14.2.4 Carbon Cycle in Marine Sediments
14.2.4.1 Sinking Organic Carbon
at Water-Sediment Interface
Marine sediments are the largest organic carbon reservoir on
Earth: 80 % being stored in coastal sediments (that represent
only 10 % of the ocean surface), 17 % in continental slope
sediments, and 1.5 % in open ocean sediments. The quantity
and chemical composition of organic matter that reaches the
water-sediment interface depends on its location (continental inputs in coastal areas vs atmospheric inputs in offshore
areas), on primary production in the euphotic zone, on water
column depth, on local hydrodynamism and on the sinking
rate of organic matter.
On average, the organic matter yielded by primary production that reaches sediment surface amounts up to 25–50 %
in coastal areas, but does not exceed 1 % in abyssal zones. It is
estimated that only 1–0.1 % of primary production is
sequestered in marine sediments. This heterogeneous organic
material free of nitrogen and phosphorus, is subject to the
action of microorganisms as it reaches the water-sediment
interface.
14.2.4.2 Sediment Ecosystem
Sediment ecosystem is very complex. It consists of:
1. A mineral phase that will favor prokaryote and organic
substrate fixation;
2. An aqueous phase that plays an essential role in
exchanges of dissolved elements (e.g., dioxygen,
nutrients) between sediment and overlying waters;
3. An organic phase constituted of dissolved compounds in
interstitial waters and as particles of various sizes (up to
90 % of total organic matter), offering substrates and
binding sites for prokaryotes;
4. Living organisms: animals (zooplankton) and primary
producers (phytobenthos) whose distribution within
sediments depends on physical and chemical gradients,
e.g., organic matter and nutrient concentrations, partial
pressure of dioxygen and other dissolved gases (CO 2 , N 2 ,
CH 4 ), oxido-reduction potential, pH, temperature,
salinity, light.
Zoobenthos in sediments can be divided up in several size
classes: mega- and macrofauna encompassing organisms
larger than 1 mm, and meiofauna which comprise organisms
smaller than 1 mm and that are retained on a sieve of
63–40 μm mesh size. Microorganisms are collected on filters
of lower porosity.
When the amount of light energy reaching the sediment
surface is sufficient, phototrophic organisms constituting the
phytobenthos can develop, frequently forming microbial mats.
The number of individuals that constitute macrobenthic,
meiobenthic, and bacterial communities considerably varies,
depending on sediment structure, organic matter inputs, and
depth. Indeed, organic matter is not evenly distributed in
sediment, in particular under the action of bioturbation
(Box 14.3). This distribution results in “hot spot”
formations, zones where biological activities are more
intense in connection with organic matter accumulation.
Macrofauna is mainly located in the first 20 cm, but some
organisms thrive more deeply, such as the fish
Boleophthalmus boddarti whose burrow may extend up to
2 m. Most of meiofauna organisms are mainly distributed in
the first 2 cm depth.
The fate of organic matter in sediments is largely under the
control of prokaryotes (Jørgensen 2000). Counting them by
epifluorescence microscopy in the upper 2 cm of sediment,
yields concentrations between 10
8 and 10
11 cells per gram of
dry weight sediment in areas rich in organic matter.
Even if prokaryotes play a major role in organic carbon
mineralization in sediments, the action of other organisms
(unicellular eukaryotes, meiofauna, macrofauna) must
also be taken into account (Box 14.3 and cf. Sects. 13.3.3
and 13.4.2).
Box 14.3: Macrobenthic Functional Groups Responsible
of Sediment Reworking
Georges Stora
Macrobenthic organisms can be classified into five
functional groups:
The biodiffusors (Box Fig. 14.8a). They move sediment particles in a random manner over short
distances. The movement of organisms leads, more
or less rapidly, to a homogeneous distribution of the
particles in the sedimentary column.
The downward conveyors (Box Fig. 14.8b). They
are head-up vertical oriented species causing an
“active” transport of sediment through their gut
from the sediment water interface to their egestion
depth.
The upward conveyors (Box Fig. 14.8c). They are
head-down vertically oriented species that remove
sediment at depth in the substratum and expel it as
faeces at the sediment surface. They cause an “active”
transport of sediment from the bottom up through their
gut and a “passive” transport all around them from the
interface to the bottom of their feeding zone, due to
sediment discharge at the sediment-water interface
G. Stora
Institut Me ´diterrane ´en d’Oce ´anologie (MIO), UM 110, CNRS
7294 IRD 235, Universite ´ de Toulon, Aix-Marseille Universite ´,
Campus de Luminy, 13288 Marseille Cedex 9, France
(continued)
14 Biogeochemical Cycles
537
14.2.4.1 Sinking Organic Carbon
at Water-Sediment Interface
Marine sediments are the largest organic carbon reservoir on
Earth: 80 % being stored in coastal sediments (that represent
only 10 % of the ocean surface), 17 % in continental slope
sediments, and 1.5 % in open ocean sediments. The quantity
and chemical composition of organic matter that reaches the
water-sediment interface depends on its location (continental inputs in coastal areas vs atmospheric inputs in offshore
areas), on primary production in the euphotic zone, on water
column depth, on local hydrodynamism and on the sinking
rate of organic matter.
On average, the organic matter yielded by primary production that reaches sediment surface amounts up to 25–50 %
in coastal areas, but does not exceed 1 % in abyssal zones. It is
estimated that only 1–0.1 % of primary production is
sequestered in marine sediments. This heterogeneous organic
material free of nitrogen and phosphorus, is subject to the
action of microorganisms as it reaches the water-sediment
interface.
14.2.4.2 Sediment Ecosystem
Sediment ecosystem is very complex. It consists of:
1. A mineral phase that will favor prokaryote and organic
substrate fixation;
2. An aqueous phase that plays an essential role in
exchanges of dissolved elements (e.g., dioxygen,
nutrients) between sediment and overlying waters;
3. An organic phase constituted of dissolved compounds in
interstitial waters and as particles of various sizes (up to
90 % of total organic matter), offering substrates and
binding sites for prokaryotes;
4. Living organisms: animals (zooplankton) and primary
producers (phytobenthos) whose distribution within
sediments depends on physical and chemical gradients,
e.g., organic matter and nutrient concentrations, partial
pressure of dioxygen and other dissolved gases (CO 2 , N 2 ,
CH 4 ), oxido-reduction potential, pH, temperature,
salinity, light.
Zoobenthos in sediments can be divided up in several size
classes: mega- and macrofauna encompassing organisms
larger than 1 mm, and meiofauna which comprise organisms
smaller than 1 mm and that are retained on a sieve of
63–40 μm mesh size. Microorganisms are collected on filters
of lower porosity.
When the amount of light energy reaching the sediment
surface is sufficient, phototrophic organisms constituting the
phytobenthos can develop, frequently forming microbial mats.
The number of individuals that constitute macrobenthic,
meiobenthic, and bacterial communities considerably varies,
depending on sediment structure, organic matter inputs, and
depth. Indeed, organic matter is not evenly distributed in
sediment, in particular under the action of bioturbation
(Box 14.3). This distribution results in “hot spot”
formations, zones where biological activities are more
intense in connection with organic matter accumulation.
Macrofauna is mainly located in the first 20 cm, but some
organisms thrive more deeply, such as the fish
Boleophthalmus boddarti whose burrow may extend up to
2 m. Most of meiofauna organisms are mainly distributed in
the first 2 cm depth.
The fate of organic matter in sediments is largely under the
control of prokaryotes (Jørgensen 2000). Counting them by
epifluorescence microscopy in the upper 2 cm of sediment,
yields concentrations between 10
8 and 10
11 cells per gram of
dry weight sediment in areas rich in organic matter.
Even if prokaryotes play a major role in organic carbon
mineralization in sediments, the action of other organisms
(unicellular eukaryotes, meiofauna, macrofauna) must
also be taken into account (Box 14.3 and cf. Sects. 13.3.3
and 13.4.2).
Box 14.3: Macrobenthic Functional Groups Responsible
of Sediment Reworking
Georges Stora
Macrobenthic organisms can be classified into five
functional groups:
The biodiffusors (Box Fig. 14.8a). They move sediment particles in a random manner over short
distances. The movement of organisms leads, more
or less rapidly, to a homogeneous distribution of the
particles in the sedimentary column.
The downward conveyors (Box Fig. 14.8b). They
are head-up vertical oriented species causing an
“active” transport of sediment through their gut
from the sediment water interface to their egestion
depth.
The upward conveyors (Box Fig. 14.8c). They are
head-down vertically oriented species that remove
sediment at depth in the substratum and expel it as
faeces at the sediment surface. They cause an “active”
transport of sediment from the bottom up through their
gut and a “passive” transport all around them from the
interface to the bottom of their feeding zone, due to
sediment discharge at the sediment-water interface
G. Stora
Institut Me ´diterrane ´en d’Oce ´anologie (MIO), UM 110, CNRS
7294 IRD 235, Universite ´ de Toulon, Aix-Marseille Universite ´,
Campus de Luminy, 13288 Marseille Cedex 9, France
(continued)
14 Biogeochemical Cycles
537