A Simple Approach to Modeling Iron Limitation
of Primary Production in the Gulf of Alaska
J. Fiechter1, A. Moore1, C. Edwards1,
K. Bruland1, E. Di Lorenzo2, T. Powell3,
C. Lewis4, E. Curchitser5, and K. Hedstrom6
1
Dept. of Ocean Sciences, University of California, Santa Cruz
2 School of Earth and Atmos. Sciences, Georgia Institute of Technology
3 Dept. of Integrative Biology, University of California, Berkeley
4 NATO Undersea Research Center, La Spezia
5 Institute of Marine and Coastal Sciences, Rutgers University
6 Arctic Region Supercomputing Center, Fairbanks
ROMS Workshop, Los Angeles, 2 October 2007
Outline
• Ocean circulation model: CGOA domain
• Ecosystem model: NPZD + iron limitation
• Results
1) monthly “climatology” (1995-2004)
2) Intra-annual variability (2001)
3) Interannual variability (1995-2004)
• Summary
Coastal Gulf of Alaska Ocean Circulation Model
• ROMS: ~ 10 km horizontal resolution, 42 sigma levels
• One-way offline nesting with North East Pacific ROMS
• Monthly mean atmospheric and open boundary forcing
• 10-year simulation (1995 through 2004)
NPZD Model with Iron Limitation
• NPZD Model (Powell et al. formulation in ROMS)
Nitrate-limited phytop. growth rate: U N*  Vm
N
e ( W z  P P )
N  kN
• Iron Limitation Model
Dissolved (available) Iron:
Phytop.-associated Iron:
Iron uptake: I Fe 
  GZ

Fd

 Fp  RFe 
 pm   U N   I Fe
t

  P

Fp
GZ


 Fp U N 
 pm   I Fe
t
P


R0-R PC : N 
t Fe
Optimal Fe:C:
R0  bFda
Realized Fe:C: R 
R2
*
U
Iron-limited phytop. growth: U N  2
N
2
R  k Fe
Fp
PC : N 
Nitrate and Dissolved Iron Initial Conditions
Nitrate Climatology
Dissolved Iron
WOA 2001, Monthly
1° x 1° Spatial Resolution
Annual “Climatology”
VERTEX (Martin et al., 1989)
10 Year (1995–2004) Simulation, Monthly Averages
RESULTS: PART I
MONTHLY “CLIMATOLOGY” (1995-2004)
SURFACE MAPS
Monthly “Climatology” (1995-2004)
Simulated and Observed Chlorophyll, Spring Bloom
MODEL
OBSERVATIONS (SEAWIFS)
Monthly “Climatology” (1995-2004)
Simulated and Observed Chlorophyll, Summer
MODEL
OBSERVATIONS (SEAWIFS)
Monthly “Climatology” (1995-2004)
Nitrate and Iron Limitation on Phytoplankton Growth
Monthly “Climatology” (1995-2004)
SSH and Chlorophyll Variability, Spring Bloom
SURFACE CHLOROPHYLL
SEA SURFACE HEIGHT
RESULTS: PART II
INTRA-ANNUAL VARIABILITY (2001)
GAK STATIONS (CROSS-SHELF)
Kenai
Peninsula
IS
MS
OS
GAK Intra-annual Variability (2001): Chlorophyll Profiles
GAK Intra-annual Variability (2001): Nitrate Profiles
RESULTS: PART III
INTERANNUAL VARIABILITY (1995-2004)
GAK LINE (CROSS-SHELF)
Kenai
Peninsula
IS
MS
OS
GAK Line Interannual Variability (1995-2004)
Simulated and Observed Sea Surface Height
MODEL
AVISO
GAK Line Interannual Variability (1995-2004)
Simulated and Observed Surface Nitrate
MODEL
GLOBEC
GAK Line Interannual Variability (1995-2004)
Simulated and Observed Surface Chlorophyll
MODEL
SEAWIFS
Summary
• Simple approach to iron limitation on phytoplankton growth
 Nitrate budget for N, P, Z, D
 Iron budget for dissolved and P-associated Fe
 Phytoplankton growth limited by realized Fe:C ratio
• Reproduces seasonal and cross-shelf variability




Primary production dominated by spring bloom (APR-JUN)
Inner- and mid-shelf dominated by seasonal variability
Outer-shelf dominated by mesoscale variability
Outstanding issues: PAR, Fe cross-shelf gradient, tides
• Future work
 14-component ecosystem model (NEMURO + Fe limitation)
 Adjoint sensitivity studies with NPZD + Fe limitation
CGOA ROMS + NEMURO w/ IRON LIMITATION:
MAY “CLIMATOLOGY” (1995-2004)
PHYTOPLANKTON AND ZOOPLANKTON COMMUNITY STRUCTURE