Chapter 17 Biology of Posidonia
397
(Lepoint et al., 2002b). Nitrate is only an important
source of nitrogen during winter and early spring,
a period of intense incorporation (Alcoverro et al.,
2000).
Nutrients incorporated during periods of low
growth (but high nutrient availability) (i.e. winter)
are stored in rhizomes and new leaves (Alcoverro et
al., 2000; Invers et al., 2002). Nitrogen concentrations in leaves are at a maximum in winter. These
nutrients can be remobilized and translocated to
growing tissues during high growth period (i.e.
spring), explaining the decrease of nitrogen concentrations (Alcoverro et al., 2000). Invers et al. (2002)
showed that this storage is mainly in an organic form
(free amino acids) providing 33% of the nitrogen requirements for spring growth.
In addition to the uptake from external medium,
annual nitrogen demand is partially satisfied by resorption of nitrogen from adult leaves and recycling
of this nitrogen to ensure the growth of young leaves
completes nutrient supply, contributing up to 40%
of the annual N needs of the plant (Alcoverro et al.,
2000; Lepoint et al., 2002a). Furthermore, contributions from epiphytes (Borowitzka et al., Chapter 19), other seston (Romero et al., Chapter 9)
and benthic macro- and microalgae (Mateo et al.,
Chapter 7), together with the cycling of nutrients
from decaying leaves (Gacia et al., 2002) have to be
taken into account in both nitrogen and phosphorus
supply.
Marb` a et al. (2002) showed that strong physiological integrations occur between clonal ramets in
some years. This integration allows the transfer of
photosynthates and amino acids along rhizomes over
a distance of 20–30 cm. This translocation is mainly
directed from parent ramets to young and growing
ramets. This mechanism could be very important to
ensure the N needs and expansion of young ramets
which are generally unrooted during the first months
of their growth.
In some areas, very low-nitrogen concentrations
occur both in the water column and pore water but
P. oceanica meadows display high biomass and productivity (e.g. Calvi, Corsica). During the summer
when the leaves are long (60 cm), the canopy water
(the water in between shoots) displays both cooler
temperatures and much higher nutrient content than
the upper water column. The fluxes measured from
the sediment to the canopy water are sufficient to provide 20% of the mean daily nitrogen requirements
of the P. oceanica shoots (Gobert et al., 2002).
VIII. Food Webs
Posidonia beds form extensive habitat for associated fauna and flora, both in the Mediterranean and
in southern Australia. Food webs in coastal waters are affected by their presence both, by modifying the ecosystem structure, providing habitat, as
well as producing organic matter. This production
is then transferred directly through the consumption
of leaf material by grazers or indirectly through dissolved organic matter and nutrients leaching from
seagrasses being utilized by detritivores or epiphytic
algae (Mateo et al., Chapter 7).
Assimilation of Posidonia-derived organic matter
into food webs occurs not only in the bed, but because of drift of leaf detritus, within habitats that
may be located at considerable distances from the
bed (see Mateo et al., Chapter 7). Depending on
waves and currents, and tidal action in Australia,
debris can be deposited within the seagrass bed or
exported inshore to the swashzone or beaches, to inshore basins or to the deep oceanic floor (Walker et
al., 2001). The presence of Posidonia can greatly alter food webs even in the absence of large vertebrate
grazers through the provision of substrata for epiphytic alga such as diatoms and filamentous algae
or through the production of detritus (Jernakoff and
Nielsen, 1997; Jernakoff and Nielsen, 1998). The
epiphytic community of Posidonia leaves shows a
clear depth zonation and daily (Sanchez-Jerez, 1999)
or seasonal variation (Mazzella et al., 1989; Russo
et al., 2002) and exhibits a time-sequence of leaf
colonization (Mazzella et al., 1992; Trautman and
Borowitzka, 1999; Lavery and Vanderklift, 2002).
The importance of Posidonia to food webs depends largely on the overall productivity of the
system. Thus at exposed, low-nutrient sites where
primary productivity is low, the presence of seagrass
will greatly increase invertebrate and fish production and alter food webs in comparison to unvegetated areas (Edgar and Shaw, 1995c). This is also
the case in deep basins and the swash zone where
seagrass debris accumulates (Lenanton et al., 1982).
In contrast, food webs in nutrient-rich estuaries with
dense phytoplankton populations and considerable
allochthonous inputs of organic matter may be little
affected by the presence of seagrasses. How far and
how much Posidonia production influences faunal
production has not been well quantified.
Posidonia food webs are generally in areas lacking large vertebrate grazers and appear to be fueled
397
(Lepoint et al., 2002b). Nitrate is only an important
source of nitrogen during winter and early spring,
a period of intense incorporation (Alcoverro et al.,
2000).
Nutrients incorporated during periods of low
growth (but high nutrient availability) (i.e. winter)
are stored in rhizomes and new leaves (Alcoverro et
al., 2000; Invers et al., 2002). Nitrogen concentrations in leaves are at a maximum in winter. These
nutrients can be remobilized and translocated to
growing tissues during high growth period (i.e.
spring), explaining the decrease of nitrogen concentrations (Alcoverro et al., 2000). Invers et al. (2002)
showed that this storage is mainly in an organic form
(free amino acids) providing 33% of the nitrogen requirements for spring growth.
In addition to the uptake from external medium,
annual nitrogen demand is partially satisfied by resorption of nitrogen from adult leaves and recycling
of this nitrogen to ensure the growth of young leaves
completes nutrient supply, contributing up to 40%
of the annual N needs of the plant (Alcoverro et al.,
2000; Lepoint et al., 2002a). Furthermore, contributions from epiphytes (Borowitzka et al., Chapter 19), other seston (Romero et al., Chapter 9)
and benthic macro- and microalgae (Mateo et al.,
Chapter 7), together with the cycling of nutrients
from decaying leaves (Gacia et al., 2002) have to be
taken into account in both nitrogen and phosphorus
supply.
Marb` a et al. (2002) showed that strong physiological integrations occur between clonal ramets in
some years. This integration allows the transfer of
photosynthates and amino acids along rhizomes over
a distance of 20–30 cm. This translocation is mainly
directed from parent ramets to young and growing
ramets. This mechanism could be very important to
ensure the N needs and expansion of young ramets
which are generally unrooted during the first months
of their growth.
In some areas, very low-nitrogen concentrations
occur both in the water column and pore water but
P. oceanica meadows display high biomass and productivity (e.g. Calvi, Corsica). During the summer
when the leaves are long (60 cm), the canopy water
(the water in between shoots) displays both cooler
temperatures and much higher nutrient content than
the upper water column. The fluxes measured from
the sediment to the canopy water are sufficient to provide 20% of the mean daily nitrogen requirements
of the P. oceanica shoots (Gobert et al., 2002).
VIII. Food Webs
Posidonia beds form extensive habitat for associated fauna and flora, both in the Mediterranean and
in southern Australia. Food webs in coastal waters are affected by their presence both, by modifying the ecosystem structure, providing habitat, as
well as producing organic matter. This production
is then transferred directly through the consumption
of leaf material by grazers or indirectly through dissolved organic matter and nutrients leaching from
seagrasses being utilized by detritivores or epiphytic
algae (Mateo et al., Chapter 7).
Assimilation of Posidonia-derived organic matter
into food webs occurs not only in the bed, but because of drift of leaf detritus, within habitats that
may be located at considerable distances from the
bed (see Mateo et al., Chapter 7). Depending on
waves and currents, and tidal action in Australia,
debris can be deposited within the seagrass bed or
exported inshore to the swashzone or beaches, to inshore basins or to the deep oceanic floor (Walker et
al., 2001). The presence of Posidonia can greatly alter food webs even in the absence of large vertebrate
grazers through the provision of substrata for epiphytic alga such as diatoms and filamentous algae
or through the production of detritus (Jernakoff and
Nielsen, 1997; Jernakoff and Nielsen, 1998). The
epiphytic community of Posidonia leaves shows a
clear depth zonation and daily (Sanchez-Jerez, 1999)
or seasonal variation (Mazzella et al., 1989; Russo
et al., 2002) and exhibits a time-sequence of leaf
colonization (Mazzella et al., 1992; Trautman and
Borowitzka, 1999; Lavery and Vanderklift, 2002).
The importance of Posidonia to food webs depends largely on the overall productivity of the
system. Thus at exposed, low-nutrient sites where
primary productivity is low, the presence of seagrass
will greatly increase invertebrate and fish production and alter food webs in comparison to unvegetated areas (Edgar and Shaw, 1995c). This is also
the case in deep basins and the swash zone where
seagrass debris accumulates (Lenanton et al., 1982).
In contrast, food webs in nutrient-rich estuaries with
dense phytoplankton populations and considerable
allochthonous inputs of organic matter may be little
affected by the presence of seagrasses. How far and
how much Posidonia production influences faunal
production has not been well quantified.
Posidonia food webs are generally in areas lacking large vertebrate grazers and appear to be fueled