228
J. Romero, K.-S. Lee, M. P ´
erez, M. A. Mateo, and T. Alcoverro
to obtain an oxygen release/carbon assimilation molar quotient of 1.2.
This equation will not be discussed further, as it
is merely an orientation and disregards some basic
facts, such as the (frequent) assimilation of ammonium instead of nitrate or the participation of the
other essential elements. But it has the advantage of
reminding us that plant growth is, to a certain extent, a process requiring specific ‘reagents’ at fixed
proportions. It also highlights what we can consider
the three major nutrients in the sea: carbon dioxide,
nitrate (or the reduced form of inorganic nitrogen,
ammonium), and phosphate. Inorganic carbon is, at
least in bulk concentration, much more abundant
than the other two in marine waters. Despite this, the
importance of the dissolved inorganic carbon supply
and its role in limiting or controlling plant growth is
far from being completely elucidated. However, and
since this topic is addressed elsewhere in this book
(Larkum et al., Chapter 14), nutritional aspects related with carbon will not be further discussed here.
Other elements considered as ‘macronutrients’ in
agriculture or terrestrial ecology (K, S, Ca, and Mg)
are probably not limiting in the marine environment,
given the high concentration at which they occur. In
any case, any reports examining their role in seagrass
production are not known to us.
C. Other Minor Elements That Can Act
as Nutrients
Micronutrients are considered the elements with
very low concentration in plant tissues (in the order
of hundreds of ppm or less), and probably the biologically most important are metals (Fe, Mo, Mn, and
Cu). There are abundant data on concentrations of
these elements in seagrass tissues (e.g. Ward, 1989;
see also Ralph et al., Chapter 24), and about the plant
or ecosystem response to them under some degree of
pollution (e.g. Prange and Dennison, 2000), but unfortunately little is known about their physiological
and ecological role under natural conditions.
Relatively recent experimental work has shown
that iron can limit phytoplankton growth (e.g. Martin
et al., 1994). Since iron additions have been shown
Abbreviations: ABC – ATP binding cassette; tranport protein;
ATP – adenine triphosphate; ATP’ase – a class of enzyme that
hydolyzes adenosine triphosphate; δ
15 N – ratio of
15 Nto
14 N;
DBL – diffusive boundary layer; DOP – dissolved organic phosphorus; POP – particulate organic phosphorus.
to stimulate seagrass growth (Duarte et al., 1995),
it could be supposed that iron can be a limiting element. However, the supply of trace metals can enhance nitrogen fixation, or can protect plant roots
through binding the toxic sulphide ion (Chambers et
al., 2001; Holmer et al., 2003), or even increase alkaline phosphatase activity, facilitating phosphorus
uptake (see Marb` a et al., Chapter 6). Therefore, the
possibility of iron deficiency remains uncertain.
In view of the lack of research on micronutrients,
the major focus of the rest of this chapter will be on
nitrogen and phosphorus.
D. The Importance of Nutrients
in Seagrass Ecology
The interest of seagrass ecologists in nutrient dynamics probably comes from two contrasting approaches. On the one hand, seagrass beds are sites of
high, sometimes luxuriant, primary production. On
the other hand, seagrasses frequently occur in oligotrophic waters (e.g. Mediterranean, tropical waters). The question of how seagrasses are able to
sustain such a high primary productivity under conditions of low-nutrient availability has often puzzled
the scientific community, just as it has for coral reefs.
Moreover, the fact that seagrasses are rooted plants
raises, in this context, the additional question about
how such ‘uncommon’ marine plants perform under
aquatic conditions.
On the other hand, the recent increase of nutrient
concentrations in coastal waters, following the process of anthropogenic eutrophication, has promoted
a considerable interest in the effects of increased nutrient levels on plant physiology and ecosystem functioning (see Walker et al., Chapter 23; Ralph et al.,
Chapter 24; Kenworthy et al., Chapter 25).
II. Nutrient Economy: Acquisition,
Transport, and Storage
A. Nutrient Sources
In contrast to terrestrial plants, submerged aquatic
vascular plants, including seagrasses, can take up nutrients not only through roots but also through leaves
(Iizumi and Hattori, 1982; Thursby and Harlin, 1982,
1984; Short and McRoy, 1984; Stapel et al., 1996;
Pedersen et al., 1997; Terrados and Williams, 1997;
Lee and Dunton, 1999b). Except for Phyllospadix
J. Romero, K.-S. Lee, M. P ´
erez, M. A. Mateo, and T. Alcoverro
to obtain an oxygen release/carbon assimilation molar quotient of 1.2.
This equation will not be discussed further, as it
is merely an orientation and disregards some basic
facts, such as the (frequent) assimilation of ammonium instead of nitrate or the participation of the
other essential elements. But it has the advantage of
reminding us that plant growth is, to a certain extent, a process requiring specific ‘reagents’ at fixed
proportions. It also highlights what we can consider
the three major nutrients in the sea: carbon dioxide,
nitrate (or the reduced form of inorganic nitrogen,
ammonium), and phosphate. Inorganic carbon is, at
least in bulk concentration, much more abundant
than the other two in marine waters. Despite this, the
importance of the dissolved inorganic carbon supply
and its role in limiting or controlling plant growth is
far from being completely elucidated. However, and
since this topic is addressed elsewhere in this book
(Larkum et al., Chapter 14), nutritional aspects related with carbon will not be further discussed here.
Other elements considered as ‘macronutrients’ in
agriculture or terrestrial ecology (K, S, Ca, and Mg)
are probably not limiting in the marine environment,
given the high concentration at which they occur. In
any case, any reports examining their role in seagrass
production are not known to us.
C. Other Minor Elements That Can Act
as Nutrients
Micronutrients are considered the elements with
very low concentration in plant tissues (in the order
of hundreds of ppm or less), and probably the biologically most important are metals (Fe, Mo, Mn, and
Cu). There are abundant data on concentrations of
these elements in seagrass tissues (e.g. Ward, 1989;
see also Ralph et al., Chapter 24), and about the plant
or ecosystem response to them under some degree of
pollution (e.g. Prange and Dennison, 2000), but unfortunately little is known about their physiological
and ecological role under natural conditions.
Relatively recent experimental work has shown
that iron can limit phytoplankton growth (e.g. Martin
et al., 1994). Since iron additions have been shown
Abbreviations: ABC – ATP binding cassette; tranport protein;
ATP – adenine triphosphate; ATP’ase – a class of enzyme that
hydolyzes adenosine triphosphate; δ
15 N – ratio of
15 Nto
14 N;
DBL – diffusive boundary layer; DOP – dissolved organic phosphorus; POP – particulate organic phosphorus.
to stimulate seagrass growth (Duarte et al., 1995),
it could be supposed that iron can be a limiting element. However, the supply of trace metals can enhance nitrogen fixation, or can protect plant roots
through binding the toxic sulphide ion (Chambers et
al., 2001; Holmer et al., 2003), or even increase alkaline phosphatase activity, facilitating phosphorus
uptake (see Marb` a et al., Chapter 6). Therefore, the
possibility of iron deficiency remains uncertain.
In view of the lack of research on micronutrients,
the major focus of the rest of this chapter will be on
nitrogen and phosphorus.
D. The Importance of Nutrients
in Seagrass Ecology
The interest of seagrass ecologists in nutrient dynamics probably comes from two contrasting approaches. On the one hand, seagrass beds are sites of
high, sometimes luxuriant, primary production. On
the other hand, seagrasses frequently occur in oligotrophic waters (e.g. Mediterranean, tropical waters). The question of how seagrasses are able to
sustain such a high primary productivity under conditions of low-nutrient availability has often puzzled
the scientific community, just as it has for coral reefs.
Moreover, the fact that seagrasses are rooted plants
raises, in this context, the additional question about
how such ‘uncommon’ marine plants perform under
aquatic conditions.
On the other hand, the recent increase of nutrient
concentrations in coastal waters, following the process of anthropogenic eutrophication, has promoted
a considerable interest in the effects of increased nutrient levels on plant physiology and ecosystem functioning (see Walker et al., Chapter 23; Ralph et al.,
Chapter 24; Kenworthy et al., Chapter 25).
II. Nutrient Economy: Acquisition,
Transport, and Storage
A. Nutrient Sources
In contrast to terrestrial plants, submerged aquatic
vascular plants, including seagrasses, can take up nutrients not only through roots but also through leaves
(Iizumi and Hattori, 1982; Thursby and Harlin, 1982,
1984; Short and McRoy, 1984; Stapel et al., 1996;
Pedersen et al., 1997; Terrados and Williams, 1997;
Lee and Dunton, 1999b). Except for Phyllospadix