Chapter 9 Nutrients and Seagrasses
245
(grazers, suspension-feeders, etc.) affected, at their
own individual or population levels. Finally, the interaction network can be modified, with responses at
the community/ecosystem level.
1. Relationship of Ambient Nutrient
Concentration and Uptake Rates
The increase in nutrient availability generally increases nutrient acquisition. In the case of nitrogen, both an increase in uptake (P´ erez-Llor´ ens and
Niell, 1995; Pedersen et al., 1997) and in assimilation
(i.e. glutamine synthase or nitrate reductase activity,
Touchette, 1999) have been reported, and the same is
true for phosphorus (Penhale and Thayer, 1980; Pellikaan and Nienhuis, 1988; P´ erez-Llor´ ens and Niell,
1995).
This increased nutrient acquisition is reflected by
increases in N and P nutrient content (Duarte, 1990;
Bulthuis et al., 1992; Erftemeijer et al., 1994; Alcoverro et al., 1997; Udy and Dennison, 1997a; Lee
and Dunton, 1999a; Terrados et al., 1999), and by
the increase in the tissue content of glutamine and
asparagine (Udy and Dennison, 1997a,b; Udy et al.,
1999). It has also been shown that plants exposed to
higher N concentrations had higher δ
15 N values in
their tissues. However, this shift seems to be caused
by the higher δ
15 N value of the inorganic nitrogen
of anthropogenic origin, more than by changes in
physiological discrimination during the process of N
acquisition. (Udy and Dennison, 1997b; Yamamuro
et al., 2003).
2. Flow-On Metabolic Responses
However, this increased uptake is not without consequences for other components of plant metabolism.
For example, the relative importance of roots vs.
leaves as major sites for nutrient assimilation can
shift (Thursby and Harlin, 1982, 1984). Moreover,
nutrient assimilation (especially nitrogen) requires
carbon skeletons and energy (Turpin, 1991; Huppe
and Turpin, 1994; Touchette, 1999), both of which
are supplied by photosynthesis. Therefore, high nitrogen assimilation may require increases in photosynthetic C-fixation (Agawin et al., 1996; Lee
and Dunton, 1999a). Yet nutrient additions have
been shown to stimulate plant carbon production
through physiological changes in the maximum rate
of photosynthesis, photosynthetic efficiency, and/or
increased chlorophyll concentrations (Agawin et al.,
1996; Lee and Dunton, 1999a). Alternatively, the
need for carbon skeletons can be met by mobilization of carbohydrate reserves, or by the use of fixed
carbon that could have been used to build up such
reserves; in point of fact, decrease of carbohydrate
reserves following nutrient increases has been repeatedly observed (Delgado et al., 1999; Lee and
Dunton, 1999a; Invers et al., 2004) and can eventually compromise future growth (Burkholder et al.,
1992) or overwintering (Alcoverro et al., 1999; see
Fig. 5).
3. Response of Tissues and Organs
At the individual, macroscopic level, increased leaf
growth, production, shoot height, blade length and
width, and biomass are the most commonly reported responses to increased nutrients (Orth, 1977;
Bulthuis and Woelkerling, 1981; P´ erez et al., 1991;
Tomasko and Lapointe, 1991; Murray et al., 1992;
Agawin et al., 1996; Alcoverro et al., 1997; Udy
and Dennison, 1997a; Terrados et al., 1999; Lee and
Dunton, 2000). Another individual response is the
change in the pattern of within-plant resource allocation. In effect, shoot: root ratio increases with
fertility, both following nutrient gradients (P´ erez
et al., 1994; Lee and Dunton, 2000) and in nutrient additions experiments (Powell et al., 1989; Short
et al., 1990; P´ erez et al., 1991). This behavior implies that seagrasses allocate more biomass in leaf
tissues under high-nutrient availability, but more
below-ground biomass under low-nutrient conditions. Plants under sediment nutrient deficient conditions increase biomass allocation to below-ground
tissues to expand surface area for nutrient uptake
(Gleeson, 1993; Vogt et al., 1993), while they increase carbon allocation to the above-ground tissues
as a result of nitrogen addition into sediment (Lee
and Dunton, 1999a). Changes in seagrass biomass
allocation thus reflect a kind of strategy: when nutrients are abundant, leaves seem to be the ‘preferred’
site for uptake; in contrast, when nutrients are scarce,
root uptake is maximized.
4. Influences at the Population Level
Nutrients alter population size and/or structure by
direct and indirect effects. Nutrient addition has
been shown to increase recruitment (P´ erez et al.,
1991; Fourqurean et al., 1995) and to increase
mortality, probably through enhanced intra-specific
245
(grazers, suspension-feeders, etc.) affected, at their
own individual or population levels. Finally, the interaction network can be modified, with responses at
the community/ecosystem level.
1. Relationship of Ambient Nutrient
Concentration and Uptake Rates
The increase in nutrient availability generally increases nutrient acquisition. In the case of nitrogen, both an increase in uptake (P´ erez-Llor´ ens and
Niell, 1995; Pedersen et al., 1997) and in assimilation
(i.e. glutamine synthase or nitrate reductase activity,
Touchette, 1999) have been reported, and the same is
true for phosphorus (Penhale and Thayer, 1980; Pellikaan and Nienhuis, 1988; P´ erez-Llor´ ens and Niell,
1995).
This increased nutrient acquisition is reflected by
increases in N and P nutrient content (Duarte, 1990;
Bulthuis et al., 1992; Erftemeijer et al., 1994; Alcoverro et al., 1997; Udy and Dennison, 1997a; Lee
and Dunton, 1999a; Terrados et al., 1999), and by
the increase in the tissue content of glutamine and
asparagine (Udy and Dennison, 1997a,b; Udy et al.,
1999). It has also been shown that plants exposed to
higher N concentrations had higher δ
15 N values in
their tissues. However, this shift seems to be caused
by the higher δ
15 N value of the inorganic nitrogen
of anthropogenic origin, more than by changes in
physiological discrimination during the process of N
acquisition. (Udy and Dennison, 1997b; Yamamuro
et al., 2003).
2. Flow-On Metabolic Responses
However, this increased uptake is not without consequences for other components of plant metabolism.
For example, the relative importance of roots vs.
leaves as major sites for nutrient assimilation can
shift (Thursby and Harlin, 1982, 1984). Moreover,
nutrient assimilation (especially nitrogen) requires
carbon skeletons and energy (Turpin, 1991; Huppe
and Turpin, 1994; Touchette, 1999), both of which
are supplied by photosynthesis. Therefore, high nitrogen assimilation may require increases in photosynthetic C-fixation (Agawin et al., 1996; Lee
and Dunton, 1999a). Yet nutrient additions have
been shown to stimulate plant carbon production
through physiological changes in the maximum rate
of photosynthesis, photosynthetic efficiency, and/or
increased chlorophyll concentrations (Agawin et al.,
1996; Lee and Dunton, 1999a). Alternatively, the
need for carbon skeletons can be met by mobilization of carbohydrate reserves, or by the use of fixed
carbon that could have been used to build up such
reserves; in point of fact, decrease of carbohydrate
reserves following nutrient increases has been repeatedly observed (Delgado et al., 1999; Lee and
Dunton, 1999a; Invers et al., 2004) and can eventually compromise future growth (Burkholder et al.,
1992) or overwintering (Alcoverro et al., 1999; see
Fig. 5).
3. Response of Tissues and Organs
At the individual, macroscopic level, increased leaf
growth, production, shoot height, blade length and
width, and biomass are the most commonly reported responses to increased nutrients (Orth, 1977;
Bulthuis and Woelkerling, 1981; P´ erez et al., 1991;
Tomasko and Lapointe, 1991; Murray et al., 1992;
Agawin et al., 1996; Alcoverro et al., 1997; Udy
and Dennison, 1997a; Terrados et al., 1999; Lee and
Dunton, 2000). Another individual response is the
change in the pattern of within-plant resource allocation. In effect, shoot: root ratio increases with
fertility, both following nutrient gradients (P´ erez
et al., 1994; Lee and Dunton, 2000) and in nutrient additions experiments (Powell et al., 1989; Short
et al., 1990; P´ erez et al., 1991). This behavior implies that seagrasses allocate more biomass in leaf
tissues under high-nutrient availability, but more
below-ground biomass under low-nutrient conditions. Plants under sediment nutrient deficient conditions increase biomass allocation to below-ground
tissues to expand surface area for nutrient uptake
(Gleeson, 1993; Vogt et al., 1993), while they increase carbon allocation to the above-ground tissues
as a result of nitrogen addition into sediment (Lee
and Dunton, 1999a). Changes in seagrass biomass
allocation thus reflect a kind of strategy: when nutrients are abundant, leaves seem to be the ‘preferred’
site for uptake; in contrast, when nutrients are scarce,
root uptake is maximized.
4. Influences at the Population Level
Nutrients alter population size and/or structure by
direct and indirect effects. Nutrient addition has
been shown to increase recruitment (P´ erez et al.,
1991; Fourqurean et al., 1995) and to increase
mortality, probably through enhanced intra-specific