136
N. Marb `
a, M. Holmer, and E. Gacia
Fig. 1. View of a Posidonia oceanica meadow at 4 m depth in
Cabrera (Balearic Islands, Spain) at the transition with the unvegetated sea-floor. (Photograph by E. Ballesteros).
content of structural compounds (e.g. lignin) in seagrass leaves, roots, and rhizomes (Vichkovitten and
Holmer, 2004), the decomposition of seagrass detritus may only take place when nitrogen and phosphorus is supplied from other sources to the microbial community (Pedersen et al., 1999; Mateo
et al., Chapter 7). There is thus an important competition between bacteria and seagrasses for nutrients, which may have large implications on the flux
of nutrients across the sediment–water interface and
this may affect seasonal patterns of nutrient availability in the water column. Due to the enhanced
microbial activity in seagrass sediments, oxygen as
an electron acceptor may be rapidly exhausted and
anaerobic processes may take over the mineralization of organic matter (Holmer and Nielsen, 1997).
As the availability of sulfate is high in the marine environment, sulfate reduction, and thus sulfide production, is often the most important anaerobic process in the sediments (Canfield et al., 1993).
High sulfide production may affect growth and thus
performance of seagrass meadows due to anoxia
and sulfide toxicity (Borum et al., Chapter 10).
In this chapter, we review the role of seagrass beds
on coastal biogeochemistry, (1) by examining their
effect on particulate and dissolved materials (i.e.
organic matter, dissolved inorganic carbon, carbonates, and gases) in the water column and sediments
of coastal areas, and (2) by examining the processes
Abbreviations: %OC – percent particulate organic carbon in sediments; C/N – carbon/nitrogen; DIC – dissolved inorganic carbon;
DO – dissolved oxygen; DOC – dissolved organic carbon; DOM
– dissolved organic matter; DW – dry weight; PLFA’s – phospholipid fatty acids; POM – particulate organic matter; SRR – Sulfate
reduction rate
involved in mineralization of organic matter and nutrient cycling in sediments colonized by seagrasses.
The effect of seagrass beds on ambient nutrient pools
is reviewed in detail in Romero et al., Chapter 9.
II. Role of Particulates
and Dissolved Material
A. Effects of Seagrasses on Suspended
Particle Load
Seagrass beds trap particles suspended in the water column via direct and indirect mechanisms. Leaf
canopy indirectly enhances deposition of suspended
particles by its interaction with water flow (see Koch
et al., Chapter 8), which favours sedimentation rate
(Gacia et al., 1999) and decreases resuspension of
deposited particles on seagrass sediments (Gacia and
Duarte, 2001). Yet, seagrass communities directly
trap particles suspended in the water column. Two
processes are responsible for direct particle trapping
within seagrass beds: active filtering of the particulate material by the suspended feeders associated
with the plant community (i.e. macro suspension
feeders or epibionts associated with the seagrass
leaves) and passive adherence of the suspended material onto seagrass surfaces (mainly leaves).
Abundance and biomass of macro suspension
feeders (ascidians, sponges, and bivalves) tend to
be higher in vegetated areas when compared to
bare sand. This is due to a combination of factors, including enhanced rates of recruitment within
plant canopies (Duggins et al., 1990; Bostr¨ om and
Bonsdorff, 2000), shelter from predators (Peterson
and Heck, 2001), and higher abundance of food
availability (Peterson et al., 1984). The abundance of
epifaunal suspension feeders (hydroids, bryozoans,
barnacles, amphipods, spirorbids, and protozoa) is
also higher in seagrass beds than in bare areas, since
seagrass canopy increases the available surface for
colonization. Suspension-feeder communities have
been shown to control phytoplankton populations in
shallow-semi-enclosed environments (Buss and Jakson, 1981; Alpine and Cloern, 1992). Thus, active
trapping of particulate organic matter (POM) from
the water column by suspension feeders is expected
to be higher in seagrass vegetated areas than over
bare sediments.
There is very little quantitative information for
direct particle trapping within seagrass canopies.
N. Marb `
a, M. Holmer, and E. Gacia
Fig. 1. View of a Posidonia oceanica meadow at 4 m depth in
Cabrera (Balearic Islands, Spain) at the transition with the unvegetated sea-floor. (Photograph by E. Ballesteros).
content of structural compounds (e.g. lignin) in seagrass leaves, roots, and rhizomes (Vichkovitten and
Holmer, 2004), the decomposition of seagrass detritus may only take place when nitrogen and phosphorus is supplied from other sources to the microbial community (Pedersen et al., 1999; Mateo
et al., Chapter 7). There is thus an important competition between bacteria and seagrasses for nutrients, which may have large implications on the flux
of nutrients across the sediment–water interface and
this may affect seasonal patterns of nutrient availability in the water column. Due to the enhanced
microbial activity in seagrass sediments, oxygen as
an electron acceptor may be rapidly exhausted and
anaerobic processes may take over the mineralization of organic matter (Holmer and Nielsen, 1997).
As the availability of sulfate is high in the marine environment, sulfate reduction, and thus sulfide production, is often the most important anaerobic process in the sediments (Canfield et al., 1993).
High sulfide production may affect growth and thus
performance of seagrass meadows due to anoxia
and sulfide toxicity (Borum et al., Chapter 10).
In this chapter, we review the role of seagrass beds
on coastal biogeochemistry, (1) by examining their
effect on particulate and dissolved materials (i.e.
organic matter, dissolved inorganic carbon, carbonates, and gases) in the water column and sediments
of coastal areas, and (2) by examining the processes
Abbreviations: %OC – percent particulate organic carbon in sediments; C/N – carbon/nitrogen; DIC – dissolved inorganic carbon;
DO – dissolved oxygen; DOC – dissolved organic carbon; DOM
– dissolved organic matter; DW – dry weight; PLFA’s – phospholipid fatty acids; POM – particulate organic matter; SRR – Sulfate
reduction rate
involved in mineralization of organic matter and nutrient cycling in sediments colonized by seagrasses.
The effect of seagrass beds on ambient nutrient pools
is reviewed in detail in Romero et al., Chapter 9.
II. Role of Particulates
and Dissolved Material
A. Effects of Seagrasses on Suspended
Particle Load
Seagrass beds trap particles suspended in the water column via direct and indirect mechanisms. Leaf
canopy indirectly enhances deposition of suspended
particles by its interaction with water flow (see Koch
et al., Chapter 8), which favours sedimentation rate
(Gacia et al., 1999) and decreases resuspension of
deposited particles on seagrass sediments (Gacia and
Duarte, 2001). Yet, seagrass communities directly
trap particles suspended in the water column. Two
processes are responsible for direct particle trapping
within seagrass beds: active filtering of the particulate material by the suspended feeders associated
with the plant community (i.e. macro suspension
feeders or epibionts associated with the seagrass
leaves) and passive adherence of the suspended material onto seagrass surfaces (mainly leaves).
Abundance and biomass of macro suspension
feeders (ascidians, sponges, and bivalves) tend to
be higher in vegetated areas when compared to
bare sand. This is due to a combination of factors, including enhanced rates of recruitment within
plant canopies (Duggins et al., 1990; Bostr¨ om and
Bonsdorff, 2000), shelter from predators (Peterson
and Heck, 2001), and higher abundance of food
availability (Peterson et al., 1984). The abundance of
epifaunal suspension feeders (hydroids, bryozoans,
barnacles, amphipods, spirorbids, and protozoa) is
also higher in seagrass beds than in bare areas, since
seagrass canopy increases the available surface for
colonization. Suspension-feeder communities have
been shown to control phytoplankton populations in
shallow-semi-enclosed environments (Buss and Jakson, 1981; Alpine and Cloern, 1992). Thus, active
trapping of particulate organic matter (POM) from
the water column by suspension feeders is expected
to be higher in seagrass vegetated areas than over
bare sediments.
There is very little quantitative information for
direct particle trapping within seagrass canopies.