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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 - PowerPoint PPT Presentation
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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
38 kHz
l ~4cm
Day Day DayNightNight
Dept
h be
low
surfa
ce (m
)35
70
105
140
175
210
Surface reflection
AWCP bio-acoustics - shallower smoother diel cycle in December
38 kHz
l ~4cm
Day DayNightNight
Dept
h be
low
surfa
ce (m
)35
70
105
140
175
210
Surface reflection
NCP – Pulse nitrate sampler&sensor results
0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
4.00
4.50
0.00
2.00
4.00
6.00
8.00
10.00
12.00
14.00
16.00
Pulse7 nutrients
phosphatesilicatenitrate
phos
phat
e si
licat
e uM
nitra
te u
M
NCP over deployment: 89 mg C m-2
Very sensitive to mld 2010-09-12 2010-11-01 2010-12-210
5
10
15
20
25ISUS sensor Nitrate and RAS bag nitrate
ISUS RAS
Nitr
ate
uM
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
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