Chapter 9 Nutrients and Seagrasses
239
what is the same, increasing the new to recycled production ratio (see below).
A final word of caution when considering the
role of herbivores in nutrient fluxes has to be said.
While the general view of low and moderate grazing
seems to be the rule, an increasing number of cases
of intense grazing activity, and even of overgrazing events, have been reported, by fish, sea urchins,
and water fowl (see Valentine and Heck, 1999, for
a review, and Valentine and Duffy, Chapter 20). In
all these situations, herbivores can become the major force driving key nutrient fluxes (nutrient losses
from the plant and/or nutrient export outside the seagrass bed).
C. Nutrient Regeneration Within
the Ecosystem
Nutrients that have been incorporated into the leaves
can then follow three pathways: they can be retranslocated to other plant parts (young leaves, rhizomes, etc.), they can be leached and they can remain
in the leaf tissue until leaf fall occurs. The first two
possibilities have been already addressed; we will
focus here on the third one.
Leaf litter is probably the main source for regenerated nutrients in the seagrass bed, and decay (or
decomposition) is the process by which nutrients
are returned to soluble form. Estimates of the importance of nutrient regeneration through leaf decomposition in the nutrient budget are scarce. Reported values are in the range 10–46% of the N
annual demands for leaf production (Walker and
McComb, 1985; Romero et al., 1992; Mateo and
Romero, 1997; species studied: Amphibolis antarctica, Posidonia australis, Posidonia oceanica, and
Cymodocea nodosa), and slightly higher for P where
data were available. These values depend, on the one
hand, on the seagrass capacity to absorb these nutrients, and, on the other hand, on the export rates (see
below). The range of values are much wider when
considering other communities or geographical areas, where export rates can be from near zero (e.g.
coastal lagoons) to near 100% of the primary production (e.g. surf zones, such as those occupied by
Phyllospadix spp.; see below and also Mateo et al.,
Chapter 7).
Nutrient dynamics during the decomposition process is rather complex, and clearly differs from the
kinetics of weight loss, which follow a typical negative exponential (Olson, 1963). Classically, three
phases are distinguished during leaf decay: (i) leaching, (ii) microbial, and (iii) refractory phases (Kristensen, 1994; Valiela, 1995; see also Mateo et al.,
Chapter 7). During the leaching phase, nutrients are
released in very labile forms, and some of them are
rapidly incorporated by microbes coating the leaf
detritus. When all labile substrates have been used,
bacteria begin to attack the dead leaf tissues using
exoenzymes; this implies the release of nutrients
in either inorganic or organic labile forms, part of
which are again incorporated by bacteria. Finally, in
the refractory phase, decay is very slow, although
in some cases N-rich compounds are still being decomposed, while most carbon, bound in condensed
humic compounds, is not (Kristensen, 1994). There
is probably no common pattern of nutrient content
change during leaf decay (Peduzzi and Herndl, 1991;
Romero et al., 1992; Kristensen, 1994; Mateo and
Romero, 1997; Miyajima et al., 1998; Holmer and
Olsen, 2002; see Fig. 3); however, the contention of
Harrison (1989) about nitrogen not accumulating in
leaf detritus seems to be valid, at least in general
terms. Nutrients released in dissolved forms can be
re-used by seagrasses or other primary producers;
those incorporated by bacteria can be remineralized
after detritus ingestion by detritivores; and, finally,
those bound to refractory compounds in detritus can
be either exported or stored as organic nutrients in the
seagrass sediments. In addition, the fact that the C:N
and C:P ratios of detritus are considerably higher
than those of the bacterial biomass suggests possible nutrient incorporation by bacteria from sources
other than the detritus and/or nutrient limitation of
bacterial growth (e.g. L´ opez et al., 1998). However,
experimental data indicate that nutrients in the detritus are more important in controlling decay than
nutrients in the water column (P´ erez et al., 2001).
D. Nutrient Losses from the Bed: Export
In contrast to most terrestrial systems, seagrasses
suffer substantial nutrient losses through the export,
sometimes massive, of leaf detritus. An important
part of seagrass production can exit the seagrass
bed, transported by waves and currents, to adjacent
ecosystems including terrestrial, inter-tidal, and subtidal environments (see Bell et al., Chapter 26).
Export rates reported in the literature present an
extremely wide range (from 0% to 90% of the leaf
production: see Mateo et al., Chapter 7). Data about
the amount of nutrients exported in this way are
239
what is the same, increasing the new to recycled production ratio (see below).
A final word of caution when considering the
role of herbivores in nutrient fluxes has to be said.
While the general view of low and moderate grazing
seems to be the rule, an increasing number of cases
of intense grazing activity, and even of overgrazing events, have been reported, by fish, sea urchins,
and water fowl (see Valentine and Heck, 1999, for
a review, and Valentine and Duffy, Chapter 20). In
all these situations, herbivores can become the major force driving key nutrient fluxes (nutrient losses
from the plant and/or nutrient export outside the seagrass bed).
C. Nutrient Regeneration Within
the Ecosystem
Nutrients that have been incorporated into the leaves
can then follow three pathways: they can be retranslocated to other plant parts (young leaves, rhizomes, etc.), they can be leached and they can remain
in the leaf tissue until leaf fall occurs. The first two
possibilities have been already addressed; we will
focus here on the third one.
Leaf litter is probably the main source for regenerated nutrients in the seagrass bed, and decay (or
decomposition) is the process by which nutrients
are returned to soluble form. Estimates of the importance of nutrient regeneration through leaf decomposition in the nutrient budget are scarce. Reported values are in the range 10–46% of the N
annual demands for leaf production (Walker and
McComb, 1985; Romero et al., 1992; Mateo and
Romero, 1997; species studied: Amphibolis antarctica, Posidonia australis, Posidonia oceanica, and
Cymodocea nodosa), and slightly higher for P where
data were available. These values depend, on the one
hand, on the seagrass capacity to absorb these nutrients, and, on the other hand, on the export rates (see
below). The range of values are much wider when
considering other communities or geographical areas, where export rates can be from near zero (e.g.
coastal lagoons) to near 100% of the primary production (e.g. surf zones, such as those occupied by
Phyllospadix spp.; see below and also Mateo et al.,
Chapter 7).
Nutrient dynamics during the decomposition process is rather complex, and clearly differs from the
kinetics of weight loss, which follow a typical negative exponential (Olson, 1963). Classically, three
phases are distinguished during leaf decay: (i) leaching, (ii) microbial, and (iii) refractory phases (Kristensen, 1994; Valiela, 1995; see also Mateo et al.,
Chapter 7). During the leaching phase, nutrients are
released in very labile forms, and some of them are
rapidly incorporated by microbes coating the leaf
detritus. When all labile substrates have been used,
bacteria begin to attack the dead leaf tissues using
exoenzymes; this implies the release of nutrients
in either inorganic or organic labile forms, part of
which are again incorporated by bacteria. Finally, in
the refractory phase, decay is very slow, although
in some cases N-rich compounds are still being decomposed, while most carbon, bound in condensed
humic compounds, is not (Kristensen, 1994). There
is probably no common pattern of nutrient content
change during leaf decay (Peduzzi and Herndl, 1991;
Romero et al., 1992; Kristensen, 1994; Mateo and
Romero, 1997; Miyajima et al., 1998; Holmer and
Olsen, 2002; see Fig. 3); however, the contention of
Harrison (1989) about nitrogen not accumulating in
leaf detritus seems to be valid, at least in general
terms. Nutrients released in dissolved forms can be
re-used by seagrasses or other primary producers;
those incorporated by bacteria can be remineralized
after detritus ingestion by detritivores; and, finally,
those bound to refractory compounds in detritus can
be either exported or stored as organic nutrients in the
seagrass sediments. In addition, the fact that the C:N
and C:P ratios of detritus are considerably higher
than those of the bacterial biomass suggests possible nutrient incorporation by bacteria from sources
other than the detritus and/or nutrient limitation of
bacterial growth (e.g. L´ opez et al., 1998). However,
experimental data indicate that nutrients in the detritus are more important in controlling decay than
nutrients in the water column (P´ erez et al., 2001).
D. Nutrient Losses from the Bed: Export
In contrast to most terrestrial systems, seagrasses
suffer substantial nutrient losses through the export,
sometimes massive, of leaf detritus. An important
part of seagrass production can exit the seagrass
bed, transported by waves and currents, to adjacent
ecosystems including terrestrial, inter-tidal, and subtidal environments (see Bell et al., Chapter 26).
Export rates reported in the literature present an
extremely wide range (from 0% to 90% of the leaf
production: see Mateo et al., Chapter 7). Data about
the amount of nutrients exported in this way are