Chapter 6 Seagrasses and Biogeochemistry
137
Studies in Western Australia (Lemmens et al., 1996)
show that meadows of Posidonia australis were able
to remove particles from the water column at much
faster rates (approximately once a day) than unvegetated sand bottoms, where densities of macro suspension feeders and epibionts were found to be significantly lower. These removal rates of suspended
particles were higher in P. australis and Anphibolis antarctica meadows than those of Heterozostera
tasmanica. Species-specific differences in particle
trapping rates may be due to differences in (a)
canopy surface, which limits the area of substrate for
epibiont colonization, and (b) leaf life-span, which
sets the time window for epibiont colonization and,
thus, constrains maximum epibiont biomass development (Cebri´ an et al., 1994). In addition, local environmental conditions may also restrict the development of filter feeding assemblages. In meadows of
P. australis mentioned above, the epibionts were the
dominant filtering community comprising 76% of
the filtering activity of the system (Lemmens et al.,
1996).
Direct particle trapping (including both passive
and active mechanisms) has been reported for a seagrass mixed meadow in the Philippines (Agawin
and Duarte, 2002). In situ incubations of seagrass
and bare sediment areas with labelled phytoplankton
and labelled inert particles demonstrated that water
column clearance rate was four times faster within
seagrass canopies than in bare sediments at 1.5 m
depth. The authors estimated approximately 5% of
the filtering capacity was due to the activity of protozoan epibionts (ciliates and amoeba-like organisms),
whereas the largest fraction of particles was trapped
by passive particle adherence on leaf surfaces.
Direct passive particle trapping has been estimated in seagrass meadows from South East Asia
by quantifying the inorganic particles adhered to seagrass leaf surfaces across a wide range of sediment
deposition rates (Gacia et al., 2003). These data did
not quantify the particulate organic fraction of the
passive trapping mechanisms but it provided rates
of non-carbonate mineral clearance from the water
column ranging from 0.1 to 0.6 g DW m
−2 d
−1 across
meadows. These rates, however, represented only a
minor fraction (<1%) of the total non-carbonated
inorganic material suspended in the water column,
since the studied sites supported high siltation rates
(Gacia et al., 2003). The clearance capacity of seagrass meadows by passive particle trapping depends
on, for instance, seagrass productivity, local hydrodynamic conditions, and biomass and composition
of the epibiont community, which is expected to enhance passive particle trapping by increasing the excretion of exopolymeric substances.
The above-mentioned mechanisms of particle
trapping increases the particulate organic matter
pool in seagrass sediments, since the structure of
the seagrass leaf canopy and rhizosphere prevent resuspension and erosion of the material deposited on
sediments interface.
B. Particulate Organic Matter
in Seagrass Sediments
The positive effects of seagrass beds on particle deposition, together with the retention of most particulate organic matter produced by the bed itself (see
Mateo et al., Chapter 7), enrich seagrass sediments
with particulate organic matter (POM) compared to
unvegetated areas. The percentage of POM in seagrass sediments often accounts for approx. 4% of
sediment DW (e.g. Morse et al., 1985; Boschker
et al., 2000; Hemminga and Duarte, 2000; Enr´ ıquez
et al., 2001). However, the content of POM in seagrass sediments, in general, is lower than the organic
content of coastal sediments colonized by other communities (e.g. mangroves and coral reefs; KampNielsen et al., 2002; Kennedy et al., 2004).
The sources of POM in seagrass sediments are
seston, macroalgae, epibionts, and seagrass detritus.
The relative importance of one or another source depends on different processes including meadow production, export, and decomposition rates (see Chapter 7 for extensive explanations), eutrophication,
coastal erosion, and overall water flow in the area.
Studies of the contributions of different sources of
organic matter in seagrass sediments indicate that a
significant fraction of the organic matter is refractory
(i.e. 56–84% in a P. oceanica meadow; Danovaro,
1996), while only a minor proportion (18%) is recovered in the biopolymeric fraction (lipids, carbohydrates, and proteins). Danovaro (1996) traced the origin of the labile fraction of particulate organic matter
and found that 25% of this fraction derived from benthic microphytoplankton. Other studies, however, revealed the importance of seston as organic matter
source in seagrass sediments. Gacia et al. (2002)
estimated that 43% of the organic carbon in the sediment of a 15 m depth P. oceanica meadow derived
from the seagrass and epibionts, while the remaining
fraction was provided by seston. Similar percentages
137
Studies in Western Australia (Lemmens et al., 1996)
show that meadows of Posidonia australis were able
to remove particles from the water column at much
faster rates (approximately once a day) than unvegetated sand bottoms, where densities of macro suspension feeders and epibionts were found to be significantly lower. These removal rates of suspended
particles were higher in P. australis and Anphibolis antarctica meadows than those of Heterozostera
tasmanica. Species-specific differences in particle
trapping rates may be due to differences in (a)
canopy surface, which limits the area of substrate for
epibiont colonization, and (b) leaf life-span, which
sets the time window for epibiont colonization and,
thus, constrains maximum epibiont biomass development (Cebri´ an et al., 1994). In addition, local environmental conditions may also restrict the development of filter feeding assemblages. In meadows of
P. australis mentioned above, the epibionts were the
dominant filtering community comprising 76% of
the filtering activity of the system (Lemmens et al.,
1996).
Direct particle trapping (including both passive
and active mechanisms) has been reported for a seagrass mixed meadow in the Philippines (Agawin
and Duarte, 2002). In situ incubations of seagrass
and bare sediment areas with labelled phytoplankton
and labelled inert particles demonstrated that water
column clearance rate was four times faster within
seagrass canopies than in bare sediments at 1.5 m
depth. The authors estimated approximately 5% of
the filtering capacity was due to the activity of protozoan epibionts (ciliates and amoeba-like organisms),
whereas the largest fraction of particles was trapped
by passive particle adherence on leaf surfaces.
Direct passive particle trapping has been estimated in seagrass meadows from South East Asia
by quantifying the inorganic particles adhered to seagrass leaf surfaces across a wide range of sediment
deposition rates (Gacia et al., 2003). These data did
not quantify the particulate organic fraction of the
passive trapping mechanisms but it provided rates
of non-carbonate mineral clearance from the water
column ranging from 0.1 to 0.6 g DW m
−2 d
−1 across
meadows. These rates, however, represented only a
minor fraction (<1%) of the total non-carbonated
inorganic material suspended in the water column,
since the studied sites supported high siltation rates
(Gacia et al., 2003). The clearance capacity of seagrass meadows by passive particle trapping depends
on, for instance, seagrass productivity, local hydrodynamic conditions, and biomass and composition
of the epibiont community, which is expected to enhance passive particle trapping by increasing the excretion of exopolymeric substances.
The above-mentioned mechanisms of particle
trapping increases the particulate organic matter
pool in seagrass sediments, since the structure of
the seagrass leaf canopy and rhizosphere prevent resuspension and erosion of the material deposited on
sediments interface.
B. Particulate Organic Matter
in Seagrass Sediments
The positive effects of seagrass beds on particle deposition, together with the retention of most particulate organic matter produced by the bed itself (see
Mateo et al., Chapter 7), enrich seagrass sediments
with particulate organic matter (POM) compared to
unvegetated areas. The percentage of POM in seagrass sediments often accounts for approx. 4% of
sediment DW (e.g. Morse et al., 1985; Boschker
et al., 2000; Hemminga and Duarte, 2000; Enr´ ıquez
et al., 2001). However, the content of POM in seagrass sediments, in general, is lower than the organic
content of coastal sediments colonized by other communities (e.g. mangroves and coral reefs; KampNielsen et al., 2002; Kennedy et al., 2004).
The sources of POM in seagrass sediments are
seston, macroalgae, epibionts, and seagrass detritus.
The relative importance of one or another source depends on different processes including meadow production, export, and decomposition rates (see Chapter 7 for extensive explanations), eutrophication,
coastal erosion, and overall water flow in the area.
Studies of the contributions of different sources of
organic matter in seagrass sediments indicate that a
significant fraction of the organic matter is refractory
(i.e. 56–84% in a P. oceanica meadow; Danovaro,
1996), while only a minor proportion (18%) is recovered in the biopolymeric fraction (lipids, carbohydrates, and proteins). Danovaro (1996) traced the origin of the labile fraction of particulate organic matter
and found that 25% of this fraction derived from benthic microphytoplankton. Other studies, however, revealed the importance of seston as organic matter
source in seagrass sediments. Gacia et al. (2002)
estimated that 43% of the organic carbon in the sediment of a 15 m depth P. oceanica meadow derived
from the seagrass and epibionts, while the remaining
fraction was provided by seston. Similar percentages