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. NO 3 Chlorophyll Large detritus Organic matter N2N2 NH 4 NO 3 Water column Sediment Phytoplankton NH 4 Mineralization Uptake Nitrification Grazing Mortality Zooplankton Susp. particles Aerobic mineralization Denitrification 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)