Chapter 7 Carbon Flux in Seagrasses
161
1996). Thus, below-ground production ranges from
0.001 to 20 gDW m
−2 day
−1 (Duarte et al., 1998;
Duarte and Chiscano, 1999; Kaldy and Dunton,
2000). Only recently, however, have the importance
of below-ground tissues for carbon storage and insediment biogeochemical processes been recognized
in plant production models.
B. Contribution of Epiphytes and Other
Primary Producers
In addition to understanding the factors that control
carbon flow through food webs, it is necessary to
determine the trophic importance of components of
the seagrass food web, which are often overlooked.
On a broad level, the extent to which these food webs
depend on locally-derived food resources vs. more
transient planktonic resources needs to be quantified. Although locally-generated production is high
in seagrass systems, organic matter flux through the
planktonic and filter-feeding pathways may be significant as well. At a finer level, and as will become
evident below, the relative contribution of the resident producers must be better understood: algal
epiphytes and other primary producers (e.g. drift
macroalgae, benthic macroalgae, and phytoplankton) have long been recognized as significant contributors to total seagrass bed primary production
(McRoy and McMillan, 1977), yet their quantitative
role has been neglected (Fry et al., 1987). Because
sediments within and between seagrass beds constitute a large area in the seagrass landscape, it is likely
that the role of sediment microalgae may have been
largely underestimated. Algal epiphyte contribution
alone has been reported to range from 20 to 60%
(see review by Borowitzka and Lethbridge, 1989; see
also Borowitzka et al., Chapter 19). More recently,
Moncreiff et al. (1992) reported algal production (including epiphytes, benthic diatoms, and phytoplankton) to be 87% of total system production in Mississippi Sound. As described in detail later, Moncreiff
and Sullivan (2001), Dauby (1989), and Yamamuro
(1999) have recorded the large contributions of algae in seagrass beds, and stand in contrast to many
studies which concentrate only on the importance
of seagrass carbon both within and outside seagrass
communities (see review by Stevenson, 1988 and
Section 3.4.1).
Kaldy et al. (2002), for example, showed that
benthic macroalgae accounted for most of system
net primary production (33–42%), followed by seagrasses (33–38%) and other microalgae (23–56%).
Furthermore, this is a role that may be increasing in
coastal and estuarine systems that are becoming increasingly more eutrophic due to nitrogen loadings
from adjacent watersheds (Hauxwell et al., 2003). By
incorporating the sediment microalgae into future
studies, a more complete picture of trophic dynamics can be developed that will allow us to generate a
landscape perspective (see Bell et al., Chapter 26) of
the flux of carbon and nutrients, related to variables
such as depth, irradiance, nutrient availability, and
sediment type.
C. Seagrass Production Measurements
For decades, seagrass productivity estimates have
been based on the hole-punching (Zieman, 1974).
Alternatively, for plants with very narrow blades, cut
and harvest approaches have been employed (Virnstein, 1982). Neither method accounts for belowground production, which as mentioned earlier, can
be greater than 50% of total seagrass production.
Other problems include tissue loss by grazing or mechanical damage and physiological disturbance associated with clipping (Tomasko and Dunton, 1995;
Kowalski et al., 2001).
More sophisticated approaches to net production
measurements have included in situ measurement
of whole plant photosynthetic oxygen evolution and
respiration using chambers in combination with continuous measurements of underwater irradiance at
canopy level (Herzka and Dunton, 1997, 1998).
This approach is not only extremely labor intensive
but requires comprehensive knowledge of seagrass
photosynthetic response to temperature and light.
Alternatively, various researchers have employed a
whole plant approach based on laboratory determinations of plant photosynthetic parameters (Zimmerman et al., 1989; Fourqurean and Zieman, 1991). In
either case, the method is labor intensive and requires
continuous in situ measurement of underwater irradiance to calculate daily or annual net production.
These physiological measurements normally measure oxygen evolution or carbon uptake, and the inherent errors related to internal recycling or storage
of gases, contributions by other organisms (such as
bacteria, epiphytes, and macroalgae), and the photosynthetic quotient make either approach problematic
(see Mateo et al., 2001).
161
1996). Thus, below-ground production ranges from
0.001 to 20 gDW m
−2 day
−1 (Duarte et al., 1998;
Duarte and Chiscano, 1999; Kaldy and Dunton,
2000). Only recently, however, have the importance
of below-ground tissues for carbon storage and insediment biogeochemical processes been recognized
in plant production models.
B. Contribution of Epiphytes and Other
Primary Producers
In addition to understanding the factors that control
carbon flow through food webs, it is necessary to
determine the trophic importance of components of
the seagrass food web, which are often overlooked.
On a broad level, the extent to which these food webs
depend on locally-derived food resources vs. more
transient planktonic resources needs to be quantified. Although locally-generated production is high
in seagrass systems, organic matter flux through the
planktonic and filter-feeding pathways may be significant as well. At a finer level, and as will become
evident below, the relative contribution of the resident producers must be better understood: algal
epiphytes and other primary producers (e.g. drift
macroalgae, benthic macroalgae, and phytoplankton) have long been recognized as significant contributors to total seagrass bed primary production
(McRoy and McMillan, 1977), yet their quantitative
role has been neglected (Fry et al., 1987). Because
sediments within and between seagrass beds constitute a large area in the seagrass landscape, it is likely
that the role of sediment microalgae may have been
largely underestimated. Algal epiphyte contribution
alone has been reported to range from 20 to 60%
(see review by Borowitzka and Lethbridge, 1989; see
also Borowitzka et al., Chapter 19). More recently,
Moncreiff et al. (1992) reported algal production (including epiphytes, benthic diatoms, and phytoplankton) to be 87% of total system production in Mississippi Sound. As described in detail later, Moncreiff
and Sullivan (2001), Dauby (1989), and Yamamuro
(1999) have recorded the large contributions of algae in seagrass beds, and stand in contrast to many
studies which concentrate only on the importance
of seagrass carbon both within and outside seagrass
communities (see review by Stevenson, 1988 and
Section 3.4.1).
Kaldy et al. (2002), for example, showed that
benthic macroalgae accounted for most of system
net primary production (33–42%), followed by seagrasses (33–38%) and other microalgae (23–56%).
Furthermore, this is a role that may be increasing in
coastal and estuarine systems that are becoming increasingly more eutrophic due to nitrogen loadings
from adjacent watersheds (Hauxwell et al., 2003). By
incorporating the sediment microalgae into future
studies, a more complete picture of trophic dynamics can be developed that will allow us to generate a
landscape perspective (see Bell et al., Chapter 26) of
the flux of carbon and nutrients, related to variables
such as depth, irradiance, nutrient availability, and
sediment type.
C. Seagrass Production Measurements
For decades, seagrass productivity estimates have
been based on the hole-punching (Zieman, 1974).
Alternatively, for plants with very narrow blades, cut
and harvest approaches have been employed (Virnstein, 1982). Neither method accounts for belowground production, which as mentioned earlier, can
be greater than 50% of total seagrass production.
Other problems include tissue loss by grazing or mechanical damage and physiological disturbance associated with clipping (Tomasko and Dunton, 1995;
Kowalski et al., 2001).
More sophisticated approaches to net production
measurements have included in situ measurement
of whole plant photosynthetic oxygen evolution and
respiration using chambers in combination with continuous measurements of underwater irradiance at
canopy level (Herzka and Dunton, 1997, 1998).
This approach is not only extremely labor intensive
but requires comprehensive knowledge of seagrass
photosynthetic response to temperature and light.
Alternatively, various researchers have employed a
whole plant approach based on laboratory determinations of plant photosynthetic parameters (Zimmerman et al., 1989; Fourqurean and Zieman, 1991). In
either case, the method is labor intensive and requires
continuous in situ measurement of underwater irradiance to calculate daily or annual net production.
These physiological measurements normally measure oxygen evolution or carbon uptake, and the inherent errors related to internal recycling or storage
of gases, contributions by other organisms (such as
bacteria, epiphytes, and macroalgae), and the photosynthetic quotient make either approach problematic
(see Mateo et al., 2001).