Net Community Production at the Southern Ocean Times Series

Tom Trull, Ben Weedingand the IMOS SOTS team

~400 miles from shore, ~4600m water depth Subantarctic Motivations:control of atmospheric CO2

nutrient supply outside S. Ocean

Results from Pulse Mooring: sensor-based NCP sample-based NCP

Started 1997 Expanded 2010

SOTS: west flowing limb of super-gyre, upper limb of overturning

Ridgway and Dunn, 2007

Anta

rctic

a

ACC

MOC

Currents at 200m

5 – 30 cm s-1

SAMWAAIW

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

SOTS: SAZ Sediment Trap MooringStiff subsurface designPaired traps and

current meters at 1000, 2000, 3800m

McLane Parflux funnels

Indented rotating sphere zooplankton excluding in-situ settling columns

SOTS: Pulse BGC Mooring• 1 m diameter, 0.5m freeboard float• Elastic decoupler, inertial mass, S-

tether, integrated instrument package• Aanderaa Optode 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 screenNo filtrationBackflushing with mercuric chloride instead of acid

Samples collected in pairs:

HgCl2 for nutrients, DIC, Alk, 13C-DIC

Buffered/Si-enriched glutaraldehyde for microscopy

SOTS: SOFS Air-Sea Flux Mooring

• Winds• Atmospheric Pressure• ADCP currents• Accelerometer waves• NOAA pCO2 • Sea Surface T,S,O2, Fl-BB• AWCP zooplankton

Oxygen based Net Community Production

Mixed layer

Subsurface ocean

NCP = Photosynthesis-Respiration

Bubbleinjection

Air – Sea Diffusion

Entrainment and Eddy Diffusion

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

Nitrogen gas as the physical exchange tracer1. Assume biologically inert in these oxygen rich, cold waters, <13oC

2. Assume little gradient in saturation state with depth, i.e. entrainment brings in close to saturated waters, (Emerson et al., 2008)

3. 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:

Ship and Argo profiles to quantify subsurface inputs

Deep convection in early spring

Subsurface [O2] range and gradients used in error analysis

Temperature Oxygen

SOTS: Seasonal Warming and Stratification

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…..

SOTS: Physical air-sea forcing and gas exchange

SOTS: Oxygen ventilation, exchange terms, NCP

NCP Error AnalysisTable 1. Net Community Production Error Sources and Analysis 1 2 Model versions NCP (mol O2 m-2) Standard, without vertical exchange 1.41

Standard, with vertical exchange 3.70 Corrected, with vertical exchange and advective corrections 2.20 3 Error Analysis Parameters Uncertainty Effect on NCP Optode O2 values (μmol kg-1) +1.75 +21% -1.75 -21%

Gas transfer coefficient +30% -23%

-30% +23%

Bubble parameter β = 1 x10 -7% (ratio of collapsing to diffusing effects) x0.1 +4%

Vertical eddy diffusivity Kz =1x10-4 m2 s-1 x0.3 0% x10 +6%

Deep [O2] value = 264 μmol kg-1 +5 -43%

-5 +43%

Corrected Model

Comparison to other SAZ NCP estimates

Table 2. Comparison of Subantarctic Zone seasonal NCP estimates 1 Method Reference NCP (mol O2 m-2) Moored O2/N2 This paper 2.2 ±1.2 Underway O2/Ar [Cassar et al., 2007] 4.4 ±15% Satellite remote sensing1, and ecological modeling2

[Behrenfeld and Falkowski, 1997]1, [Laws et al., 2000]2

2.4 ±40%

Surface nitrate depletion from ship surveys

[Lourey and Trull, 2001] 4.6 ±15%

1. monthly VGPM net primary production (Oregon State University Ocean Productivity Standard Product). 2 2. exported fraction of net primary production as a function of temperature and production . 3

ConclusionsMethodology:

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?)

AWCP bio-acoustics – abrupt deep diel cycle in August

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AWCP bio-acoustics - shallower smoother diel cycle in December

38 kHz

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NCP – Pulse nitrate sampler&sensor results

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Pulse7 nutrients

phosphatesilicatenitrate

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NCP over deployment: 89 mg C m-2

Very sensitive to mld 2010-09-12 2010-11-01 2010-12-210

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25ISUS sensor Nitrate and RAS bag nitrate

ISUS RAS

Nitr

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Biofouling?Replumbed for DI baselines in July 2012Replumb for pumped mode in April 2013

Motivations: planetary metabolism

Oceans estimated to contribute half of global primary productionSome time series show decadal changes – e.g. “regime shift” at HOTRemote sensing suggests possible changes – e.g. expansion of oligotrophic gyresBut 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-nitratePreferably in tandem with micro-nutrient, microbial ecology, particle observations.

Develop new models

NCP – Understanding and Predictive Capacity may Require Biology

CiliatesDiatoms

RAS phytoplankton identification, Ruth Eriksen