396
S. Gobert, M. L. Cambridge, B. Velimirov, et al.
environmental conditions (depth, hydrodynamic
conditions, predation pressure) (Pergent et al.,
1997b), among populations (Cebri´ an and Duarte,
2001) and seasons. Primary production is higher
at the shallow edge because of higher bed density.
Depth also has a significant influence on the distribution of litter: within the beds export rates are
highest at shallower stations where the hydrodynamic forces are greater. Consequently, accumulation rates among sites show high variations (i.e. 19–
191 g C m
−2 year
−1 in Romero et al., 1994; 9–112 g
C m
−2 year
−1 in Mat` eo et al., 1997; see also Mateo
et al., Chapter 7).
Meiofaunal assemblages (Danovaro et al., 2002)
(from 0 to 4 cm in the sediment) constitute a biomass
of 1.1 g C m
−2 (i.e. 4,010
3 tons C for the P. oceanica
meadows of the Mediterranean Sea) with a mean
production of 10.0 ± 1.9 g C m
−2 year
−1 (i.e.
409 × 10
3 tons C year
−1 ). The seasonal change
in meiofaunal production, like benthic bacterial activity (Lopez et al., 1995), seems to be related
with the bio-availability of sediment organic matter
(Danovaro et al., 2002).
B. Nitrogen and Phosphorus
In both the Mediterranean and southern Australia, environments are characterized by microtidal
regimes and relatively low-nutrient environments.
Thus, paradoxically Posidonia species colonize one
of the most nutrient-poor environments in the
world (e.g. Copin-Mont´ egut, 1996; Walker, 2000).
P. oceanica can even colonize ultra-oligotrophic
habitats in the Eastern Mediterranean (if this status is defined by the nutrient concentrations and not
by the biomass). The nitrogen content in Posidonia species varies seasonally and between species
and organs (Augier et al., 1982; Duarte, 1990; Gacia et al., 2002). Nitrogen dynamics in Australian
Posidonia species have been reported for Cockburn
Sound, Shark Bay, and Geographe Bay (Hocking et
al., 1980; Walker and McComb, 1988; MacMahon
and Walker, 1998; Cambridge, 1996; Cambridge and
Hocking, 1997; Paling and McComb, 2000).
Contrary to other Mediterranean species such as
Cymodocea nodosa (P` erez and Romero, 1994), P.
oceanica dynamics are not (or little) limited by nutrient availability (e.g. Alcoverro et al., 1995, 1997a)
and P. oceanica seems better adapted to nutrientpoor conditions relative to smaller Mediterranean
seagrasses (Marb` a et al., 1996), both from morphological and growth characteristics (Marb` a et al.,
1996). P. oceanica sediments receive enough nitrogen from depostion of decaying leaves to sustain the
plants’ needs, if recycling is efficient (Gacia et al.,
2002). Phosphorus appeared to limit seagrass production in Shark Bay (Australia) (Smith and Atkinson, 1983). If limitation by nutrient availability occurs, the consequences for growth are most obvious
during summer (Alcoverro et al., 1997b; Wolterding and Larkum, 2002). Even in oligotrophic areas,
such as NW Corsica, fertilizers have little effect on
the annual dynamics of P. oceanica (Gobert, unpublished data). P. oceanica may have developed diverse
strategies to buffer and reduce the impact of environmental conditions.
As in many other temperate species, P. oceanica
has a periodic loss of nutrients due to a seasonal leaf
fall (e.g. Hemminga et al., 1991). However, nutrient acquisition (Lepoint et al., 2002a; Lepoint et al.,
2004a), storage and remobilization (Alcoverro et al.,
2000; Invers et al., 2002), photosynthate translocation (Manzanera and Romero, 2000; Marb` a et al.,
2002), and nutrient resorption and recycling (Alcoverro et al., 2000; Lepoint et al., 2002b; Lepoint
et al., 2003; Lepoint et al, 2004b) all contribute to
strategies used by Posidonia. On an annual average,
5–50% of the total nitrogen assimilation occurs in
roots and 50–79% in leaves (Invers et al., 2002; Lepoint et al., 2002a). In addition rhizomes contribute
to the total shoot nitrogen assimilation by 32–54%
between autumn and spring (Invers et al., 2002).
Below-ground organs appear as a key factor in the nitrogen economy of the plant. Roots and rhizomes are
sites for nitrogen assimilation but also as organs for
N storage both inorganic and organic (Invers et al.,
2002; Lepoint et al., 2003). The leaves, comprising
the adult leaves which have little metabolic demand,
participate in nitrogen uptake. Adult leaves quickly
transfer incorporated nitrogen to growing tissues or
storage organs (rhizomes or young leaves) (Lepoint et al., 2002b). Like other marine phanerogams,
P. oceanica plants are able to take up inorganic
nitrogen as nitrate or ammonium (Touchette and
Burkholder, 2000a; Romero et al., Chapter 9), even
at very low concentrations in the water (i.e. 0.05
µM) (Lepoint et al., 2002b). Incorporation is not
limited to periods when nutrient concentrations are
high in the environment (i.e. winter, early spring)
but occurs year-round. Nitrate and ammonium uptake appear equally important on an annual scale
S. Gobert, M. L. Cambridge, B. Velimirov, et al.
environmental conditions (depth, hydrodynamic
conditions, predation pressure) (Pergent et al.,
1997b), among populations (Cebri´ an and Duarte,
2001) and seasons. Primary production is higher
at the shallow edge because of higher bed density.
Depth also has a significant influence on the distribution of litter: within the beds export rates are
highest at shallower stations where the hydrodynamic forces are greater. Consequently, accumulation rates among sites show high variations (i.e. 19–
191 g C m
−2 year
−1 in Romero et al., 1994; 9–112 g
C m
−2 year
−1 in Mat` eo et al., 1997; see also Mateo
et al., Chapter 7).
Meiofaunal assemblages (Danovaro et al., 2002)
(from 0 to 4 cm in the sediment) constitute a biomass
of 1.1 g C m
−2 (i.e. 4,010
3 tons C for the P. oceanica
meadows of the Mediterranean Sea) with a mean
production of 10.0 ± 1.9 g C m
−2 year
−1 (i.e.
409 × 10
3 tons C year
−1 ). The seasonal change
in meiofaunal production, like benthic bacterial activity (Lopez et al., 1995), seems to be related
with the bio-availability of sediment organic matter
(Danovaro et al., 2002).
B. Nitrogen and Phosphorus
In both the Mediterranean and southern Australia, environments are characterized by microtidal
regimes and relatively low-nutrient environments.
Thus, paradoxically Posidonia species colonize one
of the most nutrient-poor environments in the
world (e.g. Copin-Mont´ egut, 1996; Walker, 2000).
P. oceanica can even colonize ultra-oligotrophic
habitats in the Eastern Mediterranean (if this status is defined by the nutrient concentrations and not
by the biomass). The nitrogen content in Posidonia species varies seasonally and between species
and organs (Augier et al., 1982; Duarte, 1990; Gacia et al., 2002). Nitrogen dynamics in Australian
Posidonia species have been reported for Cockburn
Sound, Shark Bay, and Geographe Bay (Hocking et
al., 1980; Walker and McComb, 1988; MacMahon
and Walker, 1998; Cambridge, 1996; Cambridge and
Hocking, 1997; Paling and McComb, 2000).
Contrary to other Mediterranean species such as
Cymodocea nodosa (P` erez and Romero, 1994), P.
oceanica dynamics are not (or little) limited by nutrient availability (e.g. Alcoverro et al., 1995, 1997a)
and P. oceanica seems better adapted to nutrientpoor conditions relative to smaller Mediterranean
seagrasses (Marb` a et al., 1996), both from morphological and growth characteristics (Marb` a et al.,
1996). P. oceanica sediments receive enough nitrogen from depostion of decaying leaves to sustain the
plants’ needs, if recycling is efficient (Gacia et al.,
2002). Phosphorus appeared to limit seagrass production in Shark Bay (Australia) (Smith and Atkinson, 1983). If limitation by nutrient availability occurs, the consequences for growth are most obvious
during summer (Alcoverro et al., 1997b; Wolterding and Larkum, 2002). Even in oligotrophic areas,
such as NW Corsica, fertilizers have little effect on
the annual dynamics of P. oceanica (Gobert, unpublished data). P. oceanica may have developed diverse
strategies to buffer and reduce the impact of environmental conditions.
As in many other temperate species, P. oceanica
has a periodic loss of nutrients due to a seasonal leaf
fall (e.g. Hemminga et al., 1991). However, nutrient acquisition (Lepoint et al., 2002a; Lepoint et al.,
2004a), storage and remobilization (Alcoverro et al.,
2000; Invers et al., 2002), photosynthate translocation (Manzanera and Romero, 2000; Marb` a et al.,
2002), and nutrient resorption and recycling (Alcoverro et al., 2000; Lepoint et al., 2002b; Lepoint
et al., 2003; Lepoint et al, 2004b) all contribute to
strategies used by Posidonia. On an annual average,
5–50% of the total nitrogen assimilation occurs in
roots and 50–79% in leaves (Invers et al., 2002; Lepoint et al., 2002a). In addition rhizomes contribute
to the total shoot nitrogen assimilation by 32–54%
between autumn and spring (Invers et al., 2002).
Below-ground organs appear as a key factor in the nitrogen economy of the plant. Roots and rhizomes are
sites for nitrogen assimilation but also as organs for
N storage both inorganic and organic (Invers et al.,
2002; Lepoint et al., 2003). The leaves, comprising
the adult leaves which have little metabolic demand,
participate in nitrogen uptake. Adult leaves quickly
transfer incorporated nitrogen to growing tissues or
storage organs (rhizomes or young leaves) (Lepoint et al., 2002b). Like other marine phanerogams,
P. oceanica plants are able to take up inorganic
nitrogen as nitrate or ammonium (Touchette and
Burkholder, 2000a; Romero et al., Chapter 9), even
at very low concentrations in the water (i.e. 0.05
µM) (Lepoint et al., 2002b). Incorporation is not
limited to periods when nutrient concentrations are
high in the environment (i.e. winter, early spring)
but occurs year-round. Nitrate and ammonium uptake appear equally important on an annual scale