Why focus on the mixed layer?

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 collaborators BrorJonsson, Kuan Huang, Princeton Nicolas Cassar, Princeton  Duke Pedro Monteiro and Warren Joubert, CSIR and UCT, Cape Town John 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 Photosynthesis P - R = O2 NCP = O2 efflux to atmosphere = ([O2] - [O2]sat) * gas exch. velocity CO2 + H2O --> CH2O + O2 • 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) Respiration CH2O + O2 --> CO2 + H2O 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 0.1 0.1 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 Subantarctic Zone, Australian sector Palmer LTER

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

Cape Town Hobart S. Georgia Is. Seasonal NCP mmol m-2 day-1 Dumont d’Urville SANAE

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

Why focus on the mixed layer?