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Net Community Production at the S outhern O cean T imes S eries

Subantarctic Motivations: control of atmospheric CO 2 nutrient supply outside S. Ocean Results from Pulse Mooring: sensor-based NCP sample-based NCP. ~400 miles from shore, ~4600m water depth. Net Community Production at the S outhern O cean T imes S eries. Tom Trull, Ben Weeding

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Net Community Production at the S outhern O cean T imes S eries

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  1. Subantarctic Motivations: control of atmospheric CO2 nutrient supply outside S. Ocean Results from Pulse Mooring: sensor-based NCP sample-based NCP ~400 miles from shore, ~4600m water depth Net Community Production at the Southern Ocean Times Series Tom Trull, Ben Weeding and the IMOS SOTS team Started 1997 Expanded 2010

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

  3. SOTS : Fe Fertilisation - beyond Si limitation Nitrate Silicate Watson et al, Nature, 2000 Trull et al, DSR2, 2001 Frag. kerg. L.Armand Modified from Chisholm, Science, 2000 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 • 1 m diameter, 0.5m freeboard float • Elastic decoupler, inertial mass, S-tether, integrated instrument package • AanderaaOptode O2 • Seabird T, S, Electrode O2 • Pro-Oceanus Gas Tension • Mclane RAS 24x2x500ml water samples: nutrients, DIC, Alk, Phyto ID • Wetlabs PAR, Fluo-Backscatter • ISUS UV nitrate sensor 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 • Winds • Atmospheric Pressure • ADCP currents • Accelerometer waves • NOAA pCO2 • Sea Surface T,S,O2, Fl-BB • AWCP zooplankton

  7. Oxygen based Net Community Production CO2 + H2O + nutrients= phytoplankton+ O2 Terms in mass balance: d[O2]/dt = air-sea exchange (1) + bubble injection (2) + entrainment (3) + vertical eddy diffusion (4) + biology (NCP) (5) Strategy: Estimate (1) and (2) from N2 (from GTD) scaled to O2 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 [O2] from Argo and Ship O2 profiles. Obtain NCP (5) as the remainder Bubble injection Air – Sea Diffusion Mixed layer NCP = Photosynthesis-Respiration Entrainment and Eddy Diffusion Subsurface ocean

  8. Nitrogen gas as the physical exchange tracer Assume biologically inert in these oxygen rich, cold waters, <13oC Assume little gradient in saturation state with depth, i.e. entrainment brings in close to saturated waters, (Emerson et al., 2008) Estimate N2 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 Oxygen Temperature Deep convection in early spring Subsurface [O2] range and gradients used in error analysis

  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 l ~4cm Day Night Day Night Day 35 70 105 Surface reflection Depth below surface (m) 140 175 210

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

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

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