Chapter 6 Seagrasses and Biogeochemistry
151
sediments confirming the significance of sedimentary pools for seagrass growth and phosphorus as an
important limiting nutrient in carbonate sediments
(Udy et al., 1999; Jensen and Bell, 2001). A study, in
the tropical seagrass, Cymodocea rotundata, showed
that mineralization of organic matter is important
in sustaining seagrass growth, since the regenerated
nutrients supported up to 81% of the phosphorus
incorporation (Holmer et al., 2001).
Terrigenic seagrass sediments generally have
higher phosphorus pools, where phosphorus is
bound either in organic matter or in mineral pools
and pore water only represents a minor pool,
(Pedersen et al., 1997; Perez et al., 2001; Wit et al.,
2001). The mineral-bound pools are redox sensitive,
as phosphate is bound to oxidized iron (Jensen et al.,
1995; Azzoni et al., 2001; Wit et al., 2001), and oxidized iron concentrations may show large seasonal
fluctuations due to high microbial reducing activity
in summer. In eutrophic sediments vegetated with
Ruppia cirrhosa (Azzoni et al., 2001) the seasonal
changes in iron-bound, exchangeable and pore water phosphate pools were, however, quantitatively of
minor importance due to small pool sizes and very
low concentrations of labile oxidized iron in the sediments. In eutrophic sediments sulfides compete with
phosphate for oxidized iron, lowering the overall
availability of oxidized iron (Thamdrup, 2000).
Seagrass beds are generally considered to act as
sinks of phosphorus, which to a large degree can
be considered to be recycled within the beds during
colonization and development (Pedersen et al.,
1997). Only under eutrophic conditions have seagrass beds been shown to act as sources of phosphorus to the surrounding environment, as found for
C. nodosa in the Mediterranean (Perez et al., 2001).
Under hypertrophic conditions in a Ruppia maritima
meadow sulfate reduction was found to increase
and compete with phosphate for oxidized iron. As
a result phosphate was released from the sediments
and eutrophication was increased even further and
the seagrass eventually disappeared (Heijs et al.,
2000). This is an excellent example of the complex interactions between P, Fe, and S in the marine
environment.
IV. Summary and Future Work/Outlook
The studies of seagrasses during the past two decades
clearly show the influence of seagrasses on sediment biogeochemistry and vice versa. Seagrass beds
modify chemical conditions (concentration of organic matter, carbonates, O 2 , DIC and nutrients,
and pH) in the water column and the sediments of
the coastal zone (Fig. 7), including the immediate
colonized areas and, to some extent, adjacent systems. The chemical changes promoted by seagrass
beds constrain microbial processes such as nitrogen fixation and mineralization of organic matter
in seagrass sediments (Fig. 7), which, in turn, modify the chemical conditions of the bed environment
through their end products (e.g. nutrients, CO 2 , H 2 S,
see Fig. 7). Seagrass beds, therefore, play an engineering role (sensu Jones et al., 1997) in coastal
biogeochemistry, affecting, for instance, carbon, nitrogen, and sulfur cycles (see also Mateo et al.,
Chapter 7; Romero et al., Chapter 9).
The effects of seagrass beds on coastal biogeochemistry are, however, species specific. Some
seagrasses, such as those with low below-ground
biomass only have minor effect on the sediment
processes (e.g. Z. marina at nutrient-rich locations,
where the below-ground biomass is low), whereas
others with large below-ground biomass may alter the processes significantly compared to unvegetated sediments (e.g. P. oceanica in oligotrophic
sediments). Also seasonal effects have a major influence on sediment biogeochemistry, e.g. the large
seasonal variation found in redox potentials in rhizosphere sediments of C. nodosa; however, this has
only been described so far for a few species, at a
few locations and for a limited array of biogeochemical processes. Another example is the nitrogen cycling in the seagrass meadows, which primarily has been studied for seagrasses like Zostera spp,
and where very little is known for larger and slowgrowing species. Due to divergent results obtained
with established methods, it will be important in
future to include methodological studies, probably
with
15 N, as this approach is suitable for use at community level for both field and experimental studies. Studies of seagrass meadows at the community
level are greatly needed, as seagrasses seldom show
the same growth pattern in small micro/mesocosms
compared to the field. For the nitrogen cycling
there is also a challenge to quantify nitrogen fixation and investigate the microbial ecology for more
seagrass species, and under various growth conditions to understand the nutrient cycling in seagrass
beds.
The type of sediment substrate is of major importance in understanding the effect of seagrasses
on sediment biogeochemistry. There is a major
151
sediments confirming the significance of sedimentary pools for seagrass growth and phosphorus as an
important limiting nutrient in carbonate sediments
(Udy et al., 1999; Jensen and Bell, 2001). A study, in
the tropical seagrass, Cymodocea rotundata, showed
that mineralization of organic matter is important
in sustaining seagrass growth, since the regenerated
nutrients supported up to 81% of the phosphorus
incorporation (Holmer et al., 2001).
Terrigenic seagrass sediments generally have
higher phosphorus pools, where phosphorus is
bound either in organic matter or in mineral pools
and pore water only represents a minor pool,
(Pedersen et al., 1997; Perez et al., 2001; Wit et al.,
2001). The mineral-bound pools are redox sensitive,
as phosphate is bound to oxidized iron (Jensen et al.,
1995; Azzoni et al., 2001; Wit et al., 2001), and oxidized iron concentrations may show large seasonal
fluctuations due to high microbial reducing activity
in summer. In eutrophic sediments vegetated with
Ruppia cirrhosa (Azzoni et al., 2001) the seasonal
changes in iron-bound, exchangeable and pore water phosphate pools were, however, quantitatively of
minor importance due to small pool sizes and very
low concentrations of labile oxidized iron in the sediments. In eutrophic sediments sulfides compete with
phosphate for oxidized iron, lowering the overall
availability of oxidized iron (Thamdrup, 2000).
Seagrass beds are generally considered to act as
sinks of phosphorus, which to a large degree can
be considered to be recycled within the beds during
colonization and development (Pedersen et al.,
1997). Only under eutrophic conditions have seagrass beds been shown to act as sources of phosphorus to the surrounding environment, as found for
C. nodosa in the Mediterranean (Perez et al., 2001).
Under hypertrophic conditions in a Ruppia maritima
meadow sulfate reduction was found to increase
and compete with phosphate for oxidized iron. As
a result phosphate was released from the sediments
and eutrophication was increased even further and
the seagrass eventually disappeared (Heijs et al.,
2000). This is an excellent example of the complex interactions between P, Fe, and S in the marine
environment.
IV. Summary and Future Work/Outlook
The studies of seagrasses during the past two decades
clearly show the influence of seagrasses on sediment biogeochemistry and vice versa. Seagrass beds
modify chemical conditions (concentration of organic matter, carbonates, O 2 , DIC and nutrients,
and pH) in the water column and the sediments of
the coastal zone (Fig. 7), including the immediate
colonized areas and, to some extent, adjacent systems. The chemical changes promoted by seagrass
beds constrain microbial processes such as nitrogen fixation and mineralization of organic matter
in seagrass sediments (Fig. 7), which, in turn, modify the chemical conditions of the bed environment
through their end products (e.g. nutrients, CO 2 , H 2 S,
see Fig. 7). Seagrass beds, therefore, play an engineering role (sensu Jones et al., 1997) in coastal
biogeochemistry, affecting, for instance, carbon, nitrogen, and sulfur cycles (see also Mateo et al.,
Chapter 7; Romero et al., Chapter 9).
The effects of seagrass beds on coastal biogeochemistry are, however, species specific. Some
seagrasses, such as those with low below-ground
biomass only have minor effect on the sediment
processes (e.g. Z. marina at nutrient-rich locations,
where the below-ground biomass is low), whereas
others with large below-ground biomass may alter the processes significantly compared to unvegetated sediments (e.g. P. oceanica in oligotrophic
sediments). Also seasonal effects have a major influence on sediment biogeochemistry, e.g. the large
seasonal variation found in redox potentials in rhizosphere sediments of C. nodosa; however, this has
only been described so far for a few species, at a
few locations and for a limited array of biogeochemical processes. Another example is the nitrogen cycling in the seagrass meadows, which primarily has been studied for seagrasses like Zostera spp,
and where very little is known for larger and slowgrowing species. Due to divergent results obtained
with established methods, it will be important in
future to include methodological studies, probably
with
15 N, as this approach is suitable for use at community level for both field and experimental studies. Studies of seagrass meadows at the community
level are greatly needed, as seagrasses seldom show
the same growth pattern in small micro/mesocosms
compared to the field. For the nitrogen cycling
there is also a challenge to quantify nitrogen fixation and investigate the microbial ecology for more
seagrass species, and under various growth conditions to understand the nutrient cycling in seagrass
beds.
The type of sediment substrate is of major importance in understanding the effect of seagrasses
on sediment biogeochemistry. There is a major