Chapter 6 Seagrasses and Biogeochemistry
141
has also been indicated (Roberts and Moriarty,
1987). Thus, laboratory and in situ incubations or
mass balances based on oxygen or DIC gas exchange
in the water column have serious limitations in seagrass systems. Recent studies have pointed out the
importance of taking into consideration the plant–
sediment interactions in order to get a proper balance of the metabolic processes occurring in seagrass communities (Gacia et al., 2005). Indeed, the
stoichiometric balance between oxygen production
and DIC consumption deviates from the theoretical
value (0.83; Oviatt et al., 1986) in studies of seagrass
community metabolism (Ziegler and Benner, 1998;
Barr´ on et al., unpublished data; Gacia et al., 2005).
The dynamics of the carbonic–carbonate system (see
Section II. E, this chapter), carbonate production and
dissolution (see Section II. F, this chapter), anoxic
mineralization with incomplete or partial oxidation
in the sediment, and sulphate reduction processes
(see Section III. E, this chapter) may interfere with
the photosynthetic and respiratory balances of DO
and DIC in the water column. More efforts should
be made to quantify all these different potential
interactions in order to fully evaluate the metabolism
of seagrass communities based on photosynthetic
gas exchange in the water column environment.
Seagrasses also directly influence oxygen concentration in sediment pore waters. A fraction of the O 2
transported to the rhizomes and roots, is released
into the sediments (see Borum et al., Chapter 10);
Pedersen et al. (1998) found an oxic microzone of
about 80 µm around C. rotundata roots during light
incubations. Similar findings have been reported for
other seagrasses, e.g. Z. marina (Greve et al., 2003),
Halophila ovalis (Connell et al., 1999) and could be
a common feature of seagrasses (see also Kuo and
den Hartog, Chapter 3). The transport of oxygen is
most pronounced in the light, driven by photosynthesis in the leaves. However, the oxic microzone
around C. rotundata roots was also present during
dark incubations although then it diminished to 50
µm. It was shown that the oxygen present in the
rhizosphere during darkness was supplied from the
oxic water column to roots via gas-phase diffusion
in leaves and rhizomes (Borum et al., Chapter 10).
Pedersen et al. (1998) estimated that release of oxygen by roots contributed about 10% of the total
sediment oxygen consumption. Oxygen supply by
seagrass roots to the sediment has been recognized
to be essential to maintaining non-toxic levels of
sulfide for seagrass growth and survival (Pedersen
et al., 1998; Eldrigde and Morse, 2000), whereas it
would only partially support sediment aerobic bacterial metabolism (Pedersen et al., 1998; Eldrigde and
Morse, 2000).
Seagrasses are highly productive plants but they
also support a large biomass and production of animals and microbes, and a large respiratory activity partially dependent on the seagrass itself. To
date, there are few estimates of the light compensation point for balanced metabolism (i.e. gross
primary production = community respiration) in
seagrass beds (Erftemeijer and Middelburg, 1993;
D’Avanzo et al., 1996; Herzka and Dunton, 1997),
but those available indicate very high light requirements for the whole ecosystem metabolism. Indeed, seagrass meadows may only be marginally
autotrophic ecosystems (Hemminga and Duarte,
2000). Further work should extend the information
on integrative studies of seagrass metabolism and
its implications for the balance between aerobic and
anaerobic processes in the whole coastal systems.
For instance, the metabolic activity of P. oceanica
beds has been shown to influence the concentration
of oxygen and dissolved inorganic carbon in the water column within an entire Mediterranean bay partially colonized by seagrasses (Frankignoulle et al.,
1984).
E. CO 2 –Carbonate System and pH
Carbon dioxide and pH are closely linked in the marine environment through the photosynthetic activity
of primary producers, such as seagrasses. Photosynthetic uptake of dissolved inorganic carbon results
in a displacement of the CO 2 /bicarbonate/carbonate
balance from equilibrium:
CO 2 + H 2 O ↔ H 2 CO 3 ↔ H
+
+ HCO
−
3
↔ CO
2−
3 + 2H
+
decreasing the concentration of CO 2 and increasing
that of HCO
−
3 and/or CO
2−
3 . These changes are coupled to water column pH fluctuations of low amplitude due to the buffering capacity of marine waters.
However, in seagrass meadows with high biomass
and productivity, significant daily fluctuations of pH,
carbon dioxide, and dissolved oxygen are found coupled to the photosynthetic activity of the plants. In
the NW-Mediterranean diel pH fluctuations of up to
0.5 units have been described just above and within
141
has also been indicated (Roberts and Moriarty,
1987). Thus, laboratory and in situ incubations or
mass balances based on oxygen or DIC gas exchange
in the water column have serious limitations in seagrass systems. Recent studies have pointed out the
importance of taking into consideration the plant–
sediment interactions in order to get a proper balance of the metabolic processes occurring in seagrass communities (Gacia et al., 2005). Indeed, the
stoichiometric balance between oxygen production
and DIC consumption deviates from the theoretical
value (0.83; Oviatt et al., 1986) in studies of seagrass
community metabolism (Ziegler and Benner, 1998;
Barr´ on et al., unpublished data; Gacia et al., 2005).
The dynamics of the carbonic–carbonate system (see
Section II. E, this chapter), carbonate production and
dissolution (see Section II. F, this chapter), anoxic
mineralization with incomplete or partial oxidation
in the sediment, and sulphate reduction processes
(see Section III. E, this chapter) may interfere with
the photosynthetic and respiratory balances of DO
and DIC in the water column. More efforts should
be made to quantify all these different potential
interactions in order to fully evaluate the metabolism
of seagrass communities based on photosynthetic
gas exchange in the water column environment.
Seagrasses also directly influence oxygen concentration in sediment pore waters. A fraction of the O 2
transported to the rhizomes and roots, is released
into the sediments (see Borum et al., Chapter 10);
Pedersen et al. (1998) found an oxic microzone of
about 80 µm around C. rotundata roots during light
incubations. Similar findings have been reported for
other seagrasses, e.g. Z. marina (Greve et al., 2003),
Halophila ovalis (Connell et al., 1999) and could be
a common feature of seagrasses (see also Kuo and
den Hartog, Chapter 3). The transport of oxygen is
most pronounced in the light, driven by photosynthesis in the leaves. However, the oxic microzone
around C. rotundata roots was also present during
dark incubations although then it diminished to 50
µm. It was shown that the oxygen present in the
rhizosphere during darkness was supplied from the
oxic water column to roots via gas-phase diffusion
in leaves and rhizomes (Borum et al., Chapter 10).
Pedersen et al. (1998) estimated that release of oxygen by roots contributed about 10% of the total
sediment oxygen consumption. Oxygen supply by
seagrass roots to the sediment has been recognized
to be essential to maintaining non-toxic levels of
sulfide for seagrass growth and survival (Pedersen
et al., 1998; Eldrigde and Morse, 2000), whereas it
would only partially support sediment aerobic bacterial metabolism (Pedersen et al., 1998; Eldrigde and
Morse, 2000).
Seagrasses are highly productive plants but they
also support a large biomass and production of animals and microbes, and a large respiratory activity partially dependent on the seagrass itself. To
date, there are few estimates of the light compensation point for balanced metabolism (i.e. gross
primary production = community respiration) in
seagrass beds (Erftemeijer and Middelburg, 1993;
D’Avanzo et al., 1996; Herzka and Dunton, 1997),
but those available indicate very high light requirements for the whole ecosystem metabolism. Indeed, seagrass meadows may only be marginally
autotrophic ecosystems (Hemminga and Duarte,
2000). Further work should extend the information
on integrative studies of seagrass metabolism and
its implications for the balance between aerobic and
anaerobic processes in the whole coastal systems.
For instance, the metabolic activity of P. oceanica
beds has been shown to influence the concentration
of oxygen and dissolved inorganic carbon in the water column within an entire Mediterranean bay partially colonized by seagrasses (Frankignoulle et al.,
1984).
E. CO 2 –Carbonate System and pH
Carbon dioxide and pH are closely linked in the marine environment through the photosynthetic activity
of primary producers, such as seagrasses. Photosynthetic uptake of dissolved inorganic carbon results
in a displacement of the CO 2 /bicarbonate/carbonate
balance from equilibrium:
CO 2 + H 2 O ↔ H 2 CO 3 ↔ H
+
+ HCO
−
3
↔ CO
2−
3 + 2H
+
decreasing the concentration of CO 2 and increasing
that of HCO
−
3 and/or CO
2−
3 . These changes are coupled to water column pH fluctuations of low amplitude due to the buffering capacity of marine waters.
However, in seagrass meadows with high biomass
and productivity, significant daily fluctuations of pH,
carbon dioxide, and dissolved oxygen are found coupled to the photosynthetic activity of the plants. In
the NW-Mediterranean diel pH fluctuations of up to
0.5 units have been described just above and within