Chapter 7 Carbon Flux in Seagrasses
183
sediments are reduced and therefore bacterial efficiency for remineralization drops drastically (e.g.
Stumm and Morgan, 1981; Kristensen, 2000). From
the fact that t, the elongation rate of recent rhizomes
was found to be 0.95 cm yr
−1 and this must be
matched by the sediment accretion rate, since the
rhizomes do not become exposed, it can be estimated that a reducing environment will arise within
5–7 yr; at which point decay rates would fall drastically. From Mateo and Romero (1997) the C:N
ratio of dead rhizome sheaths stabilizes at ∼6 yr,
which may well be the result of anoxia and changed
redox conditions. Although the foregoing suggests
that anoxia and redox potential in seagrass sediments
promotes organic matter preservation, the considerable amount of contradictory evidence makes it advisable to further investigate the effect of oxygen
depletion on below-ground refractory material.
4. Other Factors
The situation is made more complex by the influence of other factors especially the four following
ones, which clearly favor low redox potential in marine sediments and therefore help material preservation: (i) high temperatures, (ii) high organic supply,
(iii) low water motion, and (iv) small sediment grain
size.
It has been repeatedly demonstrated that high
temperatures increase bacterial activity in seagrass
sediments (e.g. L´ opez et al., 1995a,b; Mateo and
Romero, 1997) with concomitant oxygen consumption. On the other hand, low temperatures may promote material accumulation by reducing bacterial
activity. In terrestrial ecology, it is well established
that cold biomes present the largest accumulation
of organic matter in the soil as a consequence
of reduced decomposition rates (e.g. Swift et al.,
1979).
An abundant organic matter supply not only from
seagrass production but from micro- and macroalgal
detritus (Section II.D.1) accelerates oxygen depletion in the sediment.
Reduction of water velocity over a bed due to water friction of the seagrass canopy (Gambi et al.,
1990; Koch et al., Chapter 8) leads to seagrass vegetated areas having a sediment retention capacity up
to 15 times higher than barren areas (Gacia et al.,
1999) and accelerates burial. Also, low water motion and a small grain size of the sediment lead to
a reduction of pore-water renewal and oxygenation,
very effectively helping to maintain a low redox potential in the sediment.
Finally, Cebri´ an et al. (2000) examined the formation refractory material throughout the development
of C. nodosa beds and found that refractory accumulation increased from small patches, to large patches,
to climax beds.
C. Refractory Carbon: The Potential Seagrass
Carbon Sink
Refractory accumulation has two main consequences for the ecology of seagrass beds. First, the
capacity of climax beds to sequester organic carbon is indicative of their role as sinks in oceanic
and global models. Second, the accumulation of refractory detritus entails a loss of nutrients from the
bed since a significant amount of nutrients remain
bound to fibrous compounds (i.e. lignin, cellulose)
in the detritus (Romero et al., 1994; Mateo and
Romero, 1997; Mateo et al., 1997). Hence, refractory
accumulation also needs to be added to the export of
dead leaves out of the bed when estimating the bed’s
total dependence on imported nutrients. Yet, despite
its importance, refractory accumulation is clearly the
least studied of all the routes of seagrass production
listed in Eqs. (1) and (2).
Information on organic matter burial in seagrass
sediments began to be studied in detail during the
1990s, and the only direct estimations available so
far are basically limited to the below-ground-derived
production of the species P. oceanica (Romero et al.,
1994; Mateo et al., 1997, 2002; Mateo, submitted).
The two different time scales for seagrass carbon
sinks, described at the beginning of this section, will
be distinguished below.
1. Short-Term Sinks (i.e. Material Broken
Down Over a Period of a Year or so)
The preliminary compilation made for this chapter
showed that the percentage of seagrass production
accumulated as refractory material varies from ca. 1
to 62% and the absolute value from 3 to 207 gC m
−2
year
−1 (Fig. 3G and H). The studies on P. oceanica presented in this chapter (for beds off the islands of Medes, Calvi, and Ischia; Table 3) and estimates from two other available studies (Cebri´ an
et al., 1997; Gacia et al., 2002), indicate that the average potential carbon sink (short-term) from leafderived material for this species can be estimated as
183
sediments are reduced and therefore bacterial efficiency for remineralization drops drastically (e.g.
Stumm and Morgan, 1981; Kristensen, 2000). From
the fact that t, the elongation rate of recent rhizomes
was found to be 0.95 cm yr
−1 and this must be
matched by the sediment accretion rate, since the
rhizomes do not become exposed, it can be estimated that a reducing environment will arise within
5–7 yr; at which point decay rates would fall drastically. From Mateo and Romero (1997) the C:N
ratio of dead rhizome sheaths stabilizes at ∼6 yr,
which may well be the result of anoxia and changed
redox conditions. Although the foregoing suggests
that anoxia and redox potential in seagrass sediments
promotes organic matter preservation, the considerable amount of contradictory evidence makes it advisable to further investigate the effect of oxygen
depletion on below-ground refractory material.
4. Other Factors
The situation is made more complex by the influence of other factors especially the four following
ones, which clearly favor low redox potential in marine sediments and therefore help material preservation: (i) high temperatures, (ii) high organic supply,
(iii) low water motion, and (iv) small sediment grain
size.
It has been repeatedly demonstrated that high
temperatures increase bacterial activity in seagrass
sediments (e.g. L´ opez et al., 1995a,b; Mateo and
Romero, 1997) with concomitant oxygen consumption. On the other hand, low temperatures may promote material accumulation by reducing bacterial
activity. In terrestrial ecology, it is well established
that cold biomes present the largest accumulation
of organic matter in the soil as a consequence
of reduced decomposition rates (e.g. Swift et al.,
1979).
An abundant organic matter supply not only from
seagrass production but from micro- and macroalgal
detritus (Section II.D.1) accelerates oxygen depletion in the sediment.
Reduction of water velocity over a bed due to water friction of the seagrass canopy (Gambi et al.,
1990; Koch et al., Chapter 8) leads to seagrass vegetated areas having a sediment retention capacity up
to 15 times higher than barren areas (Gacia et al.,
1999) and accelerates burial. Also, low water motion and a small grain size of the sediment lead to
a reduction of pore-water renewal and oxygenation,
very effectively helping to maintain a low redox potential in the sediment.
Finally, Cebri´ an et al. (2000) examined the formation refractory material throughout the development
of C. nodosa beds and found that refractory accumulation increased from small patches, to large patches,
to climax beds.
C. Refractory Carbon: The Potential Seagrass
Carbon Sink
Refractory accumulation has two main consequences for the ecology of seagrass beds. First, the
capacity of climax beds to sequester organic carbon is indicative of their role as sinks in oceanic
and global models. Second, the accumulation of refractory detritus entails a loss of nutrients from the
bed since a significant amount of nutrients remain
bound to fibrous compounds (i.e. lignin, cellulose)
in the detritus (Romero et al., 1994; Mateo and
Romero, 1997; Mateo et al., 1997). Hence, refractory
accumulation also needs to be added to the export of
dead leaves out of the bed when estimating the bed’s
total dependence on imported nutrients. Yet, despite
its importance, refractory accumulation is clearly the
least studied of all the routes of seagrass production
listed in Eqs. (1) and (2).
Information on organic matter burial in seagrass
sediments began to be studied in detail during the
1990s, and the only direct estimations available so
far are basically limited to the below-ground-derived
production of the species P. oceanica (Romero et al.,
1994; Mateo et al., 1997, 2002; Mateo, submitted).
The two different time scales for seagrass carbon
sinks, described at the beginning of this section, will
be distinguished below.
1. Short-Term Sinks (i.e. Material Broken
Down Over a Period of a Year or so)
The preliminary compilation made for this chapter
showed that the percentage of seagrass production
accumulated as refractory material varies from ca. 1
to 62% and the absolute value from 3 to 207 gC m
−2
year
−1 (Fig. 3G and H). The studies on P. oceanica presented in this chapter (for beds off the islands of Medes, Calvi, and Ischia; Table 3) and estimates from two other available studies (Cebri´ an
et al., 1997; Gacia et al., 2002), indicate that the average potential carbon sink (short-term) from leafderived material for this species can be estimated as