The Abdus Salam
International Centre for Theoretical Physics
Lionel DENIS
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Benthic-pelagic coupling:
a benthic view
CEAC
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Benthic-pelagic coupling:
a benthic view
I – Historical review
II – Main coupling factors
1 – Organic Matter input in surficial sediments
2- Resuspension processes
3 – Nutrient recycling
4 – Contaminant sequestration
III – Close to the coast… the history
changes
Sources of Organic Matter to the ocean:
Input type
Oceanic Primary production
-Phytoplankton
-Macrophytes
Liquid inputs
- Rivers
- Groundwater
Atmospheric inputs
- Rain
- Dry particles
Total
Quantity
(1015gtC.y-1)
Percent
90.6
23.1
1.7
84.4
6.2
3.95
1.0
0.08
3.65
0.3
5.45
1.0
0.5
3.65
1.8
27.4
100
Microphytobenthos, Thermal vents (<0.1%)
100
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Benthic-pelagic coupling:
a benthic view
Historical review
Until the 70-80’s :
« Organic matter flux from the pelagic to the benthic
system can be considered as a constant ‘rain’ of particles
sinking vertically onto the surficial sediments. »
(Steele, 1974)
No coupling between compartments
Few works demonstrated a different mechanism :
«In Kiel Bight, sediments collected during the spring were
covered with a green layer probably originating pelagic
diatoms. »
(Remane, 1940)
« In several lakes, we have demonstrated the influence of
planktonic input in spring and automn on the development of
Chirinomidae larvae in surficial sediments.»
(Jonasson, 1964)
Coupling between compartments
Hargrave (1973) :
First described a model where sediment oxygen consumption was
directly linked to pelagic primary production.
Depth is also a key parameter
This study was based on a wide variety of systems (oligotophic,
eutrophic, coastal, lakes, …)
Hargrave’s model (1973)
Kiel Bight :
All primary production is
mineralized in surficial
sediments
Further details with developping
technology
AIR
Sediment traps
ST
80 m
ST
900 m
Depth
930 m
Location : Mediterranean Sea –
Grand Rhône Canyon - Single Depth 80 m
Large seasonal variability due to fluctuations in the primary
production in surface waters
TEMPORAL VARIABILITY
Location : Mediterranean Sea –
Grand Rhône Canyon – Several depths from 80 to 900 m
Maximal inputs at a depth of 600 m ?
Several other processes than only 1DV settling contribute
to the input of Organic Matter on surficial sediments
(vertical)
Settling
Advective
transport
Resuspension
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Benthic-pelagic coupling:
a benthic view
II
Main forcing factors
1 – Organic Matter input in surficial sediments
Organic matter input depends on
4 main parameters
Depth
Decay rate of Organic Matter
Settling velocity
Disequilibrium between production and
consumption in surface waters
Depth:
- Shallow sediments: HIGH COUPLING
- Deep- sediments: LOW COUPLING
Decay rate of organic matter:
Depends on the quality of Organic Matter.
Modified during the settling
Origin
C/N ratio
Redfield ratio:
BACTERIA
4-6
PHYTOPLANKTON
(CH2O)106(NH3)16(H3PO4)
6.6
SENESCENT PHYTOPLANKTON
C/N/P = 106/16/1
7.5
ZOOPLANKTON
SEDIMENTS (1st cm)
SEDIMENTS (10th cm)
8.5
10
40
Particle diameter:
- Larger particles have higher settling velocities
- With degradation processes, large molecules are transformed in smaller molecules
Aggregates
- Decrease the surface of contact with ambient water, hence decreasing the
opportunity of bacterial degradation ,
- Diameter increase may be consecutive to the trophic network (faeces of zooplankton
/ phytoplankton)
Diameter
(µm)
Settling
velocity(/day)
20 µm
38 m
1 µm
6 cm
0.05 µm
0.4 mm
Disequilibrium between production and
consumption in surface waters:
When production of surface waters is highly variable in time
=> pulse inputs towards deeper waters
Surface production
- Primary production
- Production of higher trophic levels
DEGRADATION MINERALIZATION
RECYCLING in the
euphotic zone
Strong gradients
Physical barrier to
vertical transfer
Surficial sediments
THERMOCLINE HALOCLINE
Dystrophic events
Vertical export from surface waters is too high /
consumption in surficial sediments
SEDIMENTWATER
INTERFACE
Surface
production
Consumption
Food limitation
Equilibrium
Equilibrium
Too much Organic Matter=>
Disequilibrium =>
Bacteria=> Anoxia => Death of
several organisms
OTHER BIOLOGICAL PROCESSES
- Coastal sediments: Filtration activity
Cloern, 1982:
202.
Does the benthos control phytoplankton biomass in San Francisco Bay? MEPS 9: 191-
- Deep-sea sediments: Migratory behavior (night/day cycles)
Several Crustacean species demonstrate a migratory behavior: Surface water
during the night, close to the sediment during daylight.
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Benthic-pelagic coupling:
a benthic view
II
Main forcing factors
1 – Organic Matter input in surficial sediments
2- Resuspension processes
Resuspension processes:
Directly linked to current velocity close to the sediment
2 main parameters to calibrate resuspension
processes
Critical shear stress
Above this value, particles are resuspended
Erosion rate
The amount of particles resuspended per unit time.
Resuspension processes:
Mainly:
Sediment particles (Inorganic, Organic aggregates, dead
organisms)
Microphytobenthos
Either fixed on particles or free but resuspended
Macrophytes
The distal part of macrophytes is regularly cut by waves and movements on
rocks
Macrobenthic organisms
Either larval stages, or adults (Polychaetes).
Inorganic resuspension
Flume experiments
Comparison Inorganic/ Organic resuspension
Fluorimeter
Sediment
Test section
Turbidimeter
Peristaltic pump
Concentration de pigments
totaux (µg.l )
totaux (µg.l )
}
}
Pigment content (µg/l)
-1
-1
90
40
-1
-1
-1
-1
-1
-1
-1
5
cm.s
10cm.s
15cm.s
20cm.s
25cm.s
30cm.s
0cm.s
80
35
70
30
M.E.S.
S.P.M.
60
25
50
20
40
Pigments Totaux
15
30
10
20
5
10
0
0
0
20
40
60
80
100
120
140
Concentration de pigments
Velocity increased step by
step
Critical erosion velocity:
15-20 cm.s-1
en suspension (mg.l )
Thau lagoon
Muddy sediments
Concentration
de matière
content (mg/l)
SPM
Comparison Inorganic/ Organic resuspension
-1
35
0,40
30
0,35
25
0,30
0,25
20
0,20
15
10
5
Free-stream
velocity
Vitesse
du courant
SPM
M.E.S.
0,15
Pigmentstotaux
Pigments
0,05
0,10
0
0,00
0
20
40
60
Temps (minutes)
Time (minutes)
80
100
Pigment content -1(µg/l)
Muddy sand
Gradual increase of
velocity
Critical erosion velocity:
16.5 cm.s-1
en Suspension (mg.l )
Gulf of Fos
-1
and
(mg/l)(cm.s
content
SPM
Vitesse
)
de courant
(cm/s)
velocity
free-stream
Concentration
de Matière
Temps(minutes)
(minutes)
Time
Macrophytes Resuspension (storms)
March
Growth
during
year n
Growth
zone
Figure 36: Morphology and growth of Laminaria saccharina
(Year 2001)
May
v
June
v
Total length (cm)
Growth
during
year n-1
April
Expected length
Measured length
Days
Figure 37: Measured l and Expected length of the macroalgae Laminaria saccharina
(Year 2001)
v
*************
Benthic-pelagic coupling:
a benthic view
II
Main forcing factors
1 – Organic Matter input in surficial sediments
2- Resuspension processes
3- Nutrient recycling
Euphotic layer
Organic Particulate Matter
CO2
Settling
Resuspension
Nutrient
recycling
NO3-, NH4+ ,
PO43-, Si(OH)4
WATER
COLUMN
SEDIMENT-WATER
INTERFACE
Accumulation
?Mineralization
SURFICIAL
SEDIMENT
Refractory Organic
Matter
*************
Benthic-pelagic coupling:
a benthic view
II
Main forcing factors
1 – Organic Matter input in surficial sediments
2- Resuspension processes
3- Nutrient recycling
4- Contaminant accumulation
Organic Particulate Matter
+ bounded contaminants
Settling
Bio-available
Contaminants
WATER
COLUMN
Resuspension
SEDIMENT-WATER
INTERFACE
Accumulation
Immobilized
? contaminants
-Bounded
- Non-toxic form
-- Non bio-available
(too deep)
Bioturbation
SURFICIAL
SEDIMENT
*************
Benthic-pelagic coupling:
a benthic view
III
Close to the coast…the
history changes…
Example:
Benthic mineralization processes and
consequences close to the mouth
of two major french rivers:
the Seine and the Rhone rivers
Problematics
Benthic mineralization in estuaries
-Benthic mineralization plays an important role in
estuarine coastal systems:
-A large part of organic matter degraded in surficial
sediments
-Serious consequences of those processes (nutrient
release / eutrophication, hypoxic events, Pollutants
accumulations, transformations or releases,...)
- Numerous problems remain because of the complexity of
such environments (natural versus human activities,
riverine / open sea influence, resuspension, coastal
installations, pollution,...)
Problematics – Estuaries
Riverine
discharge
Nutrient
concentration
Tidal
cycles
Coastal
topography
Coastal
eutrophication
Dredging
activities
Are biogeochemical data a
useful tool to identify the main
forcings in an estuarine
system?
=> Oxygen microprofiling
Coastal
hydrodynamics
Littoral
constructions
River mouth
embankment
Site presentation - Bay of Seine
N 49.30
Winter flooding period
- High river discharge
- Wave action limited
→ Dispersion of suspended matter
towards open sea (W -NW)
49.28
49.26
49.24
00
W -0.02
0.02
Spring storms Summer low water periods
0.00
0.02
0.04
0.06
0.08
0.10
0
0
0.12
0
0
5 0 km
0.16 E
0.14
N 49.30
49.28
- Large wave action / resuspension
- Low river discharge
→ Accumulation of suspended matter in
the north and south of the dykes
49.26
49.24
00
W -0.02
0.02
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0
0
0
0.14
0
5 0 km
0.16 E
Sampling strategy
N 49.30
-25 Stations
49.28
all around the mouth
49.26
of the Seine river
49.24
00
W -0.02
0.02
- 2 cruises
18-19 September 2003
0.02
0.04
0.06
0.08
0.10
0.12
0
0
0.14
2000 Flood period
Daily averaged
discharge (m3.s-1)
26-27 February 2003
0.00
0
Low
water
period
1000
0
J
F M A M J
J
2003
A S O
N D
0
5 0 km
0.16 E
Sampling strategy
-For each station
N 49.30
1- Reineck Boxcores with
overlying water
49.28
49.26
2- Subsampling with
low-diameter cores
49.24
00
3b- Core slicing (1cm) for
porosity (drying), OC and
ON (CHN autoanalyzer)
measurements in
triplicates
W -0.02
0.02
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0
0
0.14
2000 Flood period
Daily averaged
discharge (m3.s-1)
3a- Direct measurements
of 4-6 oxygen
microprofiles
0
Low
water
period
1000
0
J
F M A M J
J
2003
A S O
N D
0
5 0 km
0.16 E
Oxygen Microprofiling
All measurements were performed
Oxygen microelectrodes
- on board
- in the dark
- Polarographic Clark type
microsensors
- immediatelly after retrieval
- Tip diameter 100µm
Motorized
micromanipulator
Thermometer
Motor
controller
Computer
Picoammeter
Oxygen
microelectrodes
Sediment
core
Strirring system
(bubbling)
Typical oxygen profiles
Oxygen concentration (µM)
0
50
100
150
200
Station SAS04
250
0
September Cruise
Profile n°1
Profile n°2
2000
Muddy sediment
Profile n°3
Profile n°4
Porosity (1st cm): 0.68
4000
Oxygen concentration (µM)
6000
-5000
Station SAS24
September Cruise
Sandy sediment
Porosity
(1st
cm): 0.39
Depth in the sediment (µm)
Depth in the sediment (µm)
-2000
0
50
100
150
200
250
0
5000
10000
15000
20000
25000
Profile n°1
Profile n°2
Profile n°3
Profile n°4
Diffusive oxygen fluxes calculations
Depth in the sediment
(µm)
O xygen concentration (µM)
-400
-200 0
0
200
400
600
800
1000
50
100
150
200
250
Benthic Oxygen Demand (BOD):
C/z
BOD =  . Ds . (C/z) z=0
- Function of temperature and salinity
Station SAS04
- Modified method of Sweerts et al.
(1989):
Depth in the sediment
(µm)
Oxygen concentration (µM)
-2000
50
-1000 0
0
1000
2000
3000
4000
5000
6000 Station
100
150
200
250
C/z
SAS24
Location of the Sediment-Water
interface as a break in the oxygen
concentration gradient
Slope calculation averaged on five
successive data points in the
gradient
SA
0
SA 3
0
SA 4
0
SA 5
0
SA 6
0
SA 7
0
SA 8
0
SA 9
1
SA 0
1
SA 6
1
SA 8
1
SA 9
2
SA 0
2
SA 1
5
SA 1
5
SA 2
5
SA 3
5
SA 5
6
SA 2
6
SA 4
87
SA
SA F
S
SA 04
S
SA 06
S
SA 17
S
SA 18
S
SA 20
S2
4
Benthic Oxygen Demand
(mmol.m-2.d-1)
February
September
Stations
6.9-7.5 °C
19-21.3 °C
Sediment
temperature
25
20
15
10
5
0
Correlation BOD - Porosity
Average BOD (mmol.m -2.d-1)
R2=0.68
18
16
14
12
10
8
6
4
2
0
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Porosity
Sandy Stations
Muddy Stations
Organic Carbon (% Dry Weight)
Correlation Porosity – Organic Carbon
R2=0.92
4
3
2
1
0
0.3
0.4
0.5
0.6
Porosity
0.7
0.8
0.9
Organic Carbon & Benthic Oxygen Demand
Major differences with the Rhone river
Seine river
79 000 km2
780 km
410 m3.s-1
7m
Catchment area
Length
Mean discharge
(Poses)
(Beaucaire)
Tidal range
Rhône river
97 000 km2
810 km
1800 m3.s-1
0.1 m
Rhone river
-General hydrodynamic forcing easily
described
North
-Clear gradient of OC and consequently of
Mediterranean
benthic mineralization processes
Current
Benthic Oxygen Demand
3
43°24’
43°21’
2,5
R1
43°18’
2
1,5
R2
43°15’
1
43°12’
S
43°09’
(mmol.m-2.d-1)
4°39’
4°42’
4°45’
4°48’
4°51’
4°54’
0,5
4°57’
%
Organic
Carbon
25
20
15
10
5
0
S
R2
R1
Seine river
-Complex hydrodynamic features
- Local organic matter accumulation
-Patchwork of Benthic mineralization
processes
N 49.30
49.28
49.26
49.24
00
W -0.02
0.02
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0
0
0
0.14
0
5 0 km
0.16 E
Riverine Discharge
Tidal
Range
Daily averaged discharge (m3.s-1)
12000
10000
Seine River
8000
Rhône River
6000
Tidal
Range
4000
2000
0
94 95 96 97 98 99 00 01 02 03 94 95 96 97 98 99 00 01 02 03
Monthly averaged discharge (m3.s-1)
(1994-2003)
3000
Annual input of SPM
0.4 to 1.1 x106 t.y-1
Annual input of SPM
2000
1.7 to 22.7 x106 t.y-1
1000
0
J F M A M J J A S O N D
Continental shelf
topography
Distance from river mouth (km)
Depth (m)
0
20
0
10
20
30
40
Seine River
40
60
80
100
50
Rhône River
Conclusions
In the Bay of Seine
No general seasonal change of benthic mineralization
High variability at low spatial scale
Efficient dispersion towards west, accumulation in the
south of the southern dyke
Rhône River / Seine River Comparison
Gradual dispersion of organic matter for the Rhône River
Dispersion of organic matter for the Seine River but also
local redistribution and consequently higher impacts of
accumulation areas and coastal installations
Thank you for your attention…