Chapter 6 Seagrasses and Biogeochemistry
145
and the amount of organic matter in the sediment on
sediment redox potential evidently combine in different ways to influence sediment redox conditions
during seagrass meadow development. For instance,
the greatest positive redox potential anomaly in sediments colonized by a deep loose C. nodosa meadow
during the growing season occurred at the beginning
of summer, when leaf biomass was highest and seagrass rhizosphere was not yet completely developed
(Marb` a and Duarte, 2001). Similarly, sediment redox conditions tended to be less positive the longer
the time after C. nodosa colonization (Barr´ on et al.,
2004).
Seagrass activity, therefore, is able to attenuate
sediment anoxic conditions within the sediment layers occupied by seagrass rhizospheres. Changes in
redox potential promoted by seagrasses are evident
in sediments where seagrass roots are spaced by
several centimeters (Marb` a and Duarte, 2001), and,
hence, at spatial scales larger than those where oxygen released by seagrass roots is detected with microelectrodes (80 µm; Borum et al., Chapter 10).
Therefore, the effect of seagrasses on sediment redox
properties should result from O 2 root release, and oxidation of reduced compounds (e.g. iron, sulphide)
creating a suboxic zone.
III. Mineralization of Organic Matter and
Nutrient Cycling in Seagrass Sediments
A. Microbial Activity
The decomposition of organic matter in seagrass
sediments occurs through a consortium of microbial communities (Table 2). As oxygen penetration
is generally limited to the upper few millimetres
or centimetres of coastal sediments, the activities
of anaerobic heterotrophic microbial communities
are particularly important (Fenchel et al., 1998;
Thamdrup, 2000). Via the microbial food chain,
complex organic substrates are remineralized in several steps, as particulate organic carbon is solubilized to high molecular weight dissolved organic
carbon, which is enzymatically hydrolyzed to lower
molecular weight substrates (Fenchel et al., 1998).
These lower molecular weight substrates are transformed into fermentation products such as volatile
fatty acids, which are ultimately remineralized to
CO 2 . The pore water pools of dissolved organic
compounds, such as DOC, volatile fatty acids, and
dissolved carbohydrates in seagrass sediments are
generally enhanced compared to unvegetated sites
(Holmer and Nielsen, 1997; Burdige and Zimmerman, 2002) indicating that there is more substrate
available for the bacteria in the rhizosphere sediments. A variety of electron acceptors can be
used by microbial communities in the final steps
of remineralization. In marine sediments, sulfate
is quantitatively the most important electron acceptor under anoxic conditions (Jørgensen, 1982),
but recent studies have demonstrated that microbial
iron reduction and manganese reduction can also be
important, in particular in sediments with high reoxidation capacities (Thamdrup, 2000). Denitrification supported by nitrate from the water column or
through coupled nitrification–denitrification is also
an important process for the nitrogen cycling in marine sediments, but due to the low carbon oxidation
in this process, denitrification has a minor effect on
the marine carbon cycle (Fenchel et al., 1998).
B. Nitrogen Cycling
Seagrasses can influence nitrogen cycling in sediments by several direct and indirect mechanisms.
The production of ammonium through mineralization may be enhanced by (i) accumulation of allochthonous particulate organic material trapped
within the meadows (Kemp et al., 1983), (ii) microbial breakdown of dissolved organic nitrogen released from plant roots (Smith et al., 1988), and (iii)
decomposition of senescent plant material (Pedersen
et al., 1999). The production of ammonium through
mineralization is an important source of nitrogen for
plant growth (Pedersen and Borum, 1992; Holmer
et al., 2001), but nitrogen fixation also contributes
significantly both to the nitrogen cycling in the sediments (Table 3) and to support plant growth. In tropical seagrass beds, sediment nitrogen fixation has
been estimated to provide more than 50% of plant
nitrogen demand (review by Welsh, 2000), whereas
it tends to be much lower in temperate sediments,
usually less than 12% (review by Welsh, 2000) or
even <5% as found for a Z. marina community in
Denmark (McGlathery et al., 1998). Studies have
demonstrated a coupling between nitrogen fixation
and the photosynthetic activity of the plants via the
exudation of fixed carbon by the root system (Welsh,
2000). Nielsen et al. (2001) found that about onethird of the nitrogen fixation activity in the rhizosphere sediment was directly associated with the
145
and the amount of organic matter in the sediment on
sediment redox potential evidently combine in different ways to influence sediment redox conditions
during seagrass meadow development. For instance,
the greatest positive redox potential anomaly in sediments colonized by a deep loose C. nodosa meadow
during the growing season occurred at the beginning
of summer, when leaf biomass was highest and seagrass rhizosphere was not yet completely developed
(Marb` a and Duarte, 2001). Similarly, sediment redox conditions tended to be less positive the longer
the time after C. nodosa colonization (Barr´ on et al.,
2004).
Seagrass activity, therefore, is able to attenuate
sediment anoxic conditions within the sediment layers occupied by seagrass rhizospheres. Changes in
redox potential promoted by seagrasses are evident
in sediments where seagrass roots are spaced by
several centimeters (Marb` a and Duarte, 2001), and,
hence, at spatial scales larger than those where oxygen released by seagrass roots is detected with microelectrodes (80 µm; Borum et al., Chapter 10).
Therefore, the effect of seagrasses on sediment redox
properties should result from O 2 root release, and oxidation of reduced compounds (e.g. iron, sulphide)
creating a suboxic zone.
III. Mineralization of Organic Matter and
Nutrient Cycling in Seagrass Sediments
A. Microbial Activity
The decomposition of organic matter in seagrass
sediments occurs through a consortium of microbial communities (Table 2). As oxygen penetration
is generally limited to the upper few millimetres
or centimetres of coastal sediments, the activities
of anaerobic heterotrophic microbial communities
are particularly important (Fenchel et al., 1998;
Thamdrup, 2000). Via the microbial food chain,
complex organic substrates are remineralized in several steps, as particulate organic carbon is solubilized to high molecular weight dissolved organic
carbon, which is enzymatically hydrolyzed to lower
molecular weight substrates (Fenchel et al., 1998).
These lower molecular weight substrates are transformed into fermentation products such as volatile
fatty acids, which are ultimately remineralized to
CO 2 . The pore water pools of dissolved organic
compounds, such as DOC, volatile fatty acids, and
dissolved carbohydrates in seagrass sediments are
generally enhanced compared to unvegetated sites
(Holmer and Nielsen, 1997; Burdige and Zimmerman, 2002) indicating that there is more substrate
available for the bacteria in the rhizosphere sediments. A variety of electron acceptors can be
used by microbial communities in the final steps
of remineralization. In marine sediments, sulfate
is quantitatively the most important electron acceptor under anoxic conditions (Jørgensen, 1982),
but recent studies have demonstrated that microbial
iron reduction and manganese reduction can also be
important, in particular in sediments with high reoxidation capacities (Thamdrup, 2000). Denitrification supported by nitrate from the water column or
through coupled nitrification–denitrification is also
an important process for the nitrogen cycling in marine sediments, but due to the low carbon oxidation
in this process, denitrification has a minor effect on
the marine carbon cycle (Fenchel et al., 1998).
B. Nitrogen Cycling
Seagrasses can influence nitrogen cycling in sediments by several direct and indirect mechanisms.
The production of ammonium through mineralization may be enhanced by (i) accumulation of allochthonous particulate organic material trapped
within the meadows (Kemp et al., 1983), (ii) microbial breakdown of dissolved organic nitrogen released from plant roots (Smith et al., 1988), and (iii)
decomposition of senescent plant material (Pedersen
et al., 1999). The production of ammonium through
mineralization is an important source of nitrogen for
plant growth (Pedersen and Borum, 1992; Holmer
et al., 2001), but nitrogen fixation also contributes
significantly both to the nitrogen cycling in the sediments (Table 3) and to support plant growth. In tropical seagrass beds, sediment nitrogen fixation has
been estimated to provide more than 50% of plant
nitrogen demand (review by Welsh, 2000), whereas
it tends to be much lower in temperate sediments,
usually less than 12% (review by Welsh, 2000) or
even <5% as found for a Z. marina community in
Denmark (McGlathery et al., 1998). Studies have
demonstrated a coupling between nitrogen fixation
and the photosynthetic activity of the plants via the
exudation of fixed carbon by the root system (Welsh,
2000). Nielsen et al. (2001) found that about onethird of the nitrogen fixation activity in the rhizosphere sediment was directly associated with the