Coupled physical-biogeochemical modeling of the Louisiana Dead Zone

1. Coupled physical-biogeochemical modeling of the Louisiana Dead Zone Katja Fennel Dalhousie University katja.fennel@dal.ca Rob Hetland Texas A&M Steve DiMarco Texas A&M

2. 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

3. 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)

4. 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

5. Note differences between: • 1992 (low discharge, low N input), • 1993 (high discharge, high N input) and • 1994 (lower discharge, highest N input ).

7. winter

8. summer

10. 1992: • low discharge • low N input • 1993: • high discharge • high N input • 1994: • lower discharge • highest N input

11. 1992: • low discharge • low N input • 1993: • high discharge • high N input • 1994: • lower discharge • highest N input

13. 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)