Carbon-Based Net Primary Production and Phytoplankton

Growth Rates from Ocean Color Measurements

Toby K. Westberry1, Mike J. Behrenfeld1  Emmanuel Boss2, David A. Siegel3

1Department of Botany, Oregon State University2School of Marine Sciences, University of Maine

3Institute for Computational Earth System Science, UCSB

Modelling NPP

NPP ~ [biomass] x physiologic rate

NPP ~ [Chl] x Pbopt

NPP ~ [C] x

Scattering (cp or bbp)

Ratio of Chl to scattering (Chl:C)

General

Chl-based

C-based

C-based approachCh

l:C (m

g m

g-1) Chl:C (m

g mg

-1)

0 1 2 3

0.005

0.020

0.035

0.050

0.065

0.080

0 1 2 3

0.005

0.007

0.009

0.011

0.013

• Scattering coefficients covary with particle abundance (Stramski & Kiefer, 1991; Bishop, 1999; Babin et al., 2003)

• Scattering coefficients covary with phytoplankton carbon (Behrenfeld & Boss, 2003; Behrenfeld et al., 2005)

• Chlorophyll variations independent of C are an index of changing cellular pigmentation

Laboratory Satellite

Ig (Ein m-2 h-

1)

CBPM In a nutshell• Invert ocean color data to estimate [Chl a] & bbp(443)

(Garver & Siegel, 1997; Maritorena et al., 2001)

• Relate bbp(443) to carbon biomass (mg C m-3)(Behrenfeld et al., 2005)

• Use Chl:C to infer physiology (photoacclimation & nutrient stress)

• Propagate properties through water column

• Estimate phytoplankton growth rate () and NPP given: PAR, Chl, K490, bbp(443), Zeu, MLD

Carbon-Based Production Model (CBPM)

Depth-resolved CBPM

Nutrient-limited &/or light-limited + photoacc.

Uniform (e.g., [Chl/C]sat)

Light-limited + photoacc.

* Iterative such that values at z=zi+1 depend on values at z=zi *

z=zNO3

z=MLD

z=0PAR(z)

Light

-lim

itatio

n In

dex

Ig (Ein m-2 h-1)

~(1-e-3PAR(z))Ik ~0.6

CBPM details (2)1. Let surface values of Chl:C indicate level of nutrient-stress

-nutrient stress falls off as e-z (z=distance from nitracline)

2. Let cells photoacclimate through the water column - Iteratively calculate spectral attenuation

3. Account for light limitation

Ig (Ein m-2 h-1)

Chl :

C

(divisions d-1)

CBPM details (3)

- SeaWiFS: nLw(), PAR, Kd(490)- GSM01: Chl a, bbp(443)- FNMOC: MLD- WOA 2001: ZNO3

- Chl, C, & Chl:C- - NPP

INPUT (surface) OUTPUT ((z))

Run with 1° x1° monthly mean climatologies (1999-2004)

Dept

h (m

)De

pth

(m)

Example profiles

Eastern Pacific (20°N, -110°E, Jan)Eq. upwelling (0°N, -130°E, Aug)

NPP patterns (Jun-Aug)This work

∫NPP (mg C m-2 d-1)VGPM (Chl-based model)

∫NPP (mg C m-2 d-1)

• large spatial & temporal differences in carbon-based NPP from Chl-based results (e.g., > ±50%)

• Chl-based model interprets high Chl areas as high NPP

• differences due to photo- acclimation and nutrient-stress related changes in Chl : C

Seasonal NPP patterns (N. Atl.)

Western N. Atl

Eastern N. Atl

CBPMVGPM

Annual NPP∫NPP (Pg C) VGPM This modelAnnual 45 52Gyres 8 (18%) 13 (26%)High latitudes 15 (34%) 12 (23%)Subtropics? 18 (39%) 25 (48%)Southern Ocean(<-50°S)

2 (4%) 3 (5%)

• Although total NPP doesn’t change much (~15%), where and when it occurs does

Example NPP profiles (HOT)

- Uniform mixed layer (step function) v. in situ incubations- Discrepancies due to satellite estimates, NOT concept