Determining net community production in the Southern Ocean mixed layer using O2/Ar, and prospects for scaling

Why focus on the mixed layer?

How do you measure net community production (NCP) with O2/Ar ratios?

Why does NCP approximate carbon export?

Some strategies for scaling

Some collaboratorsBror Jonsson, Kuan Huang, PrincetonNicolas Cassar, Princeton DukePedro Monteiro and Warren Joubert, CSIR and UCT, Cape TownJohn Dunne (GFDL) and Scott Doney (WHOI/NOAA)Hugh Ducklow, MBL

Why focus on the mixed layer?

• Mixed layer is deep in the Southern Ocean, encompasses most of the euphotic zone

• Easy to sample and characterize underway• Gases in the ML are subject to exchange with air

– Gas transfer velocity gives a clock for calculating rates from concentrations

Determining net community production in the mixed layer

• Net community O2 production = flux to atmosphere

• Complication: [O2] > [O2]sat because of warming and bubble entrainment

• Measure Ar as inert analog to O2 to correct for physical supersaturation (Jenkins, Craig, Quay, Emerson, Luz, Stanley, Hamme…)

• O2/Ar can be measured on samples returned to the lab

• O2/Ar can be measured continuously using membrane inlet mass spectrometry (Kaiser, Tortell) or equilibrator inlet mass spec (Cassar)

CO2 + H2O --> CH2O + O2

CH2O + O2 --> CO2 + H2O

Photosynthesis

Respiration

P - R = O2 NCP = O2 efflux to atmosphere = ([O2] - [O2]sat) * gas exch. velocity

Base of the mixed layer

Assumptions, limitations and advantages of NCP by O2/Ar

• Assumptions– NCP is constant– Mixed layer depth is constant– No exchange of water across bottom of the ML

• Limitations– Upwelling brings O2-undersaturated waters to the surface, sea-air O2 lux

– If ML is shallow, NCP below ML adds O2 to ML– Changes in MLD and NCP introduce errors– In appropriate condtiions, biological O2 flux to atmosphere ~ 80 ± 20-% of NCP– Errors are larger when NCP is very low

• How to use O2/Ar NCP– Average over large areas, analyze gradients and covariance, be cautious when

upwelling is rapid.• Keep in mind…

– Other methods have limitations• Why O2/Ar?

– Analyze continuously with minimal attention, collect lots of data

Prospects for scaling: Palmer LTER

0.10.1 1.0 10

NCP from O2/Ar ratios in the Southern Ocean

• We have NCP data from about 70 crossings of the Southern Ocean• Additional data from other people (Tortell, Long, Kaiser…)• Many involve continuous O2/Ar measurements• Ample data for testing algorithms for NCP in the Southern Ocean

Strategy for scaling – 1 Get primary production from an appropriate algorithm Get export production/primary production from Dunne et al. (2007)

Export/ primary production

Lower export production at higher temperature

More chl, more primary production, more export

This approach does a surprisingly good job of accounting West Antarctic Peninsula data

5b. Prospects for scaling - one possible strategy• Start with

– VGPM (~ 14C production) from B+F: Depends on I, zeu, SST, chl– f ratio from Dunne et al.: Depends on SST, ln(VGPM/zeu)

• Scale VGPM by MLD/euphotic zone depth• Model NCP = f * scaled VGPM• Compare with observations for SAZ-SENSE

Scaling strategy 2 – Ask what is the NCP in a model when grid cells in a model have the same POC and MLD as observed

Scaling strategy 3 – calculate NCP from the observed relationship between chl-normalized NCP and MLD

Scaling observations

• Get chl from sensing• Get MLD from

– Climatology– ARGO floats– Model

• Calculate NCP

Palmer LTER

Subantarctic Zone, Australian sector

Scaled NCP Dec.-Jan. NCP in the Southern Ocean: comparison with data

Observations

Calculations

Hobart

Dumont d’Urville

Cape TownS. Georgia Is.

SANAE

Seasonal NCPmmol m-2 day-1

Net community production, Cape Town – SANAE –South Georgia – SANAE – Cape Town