CEAC The Abdus SalamInternational Centre for Theoretical Physics

Lionel DENIS* * * * * * * * * * * * *

Benthic-pelagic coupling: a benthic view

* * * * * * * * * * * * *

Benthic-pelagic coupling: a benthic view

 

I – Historical reviewII – Main coupling factors

1 – Organic Matter input in surficial sediments2- Resuspension processes

3 – Nutrient recycling4 – Contaminant sequestration

III – Close to the coast… the history changes

Sources of Organic Matter to the ocean:

Input typeInput type Quantity Quantity (10(101515gtC.ygtC.y-1-1))

PercentPercent

Oceanic Primary productionOceanic Primary production

-Phytoplankton -Phytoplankton

-Macrophytes-Macrophytes

23.123.1

1.71.7

84.484.4

6.26.2

90.690.6

Liquid inputsLiquid inputs

- Rivers- Rivers

- Groundwater- Groundwater

1.01.0

0.080.08

3.653.65

0.30.3

3.953.95

Atmospheric inputsAtmospheric inputs

- Rain- Rain

- Dry particles- Dry particles

1.01.0

0.50.5

3.653.65

1.81.8

5.455.45

TotalTotal 27.427.4 100100 100100

Microphytobenthos, Thermal vents (<0.1%)Microphytobenthos, Thermal vents (<0.1%)

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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

Sediment traps

ST 80 m

ST 900 m

Depth 930 m

AIR

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

Maximal inputs at a depth of 600 m ?

Location : Mediterranean Sea –

Grand Rhône Canyon – Several depths from 80 to 900 m

Several other processes than only 1DV settling contribute to the input of Organic Matter on surficial sediments

(vertical) Settling

Advective transport

Resuspension

* * * * * * * * * * * * *

Benthic-pelagic coupling: a benthic view

 

IIMain 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

PHYTOPLANKTON 6.6

BACTERIA 4-6

SENESCENT PHYTOPLANKTON 7.5

SEDIMENTS (1st cm) 10

SEDIMENTS (10th cm) 40

ZOOPLANKTON 8.5

Redfield ratio:

(CH2O)106(NH3)16(H3PO4)

C/N/P = 106/16/1

Origin C/N ratio

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 Diameter (µm)(µm)

Settling Settling velocity(/day)velocity(/day)

20 µm20 µm 38 m38 m

1 µm1 µm 6 cm6 cm

0.05 µm0.05 µm 0.4 mm0.4 mm

Disequilibrium between production and consumption in surface waters:

When production of surface waters is highly variable in time

=> pulse inputs towards deeper watersSurface production

- Primary production

- Production of higher trophic levels

Surficial sediments

DEGRADATION - MINERALIZATION Strong gradients

THERMOCLINE - HALOCLINE

Physical barrier to vertical transfer

RECYCLING in the euphotic zone

Dystrophic events

Vertical export from surface waters is too high / consumption in surficial sediments

SE

DIM

EN

T-

WA

TE

R

INT

ER

FA

CE

Surface production

Consumption

EquilibriumFood limitation

Equilibrium

Too much Organic Matter=>

Disequilibrium => Bacteria=> Anoxia => Death of

several organisms

OTHER BIOLOGICAL PROCESSES

- Coastal sediments: Filtration activity

Cloern, 1982: Does the benthos control phytoplankton biomass in San Francisco Bay? MEPS 9: 191-

202.

- 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.

* * * * * * * * * * * * *

Benthic-pelagic coupling: a benthic view

 

IIMain forcing factors

1 – Organic Matter input in surficial sediments2- Resuspension processes

Resuspension processes:

Directly linked to current velocity close to the sediment

2 main parameters to calibrate resuspension processes

Critical shear stressAbove this value, particles are resuspended

Erosion rateThe amount of particles resuspended per unit time.

Resuspension processes:

Mainly:

Sediment particles (Inorganic, Organic aggregates, dead organisms)

MicrophytobenthosEither fixed on particles or free but resuspended

MacrophytesThe distal part of macrophytes is regularly cut by waves and movements on rocks

Macrobenthic organismsEither larval stages, or adults (Polychaetes).

Inorganic resuspension Flume experiments

Test section

Fluorimeter Peristaltic pumpTurbidimeter

Sediment

Comparison Inorganic/ Organic resuspension

Thau lagoon Muddy sediments

Velocity increased step by step

Critical erosion velocity:15-20 cm.s-1

Gulf of Fos

Muddy sandGradual increase of

velocityCritical erosion velocity:

16.5 cm.s-1

0102030405060708090

0 20 40 60 80 100 120 140

Temps (minutes)C

once

ntr

atio

n d

e m

atiè

re

en s

usp

ensi

on (

mg.

l-1)

0

5

10

15

20

25

30

35

40

Con

cen

trat

ion

de

pig

men

ts

tota

ux

(µg.

l-1)

5 cm.s-1 10cm.s-1 15cm.s-1 20cm.s-1 25cm.s-1 30cm.s-1 0cm.s-1

Pigments Totaux

M.E.S.}

}

0

5

10

15

20

25

30

35

0 20 40 60 80 100Temps (minutes)

Vit

esse

de

cou

ran

t (c

m.s

-1)

Con

cen

trat

ion

de

Mat

ière

en S

usp

ensi

on (

mg.

l-1)

0,00

0,05

0,10

0,15

0,20

0,25

0,30

0,35

0,40

Con

cen

trat

ion

de

pig

men

ts

tota

ux

(µg.

l-1)

Vitesse du courant

M.E.S.

Pigments totaux

Comparison Inorganic/ Organic resuspension

S.P.M.

SP

M c

onte

nt

(mg/

l)

Pig

men

t co

nte

nt

(µg/

l)

Time (minutes)

Pig

men

t co

nte

nt

(µg/

l)

SP

M c

onte

nt

(mg/

l) a

nd

fr

ee-s

trea

m v

eloc

ity

(cm

/s)

Time (minutes)

Free-stream velocity

SPM

Pigments

Macrophytes Resuspension (storms)

Tot

al le

ngt

h (

cm)

March April May June

v v vExpected length

Measured length

Days

Figure 37: Measured l and Expected length of the macroalgae Laminaria saccharina (Year 2001)

Figure 36: Morphology and growth of Laminaria saccharina (Year 2001)

Growth during year n-1

Growth during year n

Growth zone

* * * * * * * * * * * * *

Benthic-pelagic coupling: a benthic view

 

IIMain forcing factors

1 – Organic Matter input in surficial sediments2- Resuspension processes

3- Nutrient recycling

Organic Particulate Matter

Resuspension

SURFICIAL SEDIMENT

WATER COLUMN

SEDIMENT-WATER INTERFACE

Euphotic layer

Accumulation

Settling

?

Nutrient recycling

NO3-, NH4

+ , PO4

3-, Si(OH)4

CO2

Mineralization

Refractory Organic Matter

* * * * * * * * * * * * *

Benthic-pelagic coupling: a benthic view

 

IIMain forcing factors

1 – Organic Matter input in surficial sediments2- Resuspension processes

3- Nutrient recycling4- Contaminant accumulation

Resuspension

SURFICIAL SEDIMENT

WATER COLUMN

SEDIMENT-WATER INTERFACE

Accumulation

Settling

?

Bio-available Contaminants

Immobilized contaminants

-Bounded

- Non-toxic form

-- Non bio-available (too deep)

Bioturbation

Organic Particulate Matter

+ bounded contaminants

 

IIIClose to the coast…the

history changes…

* * * * * * * * * * * * *

Benthic-pelagic coupling: a benthic view

Example:Example:

Benthic mineralization processes and Benthic mineralization processes and consequences close to the mouth consequences close to the mouth

of two major french rivers: of two major french rivers:

the Seine and the Rhone riversthe Seine and the Rhone rivers

Problematics Problematics

Benthic mineralization in estuariesBenthic 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,...)

Nutrient concentration

Tidal cycles

Coastal hydrodynamics

Littoralconstructions

River mouth embankment

Dredging activities

Coastal topography

Riverine discharge

Coastal eutrophication

Problematics – EstuariesProblematics – Estuaries

Are biogeochemical data a useful tool to identify the main

forcings in an estuarine system?

=> Oxygen microprofiling

-0.02 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16

49.24

49.26

49.28

49.30

W 0.02 E

N

0 0 0 0 0 00 5 km

Spring storms - Summer low water periods

- Large wave action / resuspension- Low river discharge

→ Accumulation of suspended matter in the north and south of the dykes

-0.02 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16

49.24

49.26

49.28

49.30

W 0.02 E

N

0 0 0 0 0 00 5 km

Winter flooding period- High river discharge

- Wave action limited

→ Dispersion of suspended matter towards open sea (W -NW)

Site presentation - Bay of SeineSite presentation - Bay of Seine

Sampling strategySampling strategy

-0.02 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16

49.24

49.26

49.28

49.30

W 0.02 E

N

0 0 0 0 0 00 5 km

-25 Stations

all around the mouth

of the Seine river

- 2 cruises

26-27 February 2003

18-19 September 2003

J F M A M J J A S O N D

2000

1000

0

Flood period

Low water period

Dai

ly a

vera

ged

d

isch

arg

e (m

3 .s-1

)

2003

Sampling strategySampling strategy

-0.02 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16

49.24

49.26

49.28

49.30

W 0.02 E

N

0 0 0 0 0 00 5 km

-For each station

1- Reineck Boxcores with overlying water

2- Subsampling with low-diameter cores

3a- Direct measurements of 4-6 oxygen microprofiles

3b- Core slicing (1cm) for porosity (drying), OC and ON (CHN autoanalyzer)

measurements in triplicates

J F M A M J J A S O N D

2000

1000

0

Flood period

Low water period

Dai

ly a

vera

ged

d

isch

arg

e (m

3 .s-1

)

2003

Oxygen MicroprofilingOxygen Microprofiling

Computer

Motor controller

Picoammeter

Thermometer

Sediment core

Motorized micromanipulator

Oxygen microelectrodes

All measurements were performed

- on board

- in the dark

- immediatelly after retrieval

Oxygen microelectrodes

- Polarographic Clark type microsensors

- Tip diameter 100µm

Strirring system(bubbling)

Typical oxygen profilesTypical oxygen profiles

-2000

0

2000

4000

6000

0 50 100 150 200 250

Oxygen concentration (µM)

Dep

th i

n t

he

sed

imen

t (µ

m)

Profile n°1

Profile n°2

Profile n°3

Profile n°4

-5000

0

5000

10000

15000

20000

25000

0 50 100 150 200 250

Oxygen concentration (µM)D

epth

in t

he

sed

imen

t (µ

m)

Profile n°1

Profile n°2

Profile n°3

Profile n°4

Station SAS04

September Cruise

Muddy sediment

Porosity (1st cm): 0.68

Station SAS24

September Cruise

Sandy sediment

Porosity (1st cm): 0.39

-400

-200

0

200400

600

800

1000

0 50 100 150 200 250

O xygen concentration (µM)

Dep

th in

the

sedi

men

t (µ

m)

Diffusive oxygen fluxes calculationsDiffusive oxygen fluxes calculations

-2000-1000

0100020003000400050006000

0 50 100 150 200 250

O xygen concentration (µM)

Dept

h in t

he se

dimen

t (µ

m)

Station SAS04

Station SAS24

Benthic Oxygen Demand (BOD):

BOD = . Ds . (C/z) z=0

- Modified method of Sweerts et al. (1989):

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

C/z

C/z

- Function of temperature and salinity

0

5

10

15

20

25

SA03SA04

SA05SA06

SA07SA08

SA09SA10

SA16SA18

SA19SA20

SA21SA51

SA52SA53

SA55SA62

SA64SA87

SAF

SAS04

SAS06

SAS17

SAS18

SAS20

SAS24

February

September

Benthic Oxygen DemandBenthic Oxygen Demand

(mmol.m(mmol.m-2-2.d.d-1-1))

Stations

Sediment temperature

6.9-7.5 °C

19-21.3 °C

0

2

4

6

8

10

12

14

16

18

0.3 0.4 0.5 0.6 0.7 0.8 0.9

Porosity

Ave

rage

BO

D (

mm

ol.m

-2.d

-1)

Correlation BOD - PorosityCorrelation BOD - Porosity

R2=0.68

Sandy Stations Muddy Stations

0

1

2

3

4

0.3 0.4 0.5 0.6 0.7 0.8 0.9

Porosity

Org

anic

Car

bon

(% D

ry W

eigh

t)Correlation Porosity – Organic CarbonCorrelation Porosity – Organic Carbon

R2=0.92

Organic Carbon Organic Carbon && Benthic Oxygen Demand Benthic Oxygen Demand

Major differences with the Rhone riverMajor differences with the Rhone river

Seine riverSeine river Rhône riverRhône river

79 000 km79 000 km22 Catchment areaCatchment area 97 000 km97 000 km22

780 km780 km LengthLength 810 km810 km

410 m410 m33.s.s-1-1 Mean dischargeMean discharge(Poses) (Beaucaire)(Poses) (Beaucaire)

1800 m1800 m33.s.s-1-1

7 m7 m Tidal rangeTidal range 0.1 m0.1 m

3

2,5

2

1,5

1

0,5

%

43°24’

43°21’

43°18’

43°15’

43°12’

43°09’4°39’ 4°42’ 4°45’ 4°48’ 4°51’ 4°54’ 4°57’

3

2,5

2

1,5

1

0,5

%

43°24’

43°21’

43°18’

43°15’

43°12’

43°09’4°39’ 4°42’ 4°45’ 4°48’ 4°51’ 4°54’ 4°57’

North Mediterranean

Current

R1

R2

S 05

10152025

S R2 R1

Benthic Oxygen Demand (mmol.m-2.d-1)

Rhone riverRhone river

Organic Carbon

-General hydrodynamic forcing easily described

-Clear gradient of OC and consequently of benthic mineralization processes

Seine riverSeine river

-0.02 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16

49.24

49.26

49.28

49.30

W 0.02 E

N

0 0 0 0 0 00 5 km

-Complex hydrodynamic features

- Local organic matter accumulation

-Patchwork of Benthic mineralization processes

J F M A M J J A S O N D

3000

2000

1000

0

Monthly averaged discharge (m3.s-1) (1994-2003)

Daily averaged discharge (m3.s-1)

Seine River Rhône River

12000

10000

8000

6000

4000

2000

0 94 95 96 97 98 99 00 01 02 03 94 95 96 97 98 99 00 01 02 03

Riverine DischargeRiverine Discharge

Annual input of SPM

0.4 to 1.1 x106 t.y-1

Annual input of SPM

1.7 to 22.7 x106 t.y-1

Tidal Range

Tidal Range

Continental shelf Continental shelf topographytopography

Seine River

Rhône River

0 10 20 30 40 50

Distance from river mouth (km)

0

20

40

60

80

100

Dep

th (

m)

ConclusionsConclusions

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

Rhône River / Seine River Comparison

No general seasonal change of benthic mineralization

High variability at low spatial scale

In the Bay of Seine

Efficient dispersion towards west, accumulation in the south of the southern dyke

Thank you for your attention…Thank you for your attention…