Chapter 9 Nutrients and Seagrasses
233
Chapter 20) should also be considered in order to
fully understand seagrass nutrient metabolism.
C. Nutrient Conservation Strategies: Internal
Recycling and Storage
A remarkable feature of seagrasses is their efficient
use of nutrients to attenuate losses, through leaf abscission and loss, and to buffer environmental fluctuations in nutrient availability. In effect, on the one
hand plants resorb nutrients (often called nutrient
reclamation) from old tissues and use them in other
plant parts. On the other hand, they can take up
nutrients in excess of immediate metabolic needs,
which is often known as ‘luxury consumption’, and
store them in different forms, before or after assimilation. These strategies reduce the dependence of
seagrasses on the external medium, and are shared
by other primary producers: some algae exhibit luxury consumption (e.g. Chapman and Craigie, 1977),
and terrestrial plants, much closer to seagrasses, exhibit nutrient storage and reclamation (Chapin and
Kedrowski, 1983; Escudero et al., 1992; Reich et al.,
1995, among others).
Little is known about specific mechanisms involved in nutrient resorption; Invers et al. (2002)
proposed that the main form of transport of resorbed
nitrogen would be amino acids. However, the role
of nutrient resorption in seagrass nutrient economy
has been repeatedly evaluated (Pedersen and Borum,
1993; Alcoverro et al., 1997; Mateo and Romero,
1997; Stapel and Hemminga, 1997; Hemminga et al.,
1999; Alcoverro et al., 2000; Stapel et al., 2001; Invers et al., 2002; these works mainly refer to Zostera
marina and Posidonia oceanica). Most of these authors have used a nutrient mass balance approach,
with different degrees of detail in sampling and/or
in elaboration of results. This approach estimates resorption as the difference in nutrient mass between
the leaf (or tissue) having the maximum and the oldest leaf, usually having the minimum, or alternatively, recently shed leaves. However, this has some
methodological uncertainties, including the lack of
knowledge about leaching (that seems to be small:
see Borum et al., 1989 and Pedersen and Borum,
1992 but these works refer only to Zostera marina
in a small geographic area) and the adequacy of the
material taken as reference for nutrient losses, i.e.
if standing leaves are used, resorption may not have
been completed yet and if shed leaves are used, microbial processes may have already changed nutrient
content. The recent use of isotopic tracers seems to
be a promising tool not only to better evaluate the
amount of nutrients resorbed, but also to determine
how they are re-used (Pedersen et al., 1997; Stapel
et al., 2001; Marb` a et al., 2002; Lepoint et al., 2002a).
So far, however, the results of these works seem to
agree, at least in general terms, with those using
the mass balance approach (compare, for example,
Lepoint et al., 2002a and Alcoverro et al., 2000 for
P. oceanica; and Stapel et al., 2001; Stapel and Hemminga, 1997; and Martins and Bandeira, 2001 for
Thalassia hemprichii).
The values for nutrient resorption obtained in the
works cited above range from 10% to 50% of annual N requirements, and slightly more for P requirements. Average values of 20.4% and 21.9%, for nitrogen and phosphorus, respectively, are mentioned
in the review by Hemminga et al. (1999). These
amounts are, in general, lower than those reported
for terrestrial plants (see, for example, Chapin and
Kedrowski, 1983; Aerts, 1996), but represent a substantial contribution to the annual nutrient budgets
of seagrasses. Resorption capacity could be linked to
leaf longevity, and thus long life-span of leaves could
be viewed as an advantage in oligotrophic waters.
This is the case for P. oceanica, which has the highest resorption rates evaluated so far (Alcoverro et al.,
2000; Lepoint et al., 2002a), and whose leaves have
very long life-spans (up to 200–300 days: Romero,
1989; Duarte, 1991). This resorption hypothesis, although consistent with that which occurs in terrestrial plants (e.g. Escudero et al., 1992), has not been
fully demonstrated in seagrasses (Hemminga et al.,
1999).
Storage is an obvious adaptive response to fluctuation in nutrient availability, particularly in large
perennial plants. Both leaves and rhizomes can act
as storage sites in seagrasses, with their relative importance depending on the local nutrient availability
regime, the species physiology, and the life-span of
those organs (Pirc and Wollenweber, 1988; Pedersen
and Borum, 1993; P´ erez et al., 1994; Kraemer and
Mazzella, 1999; Invers et al., 2002).
Nitrogen can be stored in different forms, the major form being amino acids, especially asparagine
(Udy and Dennison, 1997b; Udy et al., 1999). However, soluble protein also accounts for an important
part of nitrogen storage (Murray and Larkum, 1991),
as well as non-soluble compounds, while inorganic compounds seem of minor, if any, importance
233
Chapter 20) should also be considered in order to
fully understand seagrass nutrient metabolism.
C. Nutrient Conservation Strategies: Internal
Recycling and Storage
A remarkable feature of seagrasses is their efficient
use of nutrients to attenuate losses, through leaf abscission and loss, and to buffer environmental fluctuations in nutrient availability. In effect, on the one
hand plants resorb nutrients (often called nutrient
reclamation) from old tissues and use them in other
plant parts. On the other hand, they can take up
nutrients in excess of immediate metabolic needs,
which is often known as ‘luxury consumption’, and
store them in different forms, before or after assimilation. These strategies reduce the dependence of
seagrasses on the external medium, and are shared
by other primary producers: some algae exhibit luxury consumption (e.g. Chapman and Craigie, 1977),
and terrestrial plants, much closer to seagrasses, exhibit nutrient storage and reclamation (Chapin and
Kedrowski, 1983; Escudero et al., 1992; Reich et al.,
1995, among others).
Little is known about specific mechanisms involved in nutrient resorption; Invers et al. (2002)
proposed that the main form of transport of resorbed
nitrogen would be amino acids. However, the role
of nutrient resorption in seagrass nutrient economy
has been repeatedly evaluated (Pedersen and Borum,
1993; Alcoverro et al., 1997; Mateo and Romero,
1997; Stapel and Hemminga, 1997; Hemminga et al.,
1999; Alcoverro et al., 2000; Stapel et al., 2001; Invers et al., 2002; these works mainly refer to Zostera
marina and Posidonia oceanica). Most of these authors have used a nutrient mass balance approach,
with different degrees of detail in sampling and/or
in elaboration of results. This approach estimates resorption as the difference in nutrient mass between
the leaf (or tissue) having the maximum and the oldest leaf, usually having the minimum, or alternatively, recently shed leaves. However, this has some
methodological uncertainties, including the lack of
knowledge about leaching (that seems to be small:
see Borum et al., 1989 and Pedersen and Borum,
1992 but these works refer only to Zostera marina
in a small geographic area) and the adequacy of the
material taken as reference for nutrient losses, i.e.
if standing leaves are used, resorption may not have
been completed yet and if shed leaves are used, microbial processes may have already changed nutrient
content. The recent use of isotopic tracers seems to
be a promising tool not only to better evaluate the
amount of nutrients resorbed, but also to determine
how they are re-used (Pedersen et al., 1997; Stapel
et al., 2001; Marb` a et al., 2002; Lepoint et al., 2002a).
So far, however, the results of these works seem to
agree, at least in general terms, with those using
the mass balance approach (compare, for example,
Lepoint et al., 2002a and Alcoverro et al., 2000 for
P. oceanica; and Stapel et al., 2001; Stapel and Hemminga, 1997; and Martins and Bandeira, 2001 for
Thalassia hemprichii).
The values for nutrient resorption obtained in the
works cited above range from 10% to 50% of annual N requirements, and slightly more for P requirements. Average values of 20.4% and 21.9%, for nitrogen and phosphorus, respectively, are mentioned
in the review by Hemminga et al. (1999). These
amounts are, in general, lower than those reported
for terrestrial plants (see, for example, Chapin and
Kedrowski, 1983; Aerts, 1996), but represent a substantial contribution to the annual nutrient budgets
of seagrasses. Resorption capacity could be linked to
leaf longevity, and thus long life-span of leaves could
be viewed as an advantage in oligotrophic waters.
This is the case for P. oceanica, which has the highest resorption rates evaluated so far (Alcoverro et al.,
2000; Lepoint et al., 2002a), and whose leaves have
very long life-spans (up to 200–300 days: Romero,
1989; Duarte, 1991). This resorption hypothesis, although consistent with that which occurs in terrestrial plants (e.g. Escudero et al., 1992), has not been
fully demonstrated in seagrasses (Hemminga et al.,
1999).
Storage is an obvious adaptive response to fluctuation in nutrient availability, particularly in large
perennial plants. Both leaves and rhizomes can act
as storage sites in seagrasses, with their relative importance depending on the local nutrient availability
regime, the species physiology, and the life-span of
those organs (Pirc and Wollenweber, 1988; Pedersen
and Borum, 1993; P´ erez et al., 1994; Kraemer and
Mazzella, 1999; Invers et al., 2002).
Nitrogen can be stored in different forms, the major form being amino acids, especially asparagine
(Udy and Dennison, 1997b; Udy et al., 1999). However, soluble protein also accounts for an important
part of nitrogen storage (Murray and Larkum, 1991),
as well as non-soluble compounds, while inorganic compounds seem of minor, if any, importance