with a slow rhizome spread (0.3–21 cm year
−1 ) (Boudouresque et
al., 1984; Meinesz and Lef` evre, 1984; Kirkman and
Kuo, 1990; Marb` a and Duarte, 1998; Marb` a and
Walker, 1999; Paling and McComb, 2000). The loss
of Western Australian Posidonia meadows is caused
by a reduction in light reaching the meadows, either
through decreased water clarity or shading by epiphytic or unattached algae (Cambridge et al., 1986;
Walker and McComb, 1992; Kendrick et al., 2002;
Walker et al., Chapter 23). Some mechanical transplantation of Posidonia species has been conducted
with some success (Paling et al., 2001).
The main strategies for marine environmental
management in Australia (7.7 × 10
6 km
2 and 18 millions permanent residents) include maintenance of
water quality, prohibiting or regulating destructive
and unsustainable activities, zoning for particular
uses to separate and control incompatible uses, protection of vulnerable and threatened species and
regulation of fisheries through licenses, size limits,
quotas, etc. Australia is a world leader in using Marine Protected Areas (Fogarty et al., 2000; Kenworthy et al., Chapter 25) but our understanding of the
marine environment is still patchy.
The Mediterranean Sea (3 × 10
6 km
2 and 44 million permanent residents) is known for the considerable diversity of its fauna and flora as well as for
the high rate of species endemism. But generally,
Mediterranean countries have coastlines that are the
most intensively used in the world for tourism and related recreational activities. The coastline (including
islands) receives some one hundred million visitors
per year from all nationalities. As such, the Mediterranean coast and nearshore waters experience the increasing impact of tourism but also the effects of resident demographic growth. In addition, the coastal