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)