On the small range of annually averaged net community production rates in the open ocean

Michael Bender and Bror Jonsson

With support from:

Pedro Monteiro and Warren Joubert (CSIR, Stellenbosch)/Agulhas programBronte Tilbrook (CSIRO, Hobart)/Astrolabe+Aurora Australis programsMatt Reuer, Colorado College (many ship of opportunity cruises)Jan Kaiser (UEA)/Atlantic Meridional Transect programJan Kaiser and Rachel Stanley (WHOI) (Ka’Imimoana program; Thompson cruise)Nicolas Cassar, SAZ/SENSE and moreMany individual cruises and cruise participants

Distribution of carbon export: BGCCSM model (Moore, Doney…)Units: moles m-2 yr-1

• Global sinking POC flux = 6 Gt C/yr at 111 m ~ 10 Gt C/yr at 1% light levelOcean measurements indicate less variability

Main points

• Wherever measured adequately, NCP and carbon export are ~ 1-5 moles C m-2 year-1

– “Measured adequately” means extensive repeat sampling or survey over a very broad area

– The South Pacific subtropical gyre may be exceptionally low• Some speculations on the small range of variability

Background: biological and chemical cycles in the world’s oceans

Euphotic zone

CO2+H2O CH2O + O2

Carbon export

Mixed layerNCP = photosynthesis-respiration

Corg is not stored in euphotic zone; NCP ~ export

O2 flux to the atmosphere = O2supersat * gas transfer velocity

Correct for physical supersaturation based on Ar supersat

Sea-air O2 flux = carbon NCP and export

Other sources of NCP and carbon export for this work

• 15NO3- assimilation

• Seasonal drawdown of DIC or nutrients• Seasonal balance of d13C of CO2 in the upper ocean

Caveats and limitations

• In O2/Ar method, O2 fluxes into the oceans assumed due to upwelling• We need to make simple assumptions about seasonality of NCP• We do not distinguish between mixed layer and euphotic zone production• All sources of NCP and carbon export have substantial uncertainties

Net community production in the Southern Ocean• Carbon fluxes from Reuer et al. (2007) (O2/Ar)

– Subantarctic zone ~ 4 moles C m-2 year-1

– Polar Frontal Zone ~ 2 moles C m-2 year-1

– Antarctic Zone ~2 moles C m-2 yr-1

Net community production in the equatorial Pacific

• Eastern tropical Pacific (5˚S-5˚N, 90˚W – 180) ~ 5 moles C m-2 yr-1 (Aufdenkampe et al., 2001; Dugdale et al., 2011: 15NO3

- assimilation)• Western tropical Pacific (3˚N-3˚S, 170˚W – 145˚E) ~ 1.5 moles C m-2 yr-1 (Stanley et

al., 2010: O2/Ar)

Subpolar North Atlantic

• Start with 13 µmol/kg NO3- in the top 100 m. Maximum export = 9 moles m-2

• If nutrient drawdown extends to 50 m depth, maximum export = 5 moles m-2

• During the bloom, NCP ~ 1.1 moles m-2 in 3 weeks

• Our guess – annually averaged NCP ~ 5 moles C m-2 yr-1

• Quay estimate ~ 3

[NO3-] vs. time: North Atlantic Bloom study, Alkire et al. 2012

North Atlantic subtropical gyre at Bermuda• Bermuda: Annual NCP ~ 3 moles m-2 yr-1

– Jenkins, 1988, Stanley et al., 2012…: NO3- and 3He balance

– Knapp et al., 2005: 15N flux of N2 fixation– Bates and BATS project: Annual dissolved inorganic carbon drawdown in/below ML

Dissolved inorganic carbon, µmol/kg

Steinberg et al., 2004

Atlantic Meridional Transect data (Jan Kaiser; O2/Ar measurements)

Net community production, mmol m-2 day-1

• AMT 16 and 17, boreal spring and fall, 2005• North Atlantic NCP ~ 1.5 moles m-2 year-1

• South Atlantic NCP ~ 1.0 moles m-2 year-1

North Pacific subtropical gyre at Hawaii(Emerson et al., 1997)

• NCP from O2/Ar ~ 2 moles m-2 yr-1

• NCP from DIC balance and d13C balance ~ 2 moles m-2 yr-1

• Sediment traps agree well!

North and South Pacific gyres (Quay et al., 2009; d13C and DIC balance)• NCP was determined by sampling on 9 container ship cruises• Between 20-30 N, NCP ~ 3 moles m-2 yr-1

• Between 20-30 S, 2 moles m-2 yr-1

Net community production (mol m-2 yr-1) vs. latitude

NCP in most major ocean regions

• NCP is in the range of 1-4 moles m-2 yr-1 in the – Southern Ocean– Equatorial Pacific Ocean– Subpolar North Atlantic– Subtropical north and south Atllantic– Subtropical North Pacific and western South Pacific

• Is NCP much lower in the most oligotropic part of the South Pacific gyre?

Is productivity in the South Pacific subtropical gyre (chl minimum) exceptionally low? (probably yes)

• 8 summer ML samples had biological O2 supersaturation of 0.1 ± 0.2 µmol/kg (NCP ~ zero)• DIC data shows no significant drawdown in ML, small drawdown in thermocline• Below the mixed layer, NCP ~ 0.3 moles m-2 yr-1

Is productivity in the South Pacific subtropical gyre (chl minimum) exceptionally low? (probably yes)

• 8 summer ML samples had biological O2 supersaturation of 0.1 ± 0.2 µmol/kg (NCP ~ zero)• DIC data shows no significant drawdown in ML, small drawdown in thermocline• Euphotic zone NCP > 0.3 moles m-2 yr-1

CLIVAR P18 data showing no summer DIC uptakeCLIVAR P18 data showing DIC uptake and O2

production in the seasonal thermocline

Summary: why isn’t net community production more variable?

• Irradiance is not that variable (about a factor of 2 between equator and 60˚)

• Low latitude waters have high rates of gross production, low rates of NCP/GPP

• High latitude, nutrient rich waters are not that fertile because:– Growing season is short– Iron is frequently limiting– Self shading limits light penetration

• Low latitude, nutrient poor waters are relatively fertile because:– Biomass is low, light penetrates deeply– Low nutrient levels inhibit microbial consumption of DOC – Nitrogen fixation alleviates nutrient limitation – Migrating organisms bring nutrients into the euphotic zone of low nutrient waters– Organisms cleave P and maybe N from DOC

Annual mean surface insolation (Watts m-2)

250

125

25

.01

Boreal summer NO3- (µmol kg-1)

Nutrients and light vary in opposite ways

Supplementary slides

North Atlantic subtropical gyre at Bermuda• Bermuda: Annual NCP ~ 3 moles m-2 yr-1

– Jenkins, 1988, Stanley et al., 2012…: NO3- and 3He balance

– Knapp et al., 2005: 15N flux of N2 fixation– Bates and BATS project: Annual dissolved inorganic carbon drawdown in/below ML

O2 supersaturation, µmol/kg

Dissolved inorganic carbon, µmol/kg

Steinberg et al., 2004

Background: biological and chemical cycles in the world’s oceans

Warm, sunlit upper ocean (0-100 m)

Cold, dark, deep ocean (100-4000 m)

UpwellingSinking Mixing

Wind-mixed layer (0-50 m)

Background: biological and chemical cycles in the world’s oceans

Warm, sunlit upper ocean (0-100 m)

Cold, dark, deep ocean (100-4000 m)

UpwellingSinking Mixing

CO2+H2O CH2O + O2

Carbon export

CH2O + O2 CO2+H2O

Wind-mixed layer (0-50 m)