Chapter 6 Seagrasses and Biogeochemistry
139
meadows was three-fold higher than that in young
meadows (Cebri´ an et al., 2000). The role of seagrass beds as a sink, or a source of organic matter
to adjacent sediments, therefore, varies during the
colonization process.
C. Dissolved Organic Matter
and Seagrass Beds
Seagrass beds release dissolved organic matter
(DOM) by different mechanisms such as leaching
(Mateo et al., Chapter 7; Romero et al., Chapter 9)
and decomposition (Mateo et al., Chapter 7) of its
own material, and active excretion of DOM by leaves
and roots to the water column and to the sediment
pore water (Benner et al., 1986; Findlay et al., 1986).
To simplify, and because most of the studies on DOM
release provide information only on the carbon fraction, we essentially focus on dissolved organic carbon (DOC) when referring to the soluble fraction of
the organic matter associated with seagrasses.
The amount of DOC released by seagrasses has
been measured in laboratory incubations for a number of species. Brylinsky (1977) reported that less
than 5% of the inorganic carbon fixed by macrophytes is released as DOC. Halodule wrightii excretes less than 2% of the
14 C fixed to the water
column (Wetzel and Penhale, 1979; Moriarty et al.,
1986). However, the carbon released by the seagrass
ecosystems can derive from the seagrass itself, from
the epiphytes attached to the plant, and from the sediment. Penhale and Smith (1977) found that Zostera
marina shoots with epiphytes released only 0.9% of
the total carbon fixed as DOC, whereas DOC release
by clean eelgrass shoots represented 1.5% of the
total carbon.
In addition, part of the carbon fixed by seagrass
leaves is translocated to below-ground organs and a
fraction of it is released as DOC to the sediments.
Blaabjerg et al. (1998) estimated that 2–4% of the
total amount of carbon fixed by Z. marina was released as DOC into the rhizosphere. In H. wrightii,
DOC release into the sediment accounted for 11%
of the total
14 C fixed by the plant when plants were
illuminated (Moriarty et al., 1986). These estimates,
however, were obtained using
14 C technique which
can underestimate the amount of DOC released by
the seagrass ecosystems, as it only reflects the release
of carbon recently incorporated by the plant without
taking into account the release of DOC from carbon
already stored in the plant.
Recently, net DOC production by seagrass ecosystems has been estimated in situ (e.g. Velimirov, 1986;
Buzzelli et al., 1999; Ziegler and Benner, 1999a).
Ziegler and Benner (1999a) reported that the net
DOC release to the water column represented 10%
of the net primary production in a T. testudinumdominated community using in situ benthic chambers: in the light, this community released from 8 to
23 mmol C m
−2 d
−1 to the water column, while net
DOC flux in bare sediment ranged from −26 to 10
mmol C m
−2 d
−1 . Net DOC release decreased during darkness in seagrass and bare sediments (Ziegler
and Benner, 1999a). Similarly, Velimirov (1986) reported a higher DOC concentration within the P.
oceanica meadow than above bare sediments, suggesting that most DOC in the water column may
be produced by the seagrass community. Indeed, P.
oceanica communities maintained a net release of
DOC during a 1.5 year study, while the adjacent sediment oscillated between a sink or source of DOC
depending on the time of the year (Barr´ on et al.,
unpublished data).
The magnitude and quality of the DOM released
by seagrass communities varies seasonally. A T. testudinum-dominated community released DOC to the
water column during the whole year, reaching a
maximum in summer (Ziegler and Benner, 1999a).
Similar seasonality in the amount of DOC released
has been observed in P. oceanica beds (Barr´ on et al.,
submitted). The C/N ratio of DOM released by T. testudinum-dominated communities also exhibits seasonal changes. DOM fluxes were nitrogen depleted
during early summer, when seagrass productivity
was high, and nitrogen enriched during the late summer (Ziegler and Benner, 1999b). The seasonality in
DOM nutrient quality may reflect seasonal changes
among the processes (i.e. exudation vs. leaching) involved in DOM release to the water column (Ziegler
and Benner, 1999b). Exudation most likely represents the dominant process for DOC release during periods of primary production, whereas leaching
provides most of the DOC released to the water column during late summer, when net primary production is relatively low and seagrasses begin to senesce
(Ziegler and Benner, 1999b).
The release of DOC from seagrass beds enhances
bacterial activity both in the water column and
the sediments. In oligotrophic systems (e.g. a subtropical seagrass bed), an increase of bacterial activity coupled to DOC production may out-compete
phytoplankton and macrophytes for nutrients, hence
139
meadows was three-fold higher than that in young
meadows (Cebri´ an et al., 2000). The role of seagrass beds as a sink, or a source of organic matter
to adjacent sediments, therefore, varies during the
colonization process.
C. Dissolved Organic Matter
and Seagrass Beds
Seagrass beds release dissolved organic matter
(DOM) by different mechanisms such as leaching
(Mateo et al., Chapter 7; Romero et al., Chapter 9)
and decomposition (Mateo et al., Chapter 7) of its
own material, and active excretion of DOM by leaves
and roots to the water column and to the sediment
pore water (Benner et al., 1986; Findlay et al., 1986).
To simplify, and because most of the studies on DOM
release provide information only on the carbon fraction, we essentially focus on dissolved organic carbon (DOC) when referring to the soluble fraction of
the organic matter associated with seagrasses.
The amount of DOC released by seagrasses has
been measured in laboratory incubations for a number of species. Brylinsky (1977) reported that less
than 5% of the inorganic carbon fixed by macrophytes is released as DOC. Halodule wrightii excretes less than 2% of the
14 C fixed to the water
column (Wetzel and Penhale, 1979; Moriarty et al.,
1986). However, the carbon released by the seagrass
ecosystems can derive from the seagrass itself, from
the epiphytes attached to the plant, and from the sediment. Penhale and Smith (1977) found that Zostera
marina shoots with epiphytes released only 0.9% of
the total carbon fixed as DOC, whereas DOC release
by clean eelgrass shoots represented 1.5% of the
total carbon.
In addition, part of the carbon fixed by seagrass
leaves is translocated to below-ground organs and a
fraction of it is released as DOC to the sediments.
Blaabjerg et al. (1998) estimated that 2–4% of the
total amount of carbon fixed by Z. marina was released as DOC into the rhizosphere. In H. wrightii,
DOC release into the sediment accounted for 11%
of the total
14 C fixed by the plant when plants were
illuminated (Moriarty et al., 1986). These estimates,
however, were obtained using
14 C technique which
can underestimate the amount of DOC released by
the seagrass ecosystems, as it only reflects the release
of carbon recently incorporated by the plant without
taking into account the release of DOC from carbon
already stored in the plant.
Recently, net DOC production by seagrass ecosystems has been estimated in situ (e.g. Velimirov, 1986;
Buzzelli et al., 1999; Ziegler and Benner, 1999a).
Ziegler and Benner (1999a) reported that the net
DOC release to the water column represented 10%
of the net primary production in a T. testudinumdominated community using in situ benthic chambers: in the light, this community released from 8 to
23 mmol C m
−2 d
−1 to the water column, while net
DOC flux in bare sediment ranged from −26 to 10
mmol C m
−2 d
−1 . Net DOC release decreased during darkness in seagrass and bare sediments (Ziegler
and Benner, 1999a). Similarly, Velimirov (1986) reported a higher DOC concentration within the P.
oceanica meadow than above bare sediments, suggesting that most DOC in the water column may
be produced by the seagrass community. Indeed, P.
oceanica communities maintained a net release of
DOC during a 1.5 year study, while the adjacent sediment oscillated between a sink or source of DOC
depending on the time of the year (Barr´ on et al.,
unpublished data).
The magnitude and quality of the DOM released
by seagrass communities varies seasonally. A T. testudinum-dominated community released DOC to the
water column during the whole year, reaching a
maximum in summer (Ziegler and Benner, 1999a).
Similar seasonality in the amount of DOC released
has been observed in P. oceanica beds (Barr´ on et al.,
submitted). The C/N ratio of DOM released by T. testudinum-dominated communities also exhibits seasonal changes. DOM fluxes were nitrogen depleted
during early summer, when seagrass productivity
was high, and nitrogen enriched during the late summer (Ziegler and Benner, 1999b). The seasonality in
DOM nutrient quality may reflect seasonal changes
among the processes (i.e. exudation vs. leaching) involved in DOM release to the water column (Ziegler
and Benner, 1999b). Exudation most likely represents the dominant process for DOC release during periods of primary production, whereas leaching
provides most of the DOC released to the water column during late summer, when net primary production is relatively low and seagrasses begin to senesce
(Ziegler and Benner, 1999b).
The release of DOC from seagrass beds enhances
bacterial activity both in the water column and
the sediments. In oligotrophic systems (e.g. a subtropical seagrass bed), an increase of bacterial activity coupled to DOC production may out-compete
phytoplankton and macrophytes for nutrients, hence