1. Net Community Production at the Southern Ocean Times Series Tom Trull, Ben Weeding and the IMOS SOTS team ~400 miles from shore, ~4600m water depth Subantarctic Motivations: control of atmospheric CO 2 nutrient supply outside S. Ocean Results from Pulse Mooring: sensor-based NCP sample-based NCP Started 1997Expanded 2010

2. SOTS: west flowing limb of super-gyre, upper limb of overturning Ridgway and Dunn, 2007 Antarctica ACC MOC Currents at 200m 5 – 30 cm s -1 SAMW AAIW

3. SOTS : Fe Fertilisation - beyond Si limitation Trull et al, DSR2, 2001 Modified from Chisholm, Science, 2000 Watson et al, Nature, 2000 Nitrate Silicate Frag. kerg. L.Armand Modified from Chisholm, 2007

4. SOTS: SAZ Sediment Trap Mooring Stiff subsurface design Paired traps and current meters at 1000, 2000, 3800m McLane Parflux funnels Indented rotating sphere zooplankton excluding in- situ settling columns

5. SOTS: Pulse BGC Mooring Intake outside shroud through 1mm screen No filtration Backflushing with mercuric chloride instead of acid Samples collected in pairs: HgCl2 for nutrients, DIC, Alk, 13C-DIC Buffered/Si-enriched glutaraldehyde for microscopy

6. SOTS: SOFS Air-Sea Flux Mooring

7. Oxygen based Net Community Production Mixed layer Subsurface ocean NCP = Photosynthesis -Respiration Bubble injection Air – Sea Diffusion Entrainment and Eddy Diffusion CO 2 + H 2 O + nutrients= phytoplankton+ O 2 Terms in mass balance: d[O 2 ]/dt = air-sea exchange (1) + bubble injection (2) + entrainment (3) + vertical eddy diffusion (4) + biology (NCP)(5) Strategy: Estimate (1) and (2) from N 2 (from GTD) scaled to O 2 using Schmidt number and range of ratios for complete/partial bubble diffusion. Estimate terms (3) and (4) from mixed layer depth variations and literature eddy diffusivities, using constant sub-surface [O 2 ] from Argo and Ship O 2 profiles. Obtain NCP (5) as the remainder

8. Nitrogen gas as the physical exchange tracer 1.Assume biologically inert in these oxygen rich, cold waters, <13 o C 2.Assume little gradient in saturation state with depth, i.e. entrainment brings in close to saturated waters, (Emerson et al., 2008) 3.Estimate N 2 from total gas tension, by removing contributions from oxygen, water vapour, and other gases, by assuming all are similarly under- or over-saturated (Woolf and Thorpe, 1991), rather than that they are all exactly saturated as assumed by Emerson et al, 2008:

9. Ship and Argo profiles to quantify subsurface inputs Deep convection in early spring Subsurface [O 2 ] range and gradients used in error analysis Temperature Oxygen

10. SOTS: Seasonal Warming and Stratification

11. SOTS: Advective Signal Sources SST variations – local mesoscale features are sufficient to explain “events”. Correlated T-S variations are strongly density compensated. The passage of distinct water parcels past the mooring represents real environmental variability we want to capture, but can also introduce errors into NCP estimates…..

12. SOTS: Physical air-sea forcing and gas exchange

13. SOTS: Oxygen ventilation, exchange terms, NCP

14. NCP Error Analysis

15. Comparison to other SAZ NCP estimates

16. Conclusions Methodology: Oxygen mass balances are a bloody hard path to NCP! Will UV nitrate analysers be any easier? Sample return missions are important, because biology is the only path to prediction. Profiling instruments are preferable to single point measurements. Entrainment is difficult to quantify. Smooth mixed layer depth seasonality in models misses key dynamics. The steady-state approximation seems dubious except in summer. The Ocean’s Song: (teach me the lines) Springtime diel cycling may well be a key mode favouring high NCP: -escape nightime grazers by dilution -get lit up every 5 days or so – and respond quickly to daytime insolation Mode water ventilation is significant but incomplete, thus the idea that sinking particle fluxes must escape below the winter mixed layer to matter to the biological pump is an overstatement. The cause of the apparent near cessation of NCP in early summer is not yet clear: - Silica limitation of export community? - Iron limitation of production? (Anybody have a trace-metal clean water sample for us to deploy?)

17. AWCP bio-acoustics – abrupt deep diel cycle in August 38 kHz  ~4cm Day Night Depth below surface (m) 35 70 105 140 175 210 Surface reflection

18. AWCP bio-acoustics - shallower smoother diel cycle in December 38 kHz  ~4cm Day Night Depth below surface (m) 35 70 105 140 175 210 Surface reflection

19. NCP – Pulse nitrate sampler&sensor results NCP over deployment: 89 mg C m -2 Very sensitive to mld

20. Motivations: planetary metabolism Oceans estimated to contribute half of global primary production Some time series show decadal changes – e.g. “regime shift” at HOT Remote sensing suggests possible changes – e.g. expansion of oligotrophic gyres But links between biomass and production based on sparse manipulation experiments. Conventional wisdom is Sverdrup critical depth based on light/nutrient limitation but 75% of global ocean iron limited, with Fe supply not mediated by mixing alone. and export fluxes do not show seasonality consistent with biomass or light limitation Apply new methods to determine primary and net community production at high spatial and temporal resolution: SOOP, floats, gliders, moorings. O2/Ar, O2/TotalGases, uV-nitrate Preferably in tandem with micro-nutrient, microbial ecology, particle observations. Develop new models

21. NCP – Understanding and Predictive Capacity may Require Biology CiliatesDiatoms RAS phytoplankton identification, Ruth Eriksen