Chapter 9 Nutrients and Seagrasses
235
all over the plant (multidirectional transport), allows
the translocation of the so-called phloem mobile nutrients (Marschner et al., 1996, 1997). The driving
force for phloem transport is the turgor pressure gradient (created by differences in solute potential between source and sink areas of the plant, Mengel
and Kirkby, 2001). Its role as the main transport
system in seagrasses is supported by morphological evidence, such as the weak wall ingrowths found
in phloem parenchyma cells facing the sieve tubes
(conduits made of connected living cells lacking a
nucleus), which suggests that these parenchyma cells
may play an important role in solute transport (e.g.
Phyllospadix sp., Kuo and Stewart, 1995).
Resource sharing between different shoots (or
ramets) seems to be another essential component of
seagrass nutrient economy, as it is for clonal terrestrial plants (e.g. Herben and Suzuki, 2001), and it
also requires an efficient transport system. Internal
redistribution of nutrients can allow efficient management of these key elements, conducting them to
the sites with maximum demand independently of
the specific sites of acquisition (see Marb` a et al.,
2002). However, despite its apparent importance,
this mechanism has been only rarely investigated, although indirect evidence of such integrated behavior
has been obtained (e.g. Tomasko and Dawes, 1989;
Pedersen and Borum, 1992; Terrados et al., 1997b).
Using short-term experiments (4 days) with stable
isotopic tracers, Marb` a et al. (2002) showed that, developing ramets of several species may receive up to
40% of their nitrogen requirements from the neighboring ones. Resources traveled preferentially from
older to younger (apical) shoots and covered distances up to 80 cm in this 4-day period. It has to be
pointed out that resources are shared between ramets almost immediately after incorporation, meaning that retranslocation affects primary products of
synthesis. This extends and reinforces the concept
of integration for seagrass ramets in the framework
of nutrient metabolism. Further research efforts are
obviously needed in that direction.
III. Nutrient Fluxes in
Seagrass Ecosystems
In seagrass beds a variety of processes driving the
flux of matter within the ecosystem and between the
ecosystem and the rest of the marine environments
take place (see a simplified representation of these
processes for N in Fig. 2). Here we briefly summarize the nutrient fluxes from an ecosystem perspective; as it is true that N and P fluxes often work
in parallel with C fluxes (treated elsewhere in this
book, see Mateo et al., Chapter 7, and Marb` a et al.,
Chapter 6), we will focus only on specific aspects
of key importance to understand nutrient–seagrass
interactions.
A. Nutrient Inputs to Seagrass Ecosystems
A first input of nutrients takes place when the dissolved salts of N and P are extracted from the water by the complex of leaves plus epiphytes, and,
eventually, other micro- and macroalgae attached to
the rhizomes or living in the sediment surface. The
nutrients available in the water for plants include
those coming from outside the seagrass bed (what
we can consider ‘new’ nutrients) and also those released during the process of leaf decay or regenerated
by benthic remineralization and release (‘recycled’
nutrients). The importance of dissolved forms in the
overall nutrient budget of the system will depend
on their mean concentration, the fluctuations around
this mean, and on the turnover (or residence time) of
the water in the seagrass bed.
A second input of nutrients is sedimentation of
seston, whose constituents include variable amounts
of organic N and organic and inorganic P. Seston is
an important nutrient source in seagrass ecosystems,
in part because seagrass leaf canopies act as particle
traps, as they induce relatively calm hydrodynamic
conditions facilitating sedimentation (e.g. Gambi et
al., 1990; Worcester, 1995, among others; see also
Marb` a et al., Chapter 6; Koch et al., Chapter 8); this
implies that N and P inputs from the seston are much
higher under plant canopies than in comparable bare
sediments.
Although the data concerning this nutrient flux
are relatively scarce, it seems to be at least as
important as that from dissolved nutrients uptake
in the ambient water (Risgaard-Petersen et al.,
1998). It has to be acknowledged that the estimate of net nutrient inputs through sedimentation is a rather difficult task, due to a combination of reasons including spatial and temporal
heterogeneity, the complexity of the process itself (balance between sedimentation and resuspension), and the fact that part of the settled particles are originated from the bed. Using different approaches, estimates of net inputs of between
0.3 and 60 g N m
−2 yr
−1 have been proposed
(Harlin et al., 1982; Kenworthy and Thayer, 1984;
235
all over the plant (multidirectional transport), allows
the translocation of the so-called phloem mobile nutrients (Marschner et al., 1996, 1997). The driving
force for phloem transport is the turgor pressure gradient (created by differences in solute potential between source and sink areas of the plant, Mengel
and Kirkby, 2001). Its role as the main transport
system in seagrasses is supported by morphological evidence, such as the weak wall ingrowths found
in phloem parenchyma cells facing the sieve tubes
(conduits made of connected living cells lacking a
nucleus), which suggests that these parenchyma cells
may play an important role in solute transport (e.g.
Phyllospadix sp., Kuo and Stewart, 1995).
Resource sharing between different shoots (or
ramets) seems to be another essential component of
seagrass nutrient economy, as it is for clonal terrestrial plants (e.g. Herben and Suzuki, 2001), and it
also requires an efficient transport system. Internal
redistribution of nutrients can allow efficient management of these key elements, conducting them to
the sites with maximum demand independently of
the specific sites of acquisition (see Marb` a et al.,
2002). However, despite its apparent importance,
this mechanism has been only rarely investigated, although indirect evidence of such integrated behavior
has been obtained (e.g. Tomasko and Dawes, 1989;
Pedersen and Borum, 1992; Terrados et al., 1997b).
Using short-term experiments (4 days) with stable
isotopic tracers, Marb` a et al. (2002) showed that, developing ramets of several species may receive up to
40% of their nitrogen requirements from the neighboring ones. Resources traveled preferentially from
older to younger (apical) shoots and covered distances up to 80 cm in this 4-day period. It has to be
pointed out that resources are shared between ramets almost immediately after incorporation, meaning that retranslocation affects primary products of
synthesis. This extends and reinforces the concept
of integration for seagrass ramets in the framework
of nutrient metabolism. Further research efforts are
obviously needed in that direction.
III. Nutrient Fluxes in
Seagrass Ecosystems
In seagrass beds a variety of processes driving the
flux of matter within the ecosystem and between the
ecosystem and the rest of the marine environments
take place (see a simplified representation of these
processes for N in Fig. 2). Here we briefly summarize the nutrient fluxes from an ecosystem perspective; as it is true that N and P fluxes often work
in parallel with C fluxes (treated elsewhere in this
book, see Mateo et al., Chapter 7, and Marb` a et al.,
Chapter 6), we will focus only on specific aspects
of key importance to understand nutrient–seagrass
interactions.
A. Nutrient Inputs to Seagrass Ecosystems
A first input of nutrients takes place when the dissolved salts of N and P are extracted from the water by the complex of leaves plus epiphytes, and,
eventually, other micro- and macroalgae attached to
the rhizomes or living in the sediment surface. The
nutrients available in the water for plants include
those coming from outside the seagrass bed (what
we can consider ‘new’ nutrients) and also those released during the process of leaf decay or regenerated
by benthic remineralization and release (‘recycled’
nutrients). The importance of dissolved forms in the
overall nutrient budget of the system will depend
on their mean concentration, the fluctuations around
this mean, and on the turnover (or residence time) of
the water in the seagrass bed.
A second input of nutrients is sedimentation of
seston, whose constituents include variable amounts
of organic N and organic and inorganic P. Seston is
an important nutrient source in seagrass ecosystems,
in part because seagrass leaf canopies act as particle
traps, as they induce relatively calm hydrodynamic
conditions facilitating sedimentation (e.g. Gambi et
al., 1990; Worcester, 1995, among others; see also
Marb` a et al., Chapter 6; Koch et al., Chapter 8); this
implies that N and P inputs from the seston are much
higher under plant canopies than in comparable bare
sediments.
Although the data concerning this nutrient flux
are relatively scarce, it seems to be at least as
important as that from dissolved nutrients uptake
in the ambient water (Risgaard-Petersen et al.,
1998). It has to be acknowledged that the estimate of net nutrient inputs through sedimentation is a rather difficult task, due to a combination of reasons including spatial and temporal
heterogeneity, the complexity of the process itself (balance between sedimentation and resuspension), and the fact that part of the settled particles are originated from the bed. Using different approaches, estimates of net inputs of between
0.3 and 60 g N m
−2 yr
−1 have been proposed
(Harlin et al., 1982; Kenworthy and Thayer, 1984;