182
M. A. Mateo, J. Cebri ´
an, K. Dunton, and T. Mutchler
Long-term accumulation or sink accounts for material with a biomass and nutrient content (C/N ratio) that no longer exhibit a change with time. In
the best documented case, long-term accumulation
of P. oceanica applies only to the carbon derived
from below-ground organ production, starting 4–6
years after tissue death and lasting for decades, centuries, or millennia (Romero et al., 1994; Mateo and
Romero, 1997; Mateo et al., 1997). The dynamics
of this long-term sink is only affected by geochemical diagenetic phenomena. In the biospheric context,
these kinds of sinks are usually named ‘reservoirs’
because they are considered virtually permanent. For
periods comparable to a plant’s biological cycle, this
sink would increase in size.
1. Keys Processes for Material Accumulation
There is general agreement that the two main factors leading to an accumulation of carbon in sediments are (i) incorporation of carbon into refractory geomacromolecules or humic substances and
(ii) oxygen availability (turnover) in the sediments
(e.g. Henrichs, 1992, 1993). This means on the one
hand, that carbon trapped in seagrass sediments does
not necessarily have to be highly refractory provided
the redox potential of the sediments is low enough,
and on the other hand, that highly refractory material
can remain intact even in oxidizing environments. A
number of biological and environmental agents will
modulate the extent of these two factors.
2. Chemistry of Refractory Material
Among all vascular plant components, lignin is considered to have the highest preservation potential
(Klap, 1997 and references therein). The presence
of this polymer in aquatic plants seems largely unnecessary and in fact, of all aquatic plants, seagrasses
are the only ones that contain lignin (Lewis and
Yamamoto, 1990; Kuo and den Hartog, Chapter 3).
One possible reason is the terrestrial origin of seagrasses (Larkum and den Hartog, 1989), allied to the
fact that decay-resistant material is useful at the leaf
base and in rhizomes.
Despite the pioneering studies of Klap (1997) and
Klap et al. (2000), in P. oceanica, the importance of
lignin in making seagrass below-ground organs particularly decay-resistant still needs to be adequately
addressed.
Evidence suggests that one cause for the refractory accumulation of seagrass below-ground detritus
could be the progressive impoverishment of nutrients
in the detritus and resulting slower decomposition
(Melillo et al., 1984; Romero et al., 1992; Mateo
and Romero, 1997; Sterner and Elser, 2002). L´ opez
et al. (1998) found that in several P. oceanica beds
in the NW Mediterranean, fertilization significantly
increased bacterial activity. They found that in the
fertilized plots, organic matter content was reduced
by about 33% with respect to the controls, which
suggested a potential role of nutrients.
The increasing tannin content of seagrass material during aging (Pergent, 1987) has also been proposed as another important factor that impairs microbial activity (Crouzet, 1984). Also, it has been
demonstrated that this secondary compound can act
against herbivory either by deterring herbivores or
by reducing total protein availability (e.g. Robbins
et al., 1987; Bernays et al., 1989).
3. Redox Potential and Anoxia
Anoxic conditions and oxidation reduction (redox)
potential in sediments may also play a role in
preservation of refractory carbon (Harrison, 1989).
However, this is the subject of much current debate.
For instance, it is known that many bacteria that possess cellulase activity seem to be strict anaerobes
(Kenworthy and Thayer, 1984; Roth and Hayasaka,
1984) and numerous studies have reported partial
or complete lignin degradation by both aerobic and
anaerobic decomposers (Klap, 1997 and references
therein). In two reviews of the topic, Henrichs and
Reeburgh (1987) and Henrichs (1993) conclude that
organic matter decomposition rates are not substantially different under oxic or anoxic conditions. The
situation is obviously complicated by the fact that
seagrass sediments are highly structured with a variety of different environments (Kristensen, 2000).
Lepidochronological dating (i.e. dating rhizome
remains from the number of leaf bases present), elemental analysis, and sediment redox measurements
provide evidence suggesting an important role of
anoxia in helping to retain the organic carbon from
below-ground production (Mateo, submitted). In a
P. oceanica bed, the redox potential discontinuity
(RPD) was found to be located at 5 cm from the
sediment surface (range 4–6 cm, in March) which
is close to the 150 mV of redox potential limit
(Fig. 10), i.e. the potential at which it is assumed that
M. A. Mateo, J. Cebri ´
an, K. Dunton, and T. Mutchler
Long-term accumulation or sink accounts for material with a biomass and nutrient content (C/N ratio) that no longer exhibit a change with time. In
the best documented case, long-term accumulation
of P. oceanica applies only to the carbon derived
from below-ground organ production, starting 4–6
years after tissue death and lasting for decades, centuries, or millennia (Romero et al., 1994; Mateo and
Romero, 1997; Mateo et al., 1997). The dynamics
of this long-term sink is only affected by geochemical diagenetic phenomena. In the biospheric context,
these kinds of sinks are usually named ‘reservoirs’
because they are considered virtually permanent. For
periods comparable to a plant’s biological cycle, this
sink would increase in size.
1. Keys Processes for Material Accumulation
There is general agreement that the two main factors leading to an accumulation of carbon in sediments are (i) incorporation of carbon into refractory geomacromolecules or humic substances and
(ii) oxygen availability (turnover) in the sediments
(e.g. Henrichs, 1992, 1993). This means on the one
hand, that carbon trapped in seagrass sediments does
not necessarily have to be highly refractory provided
the redox potential of the sediments is low enough,
and on the other hand, that highly refractory material
can remain intact even in oxidizing environments. A
number of biological and environmental agents will
modulate the extent of these two factors.
2. Chemistry of Refractory Material
Among all vascular plant components, lignin is considered to have the highest preservation potential
(Klap, 1997 and references therein). The presence
of this polymer in aquatic plants seems largely unnecessary and in fact, of all aquatic plants, seagrasses
are the only ones that contain lignin (Lewis and
Yamamoto, 1990; Kuo and den Hartog, Chapter 3).
One possible reason is the terrestrial origin of seagrasses (Larkum and den Hartog, 1989), allied to the
fact that decay-resistant material is useful at the leaf
base and in rhizomes.
Despite the pioneering studies of Klap (1997) and
Klap et al. (2000), in P. oceanica, the importance of
lignin in making seagrass below-ground organs particularly decay-resistant still needs to be adequately
addressed.
Evidence suggests that one cause for the refractory accumulation of seagrass below-ground detritus
could be the progressive impoverishment of nutrients
in the detritus and resulting slower decomposition
(Melillo et al., 1984; Romero et al., 1992; Mateo
and Romero, 1997; Sterner and Elser, 2002). L´ opez
et al. (1998) found that in several P. oceanica beds
in the NW Mediterranean, fertilization significantly
increased bacterial activity. They found that in the
fertilized plots, organic matter content was reduced
by about 33% with respect to the controls, which
suggested a potential role of nutrients.
The increasing tannin content of seagrass material during aging (Pergent, 1987) has also been proposed as another important factor that impairs microbial activity (Crouzet, 1984). Also, it has been
demonstrated that this secondary compound can act
against herbivory either by deterring herbivores or
by reducing total protein availability (e.g. Robbins
et al., 1987; Bernays et al., 1989).
3. Redox Potential and Anoxia
Anoxic conditions and oxidation reduction (redox)
potential in sediments may also play a role in
preservation of refractory carbon (Harrison, 1989).
However, this is the subject of much current debate.
For instance, it is known that many bacteria that possess cellulase activity seem to be strict anaerobes
(Kenworthy and Thayer, 1984; Roth and Hayasaka,
1984) and numerous studies have reported partial
or complete lignin degradation by both aerobic and
anaerobic decomposers (Klap, 1997 and references
therein). In two reviews of the topic, Henrichs and
Reeburgh (1987) and Henrichs (1993) conclude that
organic matter decomposition rates are not substantially different under oxic or anoxic conditions. The
situation is obviously complicated by the fact that
seagrass sediments are highly structured with a variety of different environments (Kristensen, 2000).
Lepidochronological dating (i.e. dating rhizome
remains from the number of leaf bases present), elemental analysis, and sediment redox measurements
provide evidence suggesting an important role of
anoxia in helping to retain the organic carbon from
below-ground production (Mateo, submitted). In a
P. oceanica bed, the redox potential discontinuity
(RPD) was found to be located at 5 cm from the
sediment surface (range 4–6 cm, in March) which
is close to the 150 mV of redox potential limit
(Fig. 10), i.e. the potential at which it is assumed that