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CEAC

CEAC. The Abdus Salam International Centre for Theoretical Physics. Lionel DENIS * * * * * * * * * * * * * Benthic-pelagic coupling: a benthic view. * * * * * * * * * * * * * Benthic-pelagic coupling: a benthic view. I – Historical review II – Main coupling factors

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  1. CEAC The Abdus Salam International Centre for Theoretical Physics Lionel DENIS * * * * * * * * * * * * * Benthic-pelagic coupling: a benthic view

  2. * * * * * * * * * * * * * 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

  3. Sources of Organic Matter to the ocean: Microphytobenthos, Thermal vents (<0.1%)

  4. * * * * * * * * * * * * * Benthic-pelagic coupling: a benthic view Historical review

  5. No coupling between compartments 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)

  6. 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)

  7. 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, …)

  8. Hargrave’s model (1973) Kiel Bight : All primary production is mineralized in surficial sediments

  9. Further details with developping technology Sediment traps AIR ST 80 m ST 900 m Depth 930 m

  10. Large seasonal variability due to fluctuations in the primary production in surface waters Location : Mediterranean Sea – Grand Rhône Canyon - Single Depth 80 m TEMPORAL VARIABILITY

  11. Maximal inputs at a depth of 600 m ? Location : Mediterranean Sea – Grand Rhône Canyon – Several depths from 80 to 900 m

  12. Several other processes than only 1DV settling contribute to the input of Organic Matter on surficial sediments (vertical) Settling Advective transport Resuspension

  13. * * * * * * * * * * * * * Benthic-pelagic coupling: a benthic view II Main forcing factors 1 – Organic Matter input in surficial sediments

  14. Organic matter input depends on 4 main parameters Depth Decay rate of Organic Matter Settling velocity Disequilibrium between production and consumption in surface waters

  15. Depth: - Shallow sediments: HIGH COUPLING - Deep- sediments: LOW COUPLING

  16. Decay rate of organic matter: Depends on the quality of Organic Matter. Modified during the settling Origin C/N ratio Redfield ratio: (CH2O)106(NH3)16(H3PO4) C/N/P = 106/16/1 BACTERIA 4-6 PHYTOPLANKTON 6.6 SENESCENT PHYTOPLANKTON 7.5 ZOOPLANKTON 8.5 SEDIMENTS (1st cm) 10 SEDIMENTS (10th cm) 40

  17. 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)

  18. 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 RECYCLING in the euphotic zone DEGRADATION - MINERALIZATION Strong gradients Physical barrier to vertical transfer THERMOCLINE - HALOCLINE Surficial sediments

  19. Dystrophic events Vertical export from surface waters is too high / consumption in surficial sediments Surface production SEDIMENT-WATER INTERFACE Consumption Too much Organic Matter=> Disequilibrium => Bacteria=> Anoxia => Death of several organisms Food limitation Equilibrium Equilibrium

  20. 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.

  21. * * * * * * * * * * * * * Benthic-pelagic coupling: a benthic view II Main forcing factors 1 – Organic Matter input in surficial sediments 2- Resuspension processes

  22. 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.

  23. 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).

  24. Inorganic resuspension Flume experiments

  25. Peristaltic pump Fluorimeter Turbidimeter Comparison Inorganic/ Organic resuspension Sediment Test section

  26. Comparison Inorganic/ Organic resuspension Thau lagoon Muddy sediments Velocity increased step by step Critical erosion velocity: 15-20 cm.s-1 } S.P.M. SPM content (mg/l) Pigment content (µg/l) } Time (minutes) Gulf of Fos Muddy sand Gradual increase of velocity Critical erosion velocity: 16.5 cm.s-1 SPM content (mg/l) and free-stream velocity (cm/s) Pigment content (µg/l) Free-stream velocity SPM Pigments Time (minutes)

  27. Macrophytes Resuspension (storms) March April May June v v v Growth during year n-1 Expected length Total length (cm) Growth during year n Measured length Growth zone Figure 36: Morphology and growth of Laminaria saccharina (Year 2001) Days Figure 37: Measured l and Expected length of the macroalgae Laminaria saccharina (Year 2001)

  28. * * * * * * * * * * * * * Benthic-pelagic coupling: a benthic view II Main forcing factors 1 – Organic Matter input in surficial sediments 2- Resuspension processes 3- Nutrient recycling

  29. CO2 Nutrient recycling NO3-, NH4+ , PO43-, Si(OH)4 Mineralization Refractory Organic Matter Euphotic layer Organic Particulate Matter WATER COLUMN Settling Resuspension SEDIMENT-WATER INTERFACE Accumulation ? SURFICIAL SEDIMENT

  30. * * * * * * * * * * * * * 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

  31. Organic Particulate Matter + bounded contaminants WATER COLUMN Settling Bio-available Contaminants Resuspension SEDIMENT-WATER INTERFACE Accumulation ? • Immobilized contaminants • Bounded • Non-toxic form • - Non bio-available (too deep) Bioturbation SURFICIAL SEDIMENT

  32. * * * * * * * * * * * * * Benthic-pelagic coupling: a benthic view III Close to the coast…the history changes…

  33. Example: Benthicmineralizationprocesses and consequencesclose to the mouth of two major french rivers: the Seine and the Rhonerivers

  34. 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,...)

  35. Problematics – Estuaries Are biogeochemical data a useful tool to identify the main forcings in an estuarine system? => Oxygen microprofiling

  36. Site presentation - Bay of Seine • Winter flooding period • High river discharge • Wave action limited • → Dispersion of suspended matter towards open sea (W -NW) • 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

  37. Sampling strategy • 25 Stations • all around the mouth • of the Seine river • - 2 cruises • 26-27 February 2003 • 18-19 September 2003 Flood period 2000 1000 0 Low water period Daily averaged discharge (m3.s-1) J F M A M J J A S O N D 2003

  38. Sampling strategy • 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 Flood period 2000 1000 0 Low water period Daily averaged discharge (m3.s-1) J F M A M J J A S O N D 2003

  39. Oxygen Microprofiling • All measurements were performed • on board • in the dark • immediatelly after retrieval • Oxygen microelectrodes • - Polarographic Clark type microsensors • Tip diameter 100µm Motorized micromanipulator Thermometer Motor controller Computer Picoammeter Oxygen microelectrodes Sediment core Strirring system (bubbling)

  40. Typical oxygen profiles Station SAS04 September Cruise Muddy sediment Porosity (1st cm): 0.68 Station SAS24 September Cruise Sandy sediment Porosity (1st cm): 0.39

  41. C/z -Function of temperature and salinity -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 Diffusive oxygen fluxes calculations Benthic Oxygen Demand (BOD): BOD =  . Ds . (C/z) z=0 Station SAS04 Station SAS24

  42. 6.9-7.5 °C Sediment temperature 19-21.3 °C Benthic Oxygen Demand (mmol.m-2.d-1) Stations

  43. Correlation BOD - Porosity R2=0.68 Sandy Stations Muddy Stations

  44. Correlation Porosity – Organic Carbon R2=0.92

  45. Organic Carbon & Benthic Oxygen Demand

  46. Major differenceswith the Rhone river

  47. 4 4 3 3 ° ° 2 2 4 4 ’ ’ 3 3 North Mediterranean Current 4 4 3 3 ° ° 2 2 1 1 ’ ’ 2 2 , , 5 5 2 2 4 4 3 3 ° ° 1 1 8 8 ’ ’ Benthic Oxygen Demand (mmol.m-2.d-1) 1 1 , , 5 5 4 4 3 3 ° ° 1 1 5 5 ’ ’ R1 1 1 4 4 3 3 ° ° 1 1 2 2 ’ ’ R2 0 0 , , 5 5 4 4 3 3 ° ° 0 0 9 9 ’ ’ S % % 4 4 ° ° 3 3 9 9 ’ ’ 4 4 ° ° 4 4 2 2 ’ ’ 4 4 ° ° 4 4 5 5 ’ ’ 4 4 ° ° 4 4 8 8 ’ ’ 4 4 ° ° 5 5 1 1 ’ ’ 4 4 ° ° 5 5 4 4 ’ ’ 4 4 ° ° 5 5 7 7 ’ ’ Rhone river • General hydrodynamic forcing easily described • -Clear gradient of OC and consequently of benthic mineralization processes Organic Carbon

  48. Seine river • Complex hydrodynamic features • Local organic matter accumulation • Patchwork of Benthic mineralization processes

  49. Tidal Range Tidal Range Riverine Discharge Daily averaged discharge (m3.s-1) 12000 10000 8000 6000 4000 2000 0 Seine River Rhône River 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 2000 1000 0 Annual input of SPM 1.7 to 22.7 x106 t.y-1 Annual input of SPM 0.4 to 1.1 x106 t.y-1 J F M A M J J A S O N D

  50. Distance from river mouth (km) 0 10 20 30 40 50 0 20 40 60 80 100 Seine River Depth (m) Rhône River Continental shelf topography

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