Chapter 7 Carbon Flux in Seagrasses
163
measurement of carbon uptake rates in both seagrasses, and large macroalgae (kelp), respectively
(Fig. 1). Challenges with the
13 C method include access to an isotope ratio mass spectrometer (IRMS),
accurate knowledge of the δ
13 C ratio of the dissolved
inorganic carbon (DIC) pool, and the labor associated with sample preparation. Despite these limitations, isotopic labels of
13 C and
15 N can provide substantial insights to critical processes affecting seagrass productivity including carbon and nitrogen cycling and storage, epiphytic effects on seagrass photosynthesis, and carbon translocation within various
tissue compartments.
Pulse-amplitude modulated (PAM) fluorescence
has been used to examine seagrass photosynthetic
characteristics (Ralph et al., 1998; Beer and Bjork,
2000; Durako and Kunzelman, 2002; Larkum et al.,
Chapter 14). Currently, PAM fluorescence not only
measures leaf photosynthetic potential (Fv/Fm), but
provides a non-invasive opportunity to assess photosynthetic electron transport (approx. equivalent to
gross photosynthesis). Although PAM fluorescence
facilitates photosynthetic response determinations,
the method does not allow linkage between photosynthetic electron transport rates and net productivity, because respiratory rates are not measured. Consequently, PAM fluorescence has been limited in its
application to primary productivity measurements,
and awaits further development of reliable P/R
ratios.
D. Light and Carbon Metabolism in
Seagrasses: Whole Plant Models
Seagrass distribution and productivity are largely
regulated by variations in light attenuation through
the water column (Fig. 2). This is especially true
in coastal regions, where declines in water quality
from human encroachment have caused worldwide
losses of seagrasses (Dennison et al., 1993; Walker
et al., Chapter 23). Decreased water transparency
from river discharge, devegetation (causing higher
resuspension of sediments), dredging activities, excess phosphorus and nitrogen loading, which promotes pelagic, epiphytic, and benthic algal blooms,
etc., reduces light availability for photosynthesis,
ultimately causing significant loss of productivity
and biomass as plants retreat from deeper waters
(Onuf, 1994; Hauxwell et al., 2003; see Walker et al.,
Chapter 23).
Negative effects of nutrient enrichment on submerged aquatic vegetation in eutrophic systems
through algal overgrowth have been widely observed
(Duarte, 1995). However, a recent study by Heck
et al. (2000) contradicted this accepted notion. Based
on well-designed field experiments that combined
the effects of top predators and nutrient additions
in a Thalassia testudinum community, Heck et al.
(2000) found that nutrient enrichment had no significant effect on epiphyte biomass or T. testudinum
productivity (see also Heck and Orth, Chapter 22).
Instead, Heck et al. (2000) determined that the manipulation of top predators in the system resulted
in the most significant effects on epiphyte biomass
and seagrass productivity, complicating the simple
paradigm between nutrient enrichment, light alteration, and seagrass productivity proposed by Duarte
(1995). This example suggests that the paradigm of
eutrophication always having negative effects needs
to be closely scrutinized (see also Marb` a et al.,
Chapter 6).
The critical role of below-ground tissues as carbohydrate storage organs, sinks for photosynthetically
evolved oxygen and osmoregulation sites was summarized by Touchette and Burkholder (2000). The
below-ground tissues of seagrasses are often a major component of the total biomass and serve as a
photosynthetic reservoir that supports growth and
maintenance of other tissues during periods of low
photosynthetic production (Pirc, 1989; Burke et al.,
1996; Alcoverro et al., 2001) and also CO 2 generated
in the roots and rhizomes may be a significant source
of carbon for the leaves (Borum et al., Chapter 10).
Reliable seagrass biomass and productivity models have been generated based on both above- and
below-ground tissues and mass carbon balance calculations (Wetzel and Neckles, 1986; Alcoverro
et al., 2001; Burd and Dunton, 2001). Zimmerman
(2003) (see also Zimmerman, Chapter 13) took another approach and developed a sophisticated model
that predicts seagrass canopy photosynthetic performance in response to a variety of variables, including canopy architecture, leaf orientation, and water
quality (although the model so far does not incorporate epiphytes). It is apparent that the development
of predictive seagrass productivity models requires
further research on carbon partitioning among seagrass compartments, especially under light-limited
conditions, and long-term in situ measurements of
seagrass productivity under recorded light and water
quality conditions.
163
measurement of carbon uptake rates in both seagrasses, and large macroalgae (kelp), respectively
(Fig. 1). Challenges with the
13 C method include access to an isotope ratio mass spectrometer (IRMS),
accurate knowledge of the δ
13 C ratio of the dissolved
inorganic carbon (DIC) pool, and the labor associated with sample preparation. Despite these limitations, isotopic labels of
13 C and
15 N can provide substantial insights to critical processes affecting seagrass productivity including carbon and nitrogen cycling and storage, epiphytic effects on seagrass photosynthesis, and carbon translocation within various
tissue compartments.
Pulse-amplitude modulated (PAM) fluorescence
has been used to examine seagrass photosynthetic
characteristics (Ralph et al., 1998; Beer and Bjork,
2000; Durako and Kunzelman, 2002; Larkum et al.,
Chapter 14). Currently, PAM fluorescence not only
measures leaf photosynthetic potential (Fv/Fm), but
provides a non-invasive opportunity to assess photosynthetic electron transport (approx. equivalent to
gross photosynthesis). Although PAM fluorescence
facilitates photosynthetic response determinations,
the method does not allow linkage between photosynthetic electron transport rates and net productivity, because respiratory rates are not measured. Consequently, PAM fluorescence has been limited in its
application to primary productivity measurements,
and awaits further development of reliable P/R
ratios.
D. Light and Carbon Metabolism in
Seagrasses: Whole Plant Models
Seagrass distribution and productivity are largely
regulated by variations in light attenuation through
the water column (Fig. 2). This is especially true
in coastal regions, where declines in water quality
from human encroachment have caused worldwide
losses of seagrasses (Dennison et al., 1993; Walker
et al., Chapter 23). Decreased water transparency
from river discharge, devegetation (causing higher
resuspension of sediments), dredging activities, excess phosphorus and nitrogen loading, which promotes pelagic, epiphytic, and benthic algal blooms,
etc., reduces light availability for photosynthesis,
ultimately causing significant loss of productivity
and biomass as plants retreat from deeper waters
(Onuf, 1994; Hauxwell et al., 2003; see Walker et al.,
Chapter 23).
Negative effects of nutrient enrichment on submerged aquatic vegetation in eutrophic systems
through algal overgrowth have been widely observed
(Duarte, 1995). However, a recent study by Heck
et al. (2000) contradicted this accepted notion. Based
on well-designed field experiments that combined
the effects of top predators and nutrient additions
in a Thalassia testudinum community, Heck et al.
(2000) found that nutrient enrichment had no significant effect on epiphyte biomass or T. testudinum
productivity (see also Heck and Orth, Chapter 22).
Instead, Heck et al. (2000) determined that the manipulation of top predators in the system resulted
in the most significant effects on epiphyte biomass
and seagrass productivity, complicating the simple
paradigm between nutrient enrichment, light alteration, and seagrass productivity proposed by Duarte
(1995). This example suggests that the paradigm of
eutrophication always having negative effects needs
to be closely scrutinized (see also Marb` a et al.,
Chapter 6).
The critical role of below-ground tissues as carbohydrate storage organs, sinks for photosynthetically
evolved oxygen and osmoregulation sites was summarized by Touchette and Burkholder (2000). The
below-ground tissues of seagrasses are often a major component of the total biomass and serve as a
photosynthetic reservoir that supports growth and
maintenance of other tissues during periods of low
photosynthetic production (Pirc, 1989; Burke et al.,
1996; Alcoverro et al., 2001) and also CO 2 generated
in the roots and rhizomes may be a significant source
of carbon for the leaves (Borum et al., Chapter 10).
Reliable seagrass biomass and productivity models have been generated based on both above- and
below-ground tissues and mass carbon balance calculations (Wetzel and Neckles, 1986; Alcoverro
et al., 2001; Burd and Dunton, 2001). Zimmerman
(2003) (see also Zimmerman, Chapter 13) took another approach and developed a sophisticated model
that predicts seagrass canopy photosynthetic performance in response to a variety of variables, including canopy architecture, leaf orientation, and water
quality (although the model so far does not incorporate epiphytes). It is apparent that the development
of predictive seagrass productivity models requires
further research on carbon partitioning among seagrass compartments, especially under light-limited
conditions, and long-term in situ measurements of
seagrass productivity under recorded light and water
quality conditions.