238
J. Romero, K.-S. Lee, M. P ´
erez, M. A. Mateo, and T. Alcoverro
(e.g. in beds with poor epifauna; see also Bell et al.,
Chapter 26).
A third source of nitrogen (‘new’ nitrogen) in seagrass beds is nitrogen fixation. Nitrogen fixation occurs in both the phyllosphere and the rhizosphere,
being, apparently, more important in the latter than
in the former. Cyanobacteria and other prokaryotes
are responsible for this fixation activity. The contribution of nitrogen-fixing organisms to the nutrient budget of the bed has mostly been evaluated in
tropical areas, where it accounts for inputs in the
range of 2.7–15 g N m
−2 yr
−1 (Capone and Taylor, 1980; O’Donohue et al., 1991; Moriarty and
O’Donohue, 1993; Blackburn et al., 1994). Fewer
data exist for temperate beds, where, apparently, Nfixation is less important (Capone, 1982; Welsh et al.,
1996; McGlathery et al., 1998, between <0.1 and
2.5 g N m
−2 yr
−1 ).
The coupling of nitrogen fixation to other biological processes in the root layer (e.g. Welsh et al., 2000;
see also Marb` a et al., Chapter 6) and its contribution to the nitrogen economy at a regional scale (e.g.
B´ ethoux and Copin-Mont´ egut, 1986) are promising
fields for future developments.
B. Nutrient Losses from the Plant
Nutrients that have been incorporated into seagrass
tissues can be lost by the following processes: leaching, leaf fall, and consumption by herbivores.
Significant amounts of dissolved nutrients are
released by seagrass leaves (McRoy and Goering, 1974; Fresi and Saggiomo, 1980; Penhale and
Thayer, 1980; Brix and Lyngby, 1985; P´ erez-Llor´ ens
et al., 1993), with values typically being between
1% and 4% of the phosphorus absorbed by the roots
(Hemminga et al., 1991). As far as we are aware, this
nutrient release has not been fully investigated, and it
probably has a double origin: losses from living and
healthy tissues and leaching of soluble compounds
from senescing leaves. This flux is generally small,
compared with other fluxes in the ecosystem, and
probably an important part of the nutrients released
are used by epiphytes.
Leaf fall is probably the most important process by
which seagrasses lose nutrients. The timing and rate
of leaf fall vary greatly between species, but in any
case the falling leaves have considerable amounts
of N and P. Available evidence indicates that N and
P content in old leaves, or in recently fallen leaves,
are considerably lower than that of newly growing
leaves, and this greatly depends (if we accept that
nutrient leaching is relatively small) on the resorption efficiency. Typically, fallen leaves have between
25 and 90% of their original nutrient content, with
an average of, approximately, 75% (inferred from
data of Hemminga et al., 1999). The fate of these
nutrients will be discussed later.
Finally, a part of the leaf biomass can be removed
by grazers. Although grazing activity is analyzed
exhaustively elsewhere (see Valentine and Duffy,
Chapter 20), it can be said that in most of the world’s
seagrass beds grazers have little influence on nutrient fluxes. This is based on the fact that grazing
removal of leaf biomass is usually less than 15% of
primary production (Cebri´ an and Duarte, 1998). In
addition, herbivores show preferences towards the
oldest tissues, having less nutrients. However, several facts should be considered that further complicate the assessment of the importance of grazers on
nutrient fluxes. Thus, the preference of herbivores
for the old tissues can curtail nutrient resorption by
the plant; moreover, this preference seems to maximize epiphyte ingestion, in which a large amount of
nutrients concentrate.
In turn, probably only a minor part of the nutrients in leaves and epiphytes ingested by herbivores
will be incorporated into the consumer biomass, due
to the low assimilation efficiency of herbivores (although scarcely measured for nutrients: Thayer et al.,
1982; Velimirov, 1984) or released in soluble inorganic or organic labile compounds. Therefore, most
of these nutrients will end up, via feces, in the detritus pool, indicating that the main role of herbivores
in this respect is to transfer N and P from the plant to
the decomposers food web. However, the impact of
grazers on seagrass nutrient economy will depend
on a key aspect of behavior: thus, species inhabiting permanently the seagrass bed (e.g. sea urchins,
gastropods, some fishes) will cause mostly nutrient
losses from the plant, limiting internal recycling (resorption from old leaves) and increasing external recycling (through decomposition, see below), but will
probably increase only marginally nutrient export
from the ecosystem (i.e. a nutrient in soluble form
or in detritus is more readily exported than a nutrient
in a leaf). In contrast, species feeding in the bed but
living or resting elsewhere (water fowls, sirenidae,
turtles, and some fishes: Thayer et al., 1982; Preen,
1995) induce a net nutrient loss from the ecosystem,
increasing the dependence on external nutrients or,
J. Romero, K.-S. Lee, M. P ´
erez, M. A. Mateo, and T. Alcoverro
(e.g. in beds with poor epifauna; see also Bell et al.,
Chapter 26).
A third source of nitrogen (‘new’ nitrogen) in seagrass beds is nitrogen fixation. Nitrogen fixation occurs in both the phyllosphere and the rhizosphere,
being, apparently, more important in the latter than
in the former. Cyanobacteria and other prokaryotes
are responsible for this fixation activity. The contribution of nitrogen-fixing organisms to the nutrient budget of the bed has mostly been evaluated in
tropical areas, where it accounts for inputs in the
range of 2.7–15 g N m
−2 yr
−1 (Capone and Taylor, 1980; O’Donohue et al., 1991; Moriarty and
O’Donohue, 1993; Blackburn et al., 1994). Fewer
data exist for temperate beds, where, apparently, Nfixation is less important (Capone, 1982; Welsh et al.,
1996; McGlathery et al., 1998, between <0.1 and
2.5 g N m
−2 yr
−1 ).
The coupling of nitrogen fixation to other biological processes in the root layer (e.g. Welsh et al., 2000;
see also Marb` a et al., Chapter 6) and its contribution to the nitrogen economy at a regional scale (e.g.
B´ ethoux and Copin-Mont´ egut, 1986) are promising
fields for future developments.
B. Nutrient Losses from the Plant
Nutrients that have been incorporated into seagrass
tissues can be lost by the following processes: leaching, leaf fall, and consumption by herbivores.
Significant amounts of dissolved nutrients are
released by seagrass leaves (McRoy and Goering, 1974; Fresi and Saggiomo, 1980; Penhale and
Thayer, 1980; Brix and Lyngby, 1985; P´ erez-Llor´ ens
et al., 1993), with values typically being between
1% and 4% of the phosphorus absorbed by the roots
(Hemminga et al., 1991). As far as we are aware, this
nutrient release has not been fully investigated, and it
probably has a double origin: losses from living and
healthy tissues and leaching of soluble compounds
from senescing leaves. This flux is generally small,
compared with other fluxes in the ecosystem, and
probably an important part of the nutrients released
are used by epiphytes.
Leaf fall is probably the most important process by
which seagrasses lose nutrients. The timing and rate
of leaf fall vary greatly between species, but in any
case the falling leaves have considerable amounts
of N and P. Available evidence indicates that N and
P content in old leaves, or in recently fallen leaves,
are considerably lower than that of newly growing
leaves, and this greatly depends (if we accept that
nutrient leaching is relatively small) on the resorption efficiency. Typically, fallen leaves have between
25 and 90% of their original nutrient content, with
an average of, approximately, 75% (inferred from
data of Hemminga et al., 1999). The fate of these
nutrients will be discussed later.
Finally, a part of the leaf biomass can be removed
by grazers. Although grazing activity is analyzed
exhaustively elsewhere (see Valentine and Duffy,
Chapter 20), it can be said that in most of the world’s
seagrass beds grazers have little influence on nutrient fluxes. This is based on the fact that grazing
removal of leaf biomass is usually less than 15% of
primary production (Cebri´ an and Duarte, 1998). In
addition, herbivores show preferences towards the
oldest tissues, having less nutrients. However, several facts should be considered that further complicate the assessment of the importance of grazers on
nutrient fluxes. Thus, the preference of herbivores
for the old tissues can curtail nutrient resorption by
the plant; moreover, this preference seems to maximize epiphyte ingestion, in which a large amount of
nutrients concentrate.
In turn, probably only a minor part of the nutrients in leaves and epiphytes ingested by herbivores
will be incorporated into the consumer biomass, due
to the low assimilation efficiency of herbivores (although scarcely measured for nutrients: Thayer et al.,
1982; Velimirov, 1984) or released in soluble inorganic or organic labile compounds. Therefore, most
of these nutrients will end up, via feces, in the detritus pool, indicating that the main role of herbivores
in this respect is to transfer N and P from the plant to
the decomposers food web. However, the impact of
grazers on seagrass nutrient economy will depend
on a key aspect of behavior: thus, species inhabiting permanently the seagrass bed (e.g. sea urchins,
gastropods, some fishes) will cause mostly nutrient
losses from the plant, limiting internal recycling (resorption from old leaves) and increasing external recycling (through decomposition, see below), but will
probably increase only marginally nutrient export
from the ecosystem (i.e. a nutrient in soluble form
or in detritus is more readily exported than a nutrient
in a leaf). In contrast, species feeding in the bed but
living or resting elsewhere (water fowls, sirenidae,
turtles, and some fishes: Thayer et al., 1982; Preen,
1995) induce a net nutrient loss from the ecosystem,
increasing the dependence on external nutrients or,