Chapter 9 Nutrients and Seagrasses
247
and nutrient demands (Williams, 1987; Fourqurean
et al., 1995). Competition between seagrasses and
macroalgae (other than epiphytes) can also be modulated by nutrients; in effect, it was observed in a
mixed bed of the native seagrass Cymodocea nodosa
and the introduced alga Caulerpa taxifolia that nutrient additions enhanced C. taxifolia growth on the
short term, but this did not lead to the exclusion of the
seagrass, since coexistence was reached in the long
term (Ceccherelli and Cinelli, 1997; Ceccherelli and
Sechi, 2002).
5. Interaction Networks
Other modifications of the interaction network have
been less explored. One of the ecosystem components susceptible to nutrient additions is the bacterial
flora, which is of major importance but has received
only marginal attention. For example, sediment nutrient enrichment in oligotrophic waters can stimulate bacterial growth and activity, with bacteria competing for nutrients with seagrass roots (L´ opez et al.,
1998) and eventually induce oxygen depletion in the
pore water. Phosphorus addition can be beneficial
for N-fixating prokaryotes, potentially confounding
the results of fertilization experiments (Powell et al.,
1989). The role of bacteria in the nutrient cycling in
seagrass beds has been studied, but much more needs
to be elucidated in order to yield an understanding
of the controls on bacterial activity.
Another aspect of the interaction network modified by nutrient availability concerns the interplay
among epiphytes, herbivores, and the plant itself. In
a relatively large area affected by nutrient increase,
the standing crop of seagrass was clearly declining,
and a die-off of the bed was observed, due to a large
increase in the herbivore population and in herbivore activity (Ruiz et al., 2001). The reason for this
is unclear, but it can be hypothesized that carbon
skeletons were diverted from secondary metabolism
to growth as nutrient availability increased, causing a decrease in chemical deterrents. An alternative
(or complementary) explanation is that the increase
of N in leaves made them more palatable to herbivores. Similar findings were reported in McGlathery
(1995). Hence, nutrients would have to shift the control of seagrass abundance from resources (bottomup) to predators (top-down). However, herbivore–
plant interaction is more complex, since it is, at least
in part, mediated by epiphytes, and herbivores may
be more attracted by algal epiphytes than by seagrass leaves (Kitting et al., 1984). However, epiphytes are also strongly affected (both in quality and
in quantity) by nutrients, and in turn can reduce light
reaching the leaf surface. Therefore, it is also possible to find positive effect of herbivores under highnutrient availability, by reducing epiphyte biomass
(Heck et al., 2000).
In short, it is clear that a complete understanding
of the consequences of nutrient enrichment for seagrass ecosystems has not yet been fully achieved, and
while physiological and other individual processes
are relatively well known, some of the responses at
the ecosystem level still need to be elucidated, mostly
those linked to biotic interactions. Yet it is clear that
this is a key point in seagrass ecology, as an issue of
basic knowledge but also as a tool to predict, mitigate, and/or manage the impacts of anthropogenic
eutrophication (see Ralph et al., Chapter 24).
V. Conclusion and Future Goals
Nutrient fluxes are essential components of the material and energy flow that sustains ecosystems.
Nutrient–seagrass interactions can be viewed under,
at least, three perspectives: (i) nutrient dynamics in
seagrass beds (how nutrients enter, how they are used
and recycled, and how they leave these ecosystems),
(ii) influences of nutrient abundance on seagrass
beds (how changes in nutrient supply alter functional
aspects of the plant and modify ecological interactions), and (iii) influence of seagrass beds on nutrient
cycles (how seagrasses modify the general nutrient
fluxes in coastal waters). This chapter has focused
mainly on the first two, and it is clear that a good understanding of them requires a multilevel approach,
encompassing perspectives ranging from physiological, and even molecular, to ecological, and even biogeochemical. The effects of nutrient additions provide a good example of this hierarchical nature of
seagrass–nutrient interactions, which includes from
changes in physiology to changes in the community
interaction network.
A large body of work has been done in this field,
which is, certainly, a favorite topic in seagrass biology. In this chapter, which was not aimed at providing
a thorough and exhaustive literature review, around
150 references (strictly concerning seagrasses) have
been cited, evidence in itself of a sustained research
effort. Although this research effort has allowed a
progressive understanding of the different issues expressed above, large gaps, concerning almost any of
the subjects addressed (e.g. within-plant transport
247
and nutrient demands (Williams, 1987; Fourqurean
et al., 1995). Competition between seagrasses and
macroalgae (other than epiphytes) can also be modulated by nutrients; in effect, it was observed in a
mixed bed of the native seagrass Cymodocea nodosa
and the introduced alga Caulerpa taxifolia that nutrient additions enhanced C. taxifolia growth on the
short term, but this did not lead to the exclusion of the
seagrass, since coexistence was reached in the long
term (Ceccherelli and Cinelli, 1997; Ceccherelli and
Sechi, 2002).
5. Interaction Networks
Other modifications of the interaction network have
been less explored. One of the ecosystem components susceptible to nutrient additions is the bacterial
flora, which is of major importance but has received
only marginal attention. For example, sediment nutrient enrichment in oligotrophic waters can stimulate bacterial growth and activity, with bacteria competing for nutrients with seagrass roots (L´ opez et al.,
1998) and eventually induce oxygen depletion in the
pore water. Phosphorus addition can be beneficial
for N-fixating prokaryotes, potentially confounding
the results of fertilization experiments (Powell et al.,
1989). The role of bacteria in the nutrient cycling in
seagrass beds has been studied, but much more needs
to be elucidated in order to yield an understanding
of the controls on bacterial activity.
Another aspect of the interaction network modified by nutrient availability concerns the interplay
among epiphytes, herbivores, and the plant itself. In
a relatively large area affected by nutrient increase,
the standing crop of seagrass was clearly declining,
and a die-off of the bed was observed, due to a large
increase in the herbivore population and in herbivore activity (Ruiz et al., 2001). The reason for this
is unclear, but it can be hypothesized that carbon
skeletons were diverted from secondary metabolism
to growth as nutrient availability increased, causing a decrease in chemical deterrents. An alternative
(or complementary) explanation is that the increase
of N in leaves made them more palatable to herbivores. Similar findings were reported in McGlathery
(1995). Hence, nutrients would have to shift the control of seagrass abundance from resources (bottomup) to predators (top-down). However, herbivore–
plant interaction is more complex, since it is, at least
in part, mediated by epiphytes, and herbivores may
be more attracted by algal epiphytes than by seagrass leaves (Kitting et al., 1984). However, epiphytes are also strongly affected (both in quality and
in quantity) by nutrients, and in turn can reduce light
reaching the leaf surface. Therefore, it is also possible to find positive effect of herbivores under highnutrient availability, by reducing epiphyte biomass
(Heck et al., 2000).
In short, it is clear that a complete understanding
of the consequences of nutrient enrichment for seagrass ecosystems has not yet been fully achieved, and
while physiological and other individual processes
are relatively well known, some of the responses at
the ecosystem level still need to be elucidated, mostly
those linked to biotic interactions. Yet it is clear that
this is a key point in seagrass ecology, as an issue of
basic knowledge but also as a tool to predict, mitigate, and/or manage the impacts of anthropogenic
eutrophication (see Ralph et al., Chapter 24).
V. Conclusion and Future Goals
Nutrient fluxes are essential components of the material and energy flow that sustains ecosystems.
Nutrient–seagrass interactions can be viewed under,
at least, three perspectives: (i) nutrient dynamics in
seagrass beds (how nutrients enter, how they are used
and recycled, and how they leave these ecosystems),
(ii) influences of nutrient abundance on seagrass
beds (how changes in nutrient supply alter functional
aspects of the plant and modify ecological interactions), and (iii) influence of seagrass beds on nutrient
cycles (how seagrasses modify the general nutrient
fluxes in coastal waters). This chapter has focused
mainly on the first two, and it is clear that a good understanding of them requires a multilevel approach,
encompassing perspectives ranging from physiological, and even molecular, to ecological, and even biogeochemical. The effects of nutrient additions provide a good example of this hierarchical nature of
seagrass–nutrient interactions, which includes from
changes in physiology to changes in the community
interaction network.
A large body of work has been done in this field,
which is, certainly, a favorite topic in seagrass biology. In this chapter, which was not aimed at providing
a thorough and exhaustive literature review, around
150 references (strictly concerning seagrasses) have
been cited, evidence in itself of a sustained research
effort. Although this research effort has allowed a
progressive understanding of the different issues expressed above, large gaps, concerning almost any of
the subjects addressed (e.g. within-plant transport