coupled physical-biogeochemical modeling of the louisiana dead zone
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Coupled physical-biogeochemical modeling
of the Louisiana Dead Zone
Katja Fennel Dalhousie Universitykatja.fennel@dal.ca
Rob Hetland Texas A&MSteve DiMarco Texas A&M
Background• Preliminary results from ongoing NOAA project (lead:
DiMarco; modelers: Hetland, Harris, Xu & Fennel); Focus on understanding interplay of physical, biological and geochemical processes that control hypoxia on the TX-LA shelf
• Goals include: realistic physical-biogeochemical model for assessment of factors such as– physical environment/forcing (e.g. stratification, wind forcing)– benthic-pelagic coupling and sediment transport on hypoxic extent AND simulation of various scenarios
• Strengthen predictive capabilities through quantitative understanding of mechanisms
• Physical model: ROMS v3.0• Resolution: 3-5 km horiz., 20 vertical layers• Forcing: 3-hourly winds; climatological surface heat and freshwater fluxes• River inputs: daily measurements of FW input by U.S. Army Corps of
Engineers• Model reproduces the two dominant modes of circulation (summer and
non-summer), weather-band variability and surface salinity fields (Hetland & DiMarco, J. Mar. Syst., 2007)
NO3
Chlorophyll
Largedetritus
Organic matter
N2 NH4 NO3
Water column
SedimentSediment
Phytoplankton
NH4Mineralization
Uptake
Nitrification
Nitrification
Grazing
MortalityZooplankton
Susp.particles
Aerobic mineralizationAerobic mineralizationDenitrificationDenitrification
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
Note differences between:• 1992 (low discharge, low N input), • 1993 (high discharge, high N input) and • 1994 (lower discharge, highest N input ).
winter
summer
1992: • low discharge• low N input
1993: • high discharge• high N input
1994: • lower discharge• highest N input
1992: • low discharge• low N input
1993: • high discharge• high N input
1994: • lower discharge• highest N input
Summary
• Coupled physical-biogeochemical model reproduces many observed features– Physical variability– Surface chlorophyll distributions– Hypoxic area (size and spatial distribution)
• Interannual variability is determined by the interplay of microbial and physical process
• Important next step: inclusion of more realistic sediment (diagenesis, resuspension and transport)
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