1. 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 program Bronte Tilbrook (CSIRO, Hobart)/Astrolabe+Aurora Australis programs Matt Reuer, Colorado College (many ship of opportunity cruises) Jan Kaiser (UEA)/Atlantic Meridional Transect program Jan Kaiser and Rachel Stanley (WHOI) (Ka’Imimoana program; Thompson cruise) Nicolas Cassar, SAZ/SENSE and more Many individual cruises and cruise participants

2. 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

3. 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

4. Background: biological and chemical cycles in the world’s oceans Euphotic zone CO 2 +H 2 O  CH 2 O + O 2 Carbon export Mixed layer NCP = photosynthesis- respiration C org is not stored in euphotic zone; NCP ~ export O 2 flux to the atmosphere = O 2 supersat * gas transfer velocity Correct for physical supersaturation based on Ar supersat Sea-air O 2 flux = carbon NCP and export

5. Other sources of NCP and carbon export for this work 15 NO 3 - assimilation Seasonal drawdown of DIC or nutrients Seasonal balance of  13 C of CO 2 in the upper ocean

6. Caveats and limitations In O 2 /Ar method, O 2 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

7. Net community production in the Southern Ocean Carbon fluxes from Reuer et al. (2007) (O 2 /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

8. 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: 15 NO 3 - assimilation) Western tropical Pacific (3˚N-3˚S, 170˚W – 145˚E) ~ 1.5 moles C m -2 yr -1 (Stanley et al., 2010: O 2 /Ar)

9. Subpolar North Atlantic Start with 13 µmol/kg NO 3 - 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 [NO 3 - ] vs. time: North Atlantic Bloom study, Alkire et al. 2012

10. North Atlantic subtropical gyre at Bermuda Bermuda: Annual NCP ~ 3 moles m -2 yr -1 – Jenkins, 1988, Stanley et al., 2012…: NO 3 - and 3 He balance – Knapp et al., 2005: 15 N flux of N 2 fixation – Bates and BATS project: Annual dissolved inorganic carbon drawdown in/below ML Dissolved inorganic carbon, µmol/kg Steinberg et al., 2004

11. Atlantic Meridional Transect data (Jan Kaiser; O 2 /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

12. North Pacific subtropical gyre at Hawaii (Emerson et al., 1997) NCP from O 2 /Ar ~ 2 moles m -2 yr -1 NCP from DIC balance and  13 C balance ~ 2 moles m -2 yr -1 Sediment traps agree well!

13. North and South Pacific gyres (Quay et al., 2009;  13 C 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

14. 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?

15. Is productivity in the South Pacific subtropical gyre (chl minimum) exceptionally low? (probably yes) 8 summer ML samples had biological O 2 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

16. Is productivity in the South Pacific subtropical gyre (chl minimum) exceptionally low? (probably yes) 8 summer ML samples had biological O 2 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 uptake CLIVAR P18 data showing DIC uptake and O 2 production in the seasonal thermocline

17. 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

18. Annual mean surface insolation (Watts m -2 ) 250 125 25.01 Boreal summer NO 3 - (µmol kg -1 ) Nutrients and light vary in opposite ways

20. Supplementary slides

21. North Atlantic subtropical gyre at Bermuda Bermuda: Annual NCP ~ 3 moles m -2 yr -1 – Jenkins, 1988, Stanley et al., 2012…: NO 3 - and 3 He balance – Knapp et al., 2005: 15 N flux of N 2 fixation – Bates and BATS project: Annual dissolved inorganic carbon drawdown in/below ML O 2 supersaturation, µmol/kg Dissolved inorganic carbon, µmol/kg Steinberg et al., 2004

22. Background: biological and chemical cycles in the world’s oceans Warm, sunlit upper ocean (0-100 m) Cold, dark, deep ocean (100-4000 m) UpwellingSinkingMixing Wind-mixed layer (0-50 m)

23. Background: biological and chemical cycles in the world’s oceans Warm, sunlit upper ocean (0-100 m) Cold, dark, deep ocean (100-4000 m) UpwellingSinkingMixing CO 2 +H 2 O  CH 2 O + O 2 Carbon export CH 2 O + O 2  CO 2 +H 2 O Wind-mixed layer (0-50 m)