Chapter 7 Carbon Flux in Seagrasses
179
or examine export of organic matter to other
habitats. Despite the numerous difficulties of applying isotope additions in such open systems, advances
continue to be made to permit isotope additions in the
field. Along with direct isotope additions via spraying (Ottosen et al., 2001; Carman and Fry, 2002; Fry
et al., 2003) or incorporation into fertilizer (Mutchler
et al., 2004), organisms themselves could be labeled
in the laboratory and ‘released’ into the field. Isotope signatures of specific beds (Stapel et al., 2001)
or unvegetated sediments could be manipulated and
the fate of the tracer monitored. Indeed, multiple isotope additions (e.g.
13 C,
15 N, and
34 S) may be applied
(Carman and Fry, 2002).
E. Other Carbon Fates—Translocation
and Seagrass Exudates
Retranslocation of carbon resources from senescent
to young leaves, between above- and below-ground
organs (e.g. Alcoverro et al., 2001) or between adjacent shoots (Marb` a et al., 2002), are mechanisms
apparently used by seagrasses for an efficient use
of their resources. A maximum of 11% of the carbon gain by the P. oceanica leaves can be accumulated as non-structural carbohydrates in the whole
plant (above- and below-ground organs, Alcoverro
et al., 2001). The fraction of this amount that is actually stored in roots and rhizomes cannot be easily
determined at this time due to different criteria by
different authors concerning the size of the living
below-ground compartment. The relevance of this
possible fate for leaf carbon is also difficult to ascertain since the carbon accumulated during periods
of positive plant carbon balance can be later mobilized to support the growth of new leaves during
periods of negative plant carbon balance (Alcoverro
et al., 2001). It has been shown that the use of stable
isotopes may have a potential in the study of carbon reserves in seagrasses (see discussion in Vizzini
et al., 2003).
Concerning clonal resource sharing, Marb` a et al.
(2002) found that from 27.1 to 80.6% of the carbon incorporated by the leaves of different seagrass
species could be exported to adjacent shoots to contribute to the growth of new and colonizing shoots.
For instance, the value estimated for P. oceanica was
26% in June at 11–15 m of depth (NW Mediterranean). During periods of reserve mobilization, carbon export to other shoots could be substantially
higher. Since an asymmetrical transport between
clones has been demonstrated (and not only in seagrasses) preferential transport of resources to pioneer ramets would, in effect, represent an important
net carbon and nutrients loss from the source individuals.
Dissolved organic compounds have been recognized as a potentially important component of carbon transfer. In an early study, Moriarty et al. (1986)
reported that the amount of dissolved carbon released into the water column by the leaves of H.
wrightii was 1% of the carbon fixed. Carbon exuded
from roots and rhizomes into the sediments was estimated as 6–17% of the
14 C fixed by the leaves. Leaf
uptake, translocation to roots and rhizomes, and exudation into the sediment occurred within 6 h. Using
two different methods to determine bacterial production, Moriarty et al. (1986) concluded that all
the excreted
14 C was utilized by bacteria growing in
the sediment. For the same species, Koepfler et al.
(1993) found that dissolved inorganic carbon (DOC)
concentration in pore-water was 25% higher in vegetated than in bare sediments. In T. testudinum, benthic net fluxes of DOC ranged from 0 to 216 mgC
m
−2 day
−1 and were attributed mainly to seagrass
exudation (Ziegler and Benner, 1999). Strong correlations between benthic DOC release and water
column respiration suggested that water column heterotrophy was largely fed by seagrass exudation. Finally, an ecosystem model in Chesapeake Bay assigned to eelgrass-derived DOC an important role as
a carbon source, accounting for up to 30% of littoral
primary production (Buzzelli et al., 1998, 1999).
IV. Ecosystem Carbon Budgets
and Carbon Sinks
The concept of ecosystem carbon budgets deserves
a brief comment here to better focus the problem.
Such a budget may compare the stocks of net or gross
carbon incorporated by a producer compartment to
the amount of carbon evolved from the detrital compartment during remineralization. Fates other than
remineralization (such as grazing, export, and immediate release of DOC) need also to be known in
order to account for all the carbon synthesized. Notice that ecosystem budgets can take into account (i)
all producers of all compartments of the ecosystem,
(ii) part of them, (iii) a single producer, or (iv) a part
of a producer. The individual budget of a certain seagrass species can be largely positive, while growing
in a globally heterotrophic ecosystem. It is suggested
179
or examine export of organic matter to other
habitats. Despite the numerous difficulties of applying isotope additions in such open systems, advances
continue to be made to permit isotope additions in the
field. Along with direct isotope additions via spraying (Ottosen et al., 2001; Carman and Fry, 2002; Fry
et al., 2003) or incorporation into fertilizer (Mutchler
et al., 2004), organisms themselves could be labeled
in the laboratory and ‘released’ into the field. Isotope signatures of specific beds (Stapel et al., 2001)
or unvegetated sediments could be manipulated and
the fate of the tracer monitored. Indeed, multiple isotope additions (e.g.
13 C,
15 N, and
34 S) may be applied
(Carman and Fry, 2002).
E. Other Carbon Fates—Translocation
and Seagrass Exudates
Retranslocation of carbon resources from senescent
to young leaves, between above- and below-ground
organs (e.g. Alcoverro et al., 2001) or between adjacent shoots (Marb` a et al., 2002), are mechanisms
apparently used by seagrasses for an efficient use
of their resources. A maximum of 11% of the carbon gain by the P. oceanica leaves can be accumulated as non-structural carbohydrates in the whole
plant (above- and below-ground organs, Alcoverro
et al., 2001). The fraction of this amount that is actually stored in roots and rhizomes cannot be easily
determined at this time due to different criteria by
different authors concerning the size of the living
below-ground compartment. The relevance of this
possible fate for leaf carbon is also difficult to ascertain since the carbon accumulated during periods
of positive plant carbon balance can be later mobilized to support the growth of new leaves during
periods of negative plant carbon balance (Alcoverro
et al., 2001). It has been shown that the use of stable
isotopes may have a potential in the study of carbon reserves in seagrasses (see discussion in Vizzini
et al., 2003).
Concerning clonal resource sharing, Marb` a et al.
(2002) found that from 27.1 to 80.6% of the carbon incorporated by the leaves of different seagrass
species could be exported to adjacent shoots to contribute to the growth of new and colonizing shoots.
For instance, the value estimated for P. oceanica was
26% in June at 11–15 m of depth (NW Mediterranean). During periods of reserve mobilization, carbon export to other shoots could be substantially
higher. Since an asymmetrical transport between
clones has been demonstrated (and not only in seagrasses) preferential transport of resources to pioneer ramets would, in effect, represent an important
net carbon and nutrients loss from the source individuals.
Dissolved organic compounds have been recognized as a potentially important component of carbon transfer. In an early study, Moriarty et al. (1986)
reported that the amount of dissolved carbon released into the water column by the leaves of H.
wrightii was 1% of the carbon fixed. Carbon exuded
from roots and rhizomes into the sediments was estimated as 6–17% of the
14 C fixed by the leaves. Leaf
uptake, translocation to roots and rhizomes, and exudation into the sediment occurred within 6 h. Using
two different methods to determine bacterial production, Moriarty et al. (1986) concluded that all
the excreted
14 C was utilized by bacteria growing in
the sediment. For the same species, Koepfler et al.
(1993) found that dissolved inorganic carbon (DOC)
concentration in pore-water was 25% higher in vegetated than in bare sediments. In T. testudinum, benthic net fluxes of DOC ranged from 0 to 216 mgC
m
−2 day
−1 and were attributed mainly to seagrass
exudation (Ziegler and Benner, 1999). Strong correlations between benthic DOC release and water
column respiration suggested that water column heterotrophy was largely fed by seagrass exudation. Finally, an ecosystem model in Chesapeake Bay assigned to eelgrass-derived DOC an important role as
a carbon source, accounting for up to 30% of littoral
primary production (Buzzelli et al., 1998, 1999).
IV. Ecosystem Carbon Budgets
and Carbon Sinks
The concept of ecosystem carbon budgets deserves
a brief comment here to better focus the problem.
Such a budget may compare the stocks of net or gross
carbon incorporated by a producer compartment to
the amount of carbon evolved from the detrital compartment during remineralization. Fates other than
remineralization (such as grazing, export, and immediate release of DOC) need also to be known in
order to account for all the carbon synthesized. Notice that ecosystem budgets can take into account (i)
all producers of all compartments of the ecosystem,
(ii) part of them, (iii) a single producer, or (iv) a part
of a producer. The individual budget of a certain seagrass species can be largely positive, while growing
in a globally heterotrophic ecosystem. It is suggested