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Courtney K. Harris Virginia Institute of Marine Sciences

Northern Gulf of Mexico: Linking Sediment and Biological Processes within the Regional Ocean Modeling System (ROMS). Courtney K. Harris Virginia Institute of Marine Sciences

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Courtney K. Harris Virginia Institute of Marine Sciences

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  1. Northern Gulf of Mexico: Linking Sediment and Biological Processes within the Regional Ocean Modeling System (ROMS) Courtney K. Harris Virginia Institute of Marine Sciences In collaboration with: Kevin Xu (Coastal Carolina University), Katja Fennel (Dalhousie University), Rob Hetland and James Kaihatu (Texas A&M University) Could not find logos from CCU and Dalhousie.

  2. New Orleans Mississippi Birdfoot Delta Atchafalaya Bay

  3. Dead zone: seasonal occurrence of hypoxia Figure 4 Distribution of frequency of occurrence of mid-summer bottom-water hypoxia over the 60- to 80-station grid from 1985–2001 (updated from Rabalais et al. 1999, Rabalais& Turner 2001b). Star indicates general location of stations C6A and C6B; transect C identified. From Rabalais, Turner, and Wiseman, 2002.

  4. Mechanism Contributing to Hypoxia From EPA Web-site: http://www.epa.gov/msbasin/hypoxia101.htm

  5. ROMS Physical, Biogeochemical, and Sediment Model Physical model: ROMS v3.0 /3.1 Resolution: 3-5 km horizontal, 20 vertical layers Forcing: 3-hourly winds; climatological surface heat fluxes; waves from SWAN run by Kaihatu. River inputs: daily measurements of FW input by U.S. Army Corps of Engineers; sediment input based on USGS rating curve. Model reproduces two dominant modes of circulation (summer and non-summer), weather-band variability and surface salinity fields (Hetland & DiMarco, J. Mar. Syst., 2007) Biogeochemistry: “FASHAM”, following Fennel et al. 2006. Sediment: Community Sediment Transport Modeling System (CSTMS), (Warner et al., 2008) Figure from Hetland and DiMarco, 2007

  6. ROMS Physical, Biogeochemical, and Sediment Model Physical model: ROMS v3.0 /3.1 Resolution: 3-5 km horizontal, 20 vertical layers Forcing: 3-hourly winds; climatological surface heat and freshwater fluxes; waves from SWAN run by Kaihatu. River inputs: daily measurements of FW input by U.S. Army Corps of Engineers; sediment input based on USGS rating curve. Model reproduces two dominant modes of circulation (summer and non-summer), weather-band variability and surface salinity fields (Hetland & DiMarco, J. Mar. Syst., 2007). Biogeochemistry: “FASHAM”, following Fennel et al. 2006. Sediment: Community Sediment Transport Modeling System (CSTMS), (Warner et al., 2008). Figure from Hetland and DiMarco, 2007

  7. Nitrification Water column Mineralization NH4 NO3 Uptake Phytoplankton Grazing Chlorophyll Zooplankton Mortality Large detritus Susp. particles Nitrification N2 NH4 NO3 Denitrification Aerobic mineralization Organic matter Sediment Biological model: nitrogen cycling in water column and simplified sedimentary processes; oxygen coupled (Fennel et al., GBC, 2006) River inputs: USGS nutrients fluxes for Mississippi and Atchafalaya Current limitations: no explicit sediment (instantaneous remineralization), no sediment transport, no P-cycle

  8. Coupled Physical – Biological Model Model ran to represent 1990 – 1999. Provided realistic estimates of hypoxic area. Next step: improve treatment of biogeochemical constituents on the sediment bed. Figure: Frequency of occurrence of hypoxia in the realistic coupled physical/biological simulation with NO3 and PON inputs for June (top), July (middle), and August (middle). Model statistics based on 10-year simulation. From Katja Fennel, Dalhousie University. June July August

  9. Coupled Physical – Biological Model Model ran to represent 1990 – 1999. Provided realistic estimates of hypoxic area. Next step: improve treatment of biogeochemical constituents on the sediment bed. Figure: Frequency of occurrence of hypoxia in the realistic coupled physical/biological simulation with NO3 and PON inputs for June (top), July (middle), and August (middle). Model statistics based on 10-year simulation. From Katja Fennel, Dalhousie University. June Rabalais, Turner, and Wiseman, 2002. July August

  10. ROMS Physical, Biogeochemical, and Sediment Model Physical model: ROMS v3.0 /3.1 Resolution: 3-5 km horizontal, 20 vertical layers Forcing: 3-hourly winds; climatological surface heat and freshwater fluxes; waves from SWAN run by Kaihatu. River inputs: daily measurements of FW input by U.S. Army Corps of Engineers; sediment input based on USGS rating curve. Model reproduces two dominant modes of circulation (summer and non-summer), weather-band variability and surface salinity fields (Hetland & DiMarco, J. Mar. Syst., 2007). Biogeochemistry: “FASHAM”, following Fennel et al. 2007. Sediment: Community Sediment Transport Modeling System (CSTMS), (Warner et al., 2008). Figure from Hetland and DiMarco, 2007

  11. Noncohesive sediment model. • Multiple grain sizes. • Bed layers account for armoring. • Two sediment sources: • - Rivers (Atchafalaya and Mississippi). • - Seabed erosion. Sediment Model: CSTMS (ROMS v3.0) Figures from Warner et al. 2008. • Sediment routine calculates: • Vertical settling. • Keeps account of sediment bed layers. • Exchange between seabed and water column (erosion and deposition).

  12. Sediment Properties Trinity Shoal Muddy Ship Shoal 20m 50m This sediment model presented at Ocean Sciences, March 2008, by Kevin Xu. 100m 300m Sandy US Seabed Data from Jeff Williams (USGS) and Chris Jenkins (INSTAAR)

  13. ‘Storm of Century’ LATEX tetrapod observation Wind Speed (m/s) and SWAN Wave Height at Tetrapod (m) Wind, Wave, Water and Sediment Discharge in 1993 (USGS discharge data from C. Demas and B. Meade)

  14. Model estimates for 1993 storm. This sediment model presented at Ocean Sciences, March 2008, by Kevin Xu.

  15. Estimates for 1993 On average, currents flow westward along coast. Wave resuspension significantly impacts sediment resuspension. Fluvial material from the Atchafalaya and Mississippi mix on the Louisiana shelf.

  16. (1) Storm of the Century (March 12 – 16, 1993) Salinity Mean Current Current-wave dominated Low-medium water discharge Strong winds (2) (1) Wave Height (3) Near-bed Suspended Sediment Three Phases

  17. Storm of the Century (3) (2) Salinity, Mean Current, Wind Wave Height Near-bed Current Suspended Sediment Onshore Transport Along-Shore Transport

  18. Storm of the Century Erosion/Deposition relative to river sediment on the sea bed on Mar/12/1993 Peak of Storm Deposition (log10 kg/m2) Post Storm Erosion • On shore sediment transport during cold fronts (Kineke et al., 2006, CSR) • Facilitate on-shore accumulation • May set the stage for summertime hypoxia.

  19. LATEX (May-June, 1993) River-dominated High water discharge Weak winds Stratified water column Horizontal sediment advection LATEX tetrapod (see Wright, et al. 1997) B B’ 30 psu isohaline B B’

  20. 2. Wave Orbital Speed Comparison of Model to LATEX Data 1. Near bottom flows 3. Sediment Concentration This Model (Wright et al, 1997, MG)

  21. Sediment Model: Conclusions • Waves increased sediment concentration and dispersal, and facilitated on-shore accumulation offshore of Atchafalaya Bay. • During the ‘Storm of the Century’, currents and waves dominated transport. Water column was well mixed and sediment was eroded from middle shelf and deposited on the inner shelf. Net shoreward flux of sediment. • During ‘Calm LATEX’conditions, river plumes dominated transport in stratified water. Currents and waves occasionally resuspended sediment. Model showed reasonable agreement to tetrapod data. • Both Mississippi and Atchafalaya sediment contributed to turbidity south of the Atchafalaya Bay.

  22. Sediment-Biology CouplingOngoing work At least 13 tracers Temperature Salinity Large floc sed. Small floc sed. Sand Nitrate Ammonium Chlorophyll Phytoplankton Zooplankton Large Detritus Small Detritus Oxygen (Warner et al., 2008) (Fennel et al., 2006, GBC)

  23. Partition & Aggregation Sea Water Settling Resuspension Sea Bed Diagenesis Sediment-Biology Coupling Bio model has Large Detritus Small Detritus Sediment model has Large Flocs Small Flocs Sediment bed needs Organic Matter Organic Matter Flocs

  24. Partition & Aggregation Sea Water Settling Resuspension Sea Bed Diagenesis Sediment-Biology Coupling Bio model has Oxygen, Ammonium, Nitrate Sediment bed needs Oxygen, Ammonium, Nitrate, “ODU” Organic Matter Flocs

  25. Coupling of Biogeochemistry and Sediment: Particulate Organic Matter • Fennel et al. (2006) specifies particulate organic matter using • Large detritus; small detritus (stored in t[i,j,k,itracer]). • These interact with other constituents. • When they settle to the bed, they are (now) instantly remineralized. • Sediment model uses • Large flocs; small flocs. • These can be resuspended (t[i,j,k,ised]); settle to the bed (bed_frac[i,j,kbed,ised], bed_mass[i,j,kbed,ised]), and re-erode. • These classes do not interact. • To couple these, we defined “bed_tracer[i,j,kbed,isb]” (mmol/km2 of bed) • This stores the deposited particulate organic matter, which can be resuspended. • The index isb identifies the constituent (large detritus, small detritus). • Particulate “bed_tracer” constituents also must be linked to a sediment class. • Each “bed_tracer” will be linked to 1 or more water column tracer(s).

  26. SEDBIO TOY One dimensional model that has particulate organic matter. Large detritus linked to large floc. Small detritus linked to small floc. When detritus settles: it adds to bed_tracer(i,j,kbed=1,itracer=“organic matter”). When organic matter erodes: it adds to t(i,j,k=1,itracer=detritus). At present, neglect any interactions between detrital classes on the bed.

  27. SEDBIO Toy Test: Conservation • One-dimensional sediment & water column. • Suspended detritus settles and adds to bed mass. • Winds increase; detritus resuspended. • Winds decrease and material redeposits. • Organic matter is conserved.

  28. Early Diagenesis Model Next: Following model developed by Soetart et al. (1996). • Add other bed_tracers (oxygen, nitrate, ammonium, and “ODU”). • These will interact with water column tracers through diffusion, burial, and erosion. • Add reaction terms to bed_tracers. • Model parameters needed for the bed_tracers (like reactivity). • Bed diffusivity will need to be added. • The goal here is to improve the water column calculations.

  29. Challenges and Issues • Run-time: computational limits are always present, this will need to track at least 15 tracers. • Need to work within cohesive bed model. • Will likely stress some parts of the sediment model that have not been tested or developed (Porosity? Biodiffusion?) • Inherent mis-match in spatial scales / temporal scales between sediment dynamics and diagenesis(?). • Will we have data to set model parameters, and for validation?

  30. Summary • The MCH (Mechanisms Controlling Hypoxia) modeling group has produced coupled physical – sediment; and physical – biological models; both of which seem to be working well. • At present, particulate organic matter in the sediment routine has been linked to water column detritus. • Efforts to link other water column tracers to the seabed will face some challenges but the goal is to improve on the current simplistic assumptions used in the biology model.

  31. THE END

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