Chapter 9 Nutrients and Seagrasses
243
seagrass (and other primary producers within the
seagrass bed) growth is met by nutrients coming
from outside the system (‘new nutrients’) or recycled within the system, where the boundaries of the
system are defined by the limits of the seagrass distribution, and include some portion of the water column and the upper layers of the sediment. The nutrients can be imported through diffusion or advection,
through sedimentation (including filter feeder activity) or, more rarely, through nitrogen fixation. Recycling includes metabolic recycling (nutrient recovery, storage, etc.) and ecosystem recycling (leaching
and in situ organic matter mineralization, either in
the water column, in the sediment surface, or within
the sediment). Under the assumption of steady state
(that may not necessarily be met), import of new
nutrient should equal nutrient losses: burial, export, and, eventually, denitrification. Paralleling classic concepts in plankton ecology (e.g. Dugdale and
Goering, 1967; Eppley and Peterson, 1979), it could
be said that the relation of the exchange component
to the recycled material (or to total, to formally adapt
to the f -ratio used in planktonic systems) provides
a rough measure of the dependence of the bed on
external nutrient sources. Knowledge of how this
dependence varies geographically, depending on the
water nutrient status or across seagrass species in
various ecosystems would be very valuable but has
never been attempted, to our knowledge. Yet is has to
be acknowledged that the determination of f -ratio in
seagrasses poses serious methodological problems,
that could be circumvented in the future using stable isotope tracers (see Mateo et al., Chapter 7). In
any case, an ecosystem approach to the interactions
between nutrients and seagrasses, although difficult,
would be desirable to better address key issues, for
example those related to seagrass conservation or
the biogeochemistry of coastal waters.
IV. Nutrient Limitation and Nutrient
Imbalances
A. An Old and Frequently Tested Paradigm
The luxuriant growth, the impressive biomass, and
the large areas covered by seagrasses, especially in
oligotrophic waters, suggest a very high-nutrient demand to sustain such growth, and point to a possible nutrient limitation. To grow successfully, these
plants need physical space (substrate), solar radiation, water, inorganic carbon, and sufficient amounts
of all the macronutrients and micronutrients. Among
potential limiting factors, and at least in shallow waters, nutrients are the top candidates to be actual
limiting resources (see, however, the role of carbon,
Larkum et al., Chapter 14).
Thus, it is by no means surprising that the hypothesis of nutrient limitation has been repeatedly
addressed, generally using one of the three following approaches: (i) correlational, through the study
of plants along nutrient availability gradients, (ii)
observational, through the analysis of nutrient concentration and/or proportions in plant tissues, (iii)
experimental, through manipulations of natural nutrient concentrations (at least from the early work of
Orth, 1977). All three approaches have their weaknesses and their strengths. While it is obvious that
the experimental approach is the most robust, it is
limited in: space (because a reduced number of plots
at a reduced number of sites are studied), time (usually experimental fertilization is done during a short
time, typically a year or less), and scope (fertilization mostly affects the plants, but hardly, for reasons
of scale, the whole ecosystem). These shortcomings
can be circumvented by increasing the field work effort (Fourqurean et al., 1995) or, more economically,
by using the two other approaches.
Following Udy and Dennison (1997a), as modified by Touchette and Burkholder, (2000), the response of seagrass to increased nutrient availability fall into four categories: (i) positive response
of growth and physiology to the additions in lownutrient habitats where nutrients are the only environmental factor limiting growth, (ii) positive
physiological response but no increase in growth,
in low-nutrient habitats where environmental factors
other than the added nutrient limit growth, (iii) neither a growth nor a physiological response in highnutrient environments where nutrient supplies are in
excess, and (iv) negative physiological response and
inhibition of growth by an added nutrient. The fourth
kind of response seems to be the least frequent (but
see Burkholder et al., 1992, 1994) and indicates an
upper limit for physiological tolerance to nutrients
(mostly, nitrate and ammonium).
Concerning responses (i) and (ii), seagrass has
been repeatedly observed to positively react to nutrient addition in several nutrient-poor ecosystems
around the world (see references in the following
section).
The first attempt to make a general hypothesis
or prediction about seagrass response to increased
nutrient availability was done by Short (1987),
243
seagrass (and other primary producers within the
seagrass bed) growth is met by nutrients coming
from outside the system (‘new nutrients’) or recycled within the system, where the boundaries of the
system are defined by the limits of the seagrass distribution, and include some portion of the water column and the upper layers of the sediment. The nutrients can be imported through diffusion or advection,
through sedimentation (including filter feeder activity) or, more rarely, through nitrogen fixation. Recycling includes metabolic recycling (nutrient recovery, storage, etc.) and ecosystem recycling (leaching
and in situ organic matter mineralization, either in
the water column, in the sediment surface, or within
the sediment). Under the assumption of steady state
(that may not necessarily be met), import of new
nutrient should equal nutrient losses: burial, export, and, eventually, denitrification. Paralleling classic concepts in plankton ecology (e.g. Dugdale and
Goering, 1967; Eppley and Peterson, 1979), it could
be said that the relation of the exchange component
to the recycled material (or to total, to formally adapt
to the f -ratio used in planktonic systems) provides
a rough measure of the dependence of the bed on
external nutrient sources. Knowledge of how this
dependence varies geographically, depending on the
water nutrient status or across seagrass species in
various ecosystems would be very valuable but has
never been attempted, to our knowledge. Yet is has to
be acknowledged that the determination of f -ratio in
seagrasses poses serious methodological problems,
that could be circumvented in the future using stable isotope tracers (see Mateo et al., Chapter 7). In
any case, an ecosystem approach to the interactions
between nutrients and seagrasses, although difficult,
would be desirable to better address key issues, for
example those related to seagrass conservation or
the biogeochemistry of coastal waters.
IV. Nutrient Limitation and Nutrient
Imbalances
A. An Old and Frequently Tested Paradigm
The luxuriant growth, the impressive biomass, and
the large areas covered by seagrasses, especially in
oligotrophic waters, suggest a very high-nutrient demand to sustain such growth, and point to a possible nutrient limitation. To grow successfully, these
plants need physical space (substrate), solar radiation, water, inorganic carbon, and sufficient amounts
of all the macronutrients and micronutrients. Among
potential limiting factors, and at least in shallow waters, nutrients are the top candidates to be actual
limiting resources (see, however, the role of carbon,
Larkum et al., Chapter 14).
Thus, it is by no means surprising that the hypothesis of nutrient limitation has been repeatedly
addressed, generally using one of the three following approaches: (i) correlational, through the study
of plants along nutrient availability gradients, (ii)
observational, through the analysis of nutrient concentration and/or proportions in plant tissues, (iii)
experimental, through manipulations of natural nutrient concentrations (at least from the early work of
Orth, 1977). All three approaches have their weaknesses and their strengths. While it is obvious that
the experimental approach is the most robust, it is
limited in: space (because a reduced number of plots
at a reduced number of sites are studied), time (usually experimental fertilization is done during a short
time, typically a year or less), and scope (fertilization mostly affects the plants, but hardly, for reasons
of scale, the whole ecosystem). These shortcomings
can be circumvented by increasing the field work effort (Fourqurean et al., 1995) or, more economically,
by using the two other approaches.
Following Udy and Dennison (1997a), as modified by Touchette and Burkholder, (2000), the response of seagrass to increased nutrient availability fall into four categories: (i) positive response
of growth and physiology to the additions in lownutrient habitats where nutrients are the only environmental factor limiting growth, (ii) positive
physiological response but no increase in growth,
in low-nutrient habitats where environmental factors
other than the added nutrient limit growth, (iii) neither a growth nor a physiological response in highnutrient environments where nutrient supplies are in
excess, and (iv) negative physiological response and
inhibition of growth by an added nutrient. The fourth
kind of response seems to be the least frequent (but
see Burkholder et al., 1992, 1994) and indicates an
upper limit for physiological tolerance to nutrients
(mostly, nitrate and ammonium).
Concerning responses (i) and (ii), seagrass has
been repeatedly observed to positively react to nutrient addition in several nutrient-poor ecosystems
around the world (see references in the following
section).
The first attempt to make a general hypothesis
or prediction about seagrass response to increased
nutrient availability was done by Short (1987),