P1252109251njamk

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Spatial and Temporal Variability in Chlorophyll,
Primary Production and Export Production in the
Southern Ocean B. Greg Mitchell
Acknowledgments
SIO Mati Kahru, Haili Wang Dariusz
Stramski Rick Reynolds Chris Hewes, Osmund
Holm-Hansen
Programs NSF JGOFSNASA Ocean
Biogeochemistry AMLR
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• A long-term goal of our group is to make
  comprehensive global measurements in support of
  ocean color satellite applications
• Ocean spectral reflectance
• phytoplankton pigments
• primary production model parameters
• Rrs(l) Lu /Ed
• Rrs(l) f/Q bb (l) /a (l) bb (l)
• P chl ? aph (l) E (l) ? (l)
• and aph are functions of E, N, T ?max a /
  aph

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In situ Measurements Water Sample
Measurements Ed, Lu, Eu MER 2048 and
PRR800 Absorption AC9 Phytoplankton,
Photosynthesis Cstar 660, 490 Detritus CTD
Soluble In vivo fluorescence Particle size
distribution FRRF variable fluorescence Fluoromet
ric chlorophyll Hydroscat bb HPLC
Pigments Total suspended solids CDOM
fluorescence matrix Mycosporine amino
acids Flow cytometry Photosynthesis -
Irradiance
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Chl algorithm for the Southern Ocean
Standard ocean color products are good for
mid-latitude Case-1 waters (e.g. CalCOFI) but not
so much for the Southern Ocean
Southern Ocean bio-optics is different (Mitchell,
Holm-Hansen, 1991) - need a different Chl
algorithm
CalCOFI surface Chl
CCAMLR 2000 match-ups with SeaWiFS standard (OC4)
Chl
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Absortion at 440 nm and backscatter slope
relationship to chlorophyll for CalCOFI and
Southern Ocean data
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VGPM Behrenfeld and Falkowski
We tuned the PBopt and Zeu parameters to Southern
Ocean Data from RACER, JGOFS and AMLR programs
Euphotic Zone Depth vs Chlorophyll
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Time series of chlorophyll concentration and
primary production estimated for the Blue Water
Zone west of Drake Passage SeaWiFS
OC4v4 / VGPM(red), SeaWiFS SPGANT /
VGPM-SPG (blue), MODIS chl_a_2 or P1
(green) MODIS chla_a_3 or P2 (orange).
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Depth
Chl-a in SFZ upstream vs downstream
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Chl-a in SFZ downstream vs upstream
Mean (for Oct-Mar, 10 years)
Max ratio
Min ratio
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• Influence of the Chl- a algorithm standard OC4v4
  vs SPGAnt
• SO is bio-optically different and requires a
  different Chl algorithm (Mitchell, Holm-Hansen
  1991).
• Standard Chl-a algorithm is close to in situ Chl
  at low values (lt0.4 mg m-3) but underestimates
  Chl-a 2-3 times at mid-level (1-10 mg m-3),

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• Inter-annual variability in Chl very complex
• Interannual variations in surface Chl are very
  high (up to 4-5 x), may be out of phase over
  neighboring areas, and likely to be related to
  changes in the locations of eddy mixing

1st PC
solid line standard Chl dotted line SPGANT
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The Southern Ocean is generally organized by
zonal variations in specific water properties
(Gordon et al., 1977). That is true, for example
of satellite derived SST, but not chlorophyll in
the Southeast Atlantic region
February mean satellite estimates of SST and
chlorophyll for 1997-200
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• Phytoplankton growth is regulated by light,
  temperature and nutrients
• In the Southern Ocean during summer, temperature
  limits growth rate, but not total biomass and
  there is sufficient light in the mixed layer to
  saturate growth
• Large regions of the Southern Ocean have low
  phytoplankton biomass but high concentrations of
  macro nutrients. These regions, called High
  Nutrient, Low Chlorophyll regions, may be limited
  by the amount of iron available
• The natural sources of iron to support open
  ocean phytoplankton are not well understood
• Our goal was to explore the potential that fluid
  dynamics of the off shore water interacting with
  bathymetry might be a mechanism to transport iron
  rich coastal water offshore
• Our study was focused on the physical, chemical
  and biological processes near the Shackleton
  Fracture Zone in the Southern Drake Passage

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Hypotheses  Strong chlorophyll gradients observed
in the southern Drake Passage near SFZ are
associated with gradients in plankton community
structure, physiology and trophic coupling caused
by the relative availability of iron.   Iron
gradients in the southern Drake Passage near SFZ
are established by mixing between eastward
flowing BWZ water and northward flowing water
modified on the Antarctic Peninsula shelf.
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Interdisciplinary Research Team Collaborative
Research Plankton Community Structure and Iron
Distribution in the Southern Drake
Passage Analysis Investigator(s) Physical
Environment ADCP, Hydrography,
Drifters Zhou, Gille Chemistry Trace
metals Fe, Al, Mn Measures, Barbeau
Macronutrients, N, P, Si Holm-Hansen,
Mitchell Biology and Optics Primary
Production (PvsE) Mitchell, Holm-Hansen
Phytoplankton community composition Barbeau,
Hewes, Mitchell Active Fluorescence (FRRF),
Optics Mitchell Biomass (chl-a, HPLC,
POC/PON) Holm-Hansen, Mitchell, Hewes
Bacterioplankton, microheterotrophs Azam, Hewes,
Barbeau Zooplankton acoustic
backscatter Zhou Experiments Azam,
Barbeau, Holm-Hansen Hewes, Measures,
Mitchell Satellite Analysis (Separate NASA
funding) Chl, Surface Temperature
(SST) Mitchell, Kahru Eddy energy, surface
velocities, winds Gille
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ACC
SAF
PF
SACCF

• Satellite derived mean Chl-a for 10 years
  (Oct-Mar of 1996-2006) using OCTS, SW, Aqua data
• Superposed are the MEAN positions of major
  hydrographic fronts (Orsi et al., 1995)
  Subantarctic Front (SAF), Polar Front (PF),
  Southern ACC Front (SAACF)

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Chlorophyll-specific absorption by phytoplankton
in three distinct water types
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Methodological digression The classic paradigm of
a constant initial slope of the PvsE curve is not
valid. Often the curve has a sigmoid shape where
the maximum quantum yield is not at the limit as
light goes to zero. Better resolution of the low
light domain may have information about
acclimation and photo-respiration
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Chlorophyll-specific maximum fixation of carbon,
and the maximum quantum yield of photosynthesis
were highest in shelf waters or mixed waters on
the frontal mixing gradients.
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The Shackleton Fracture Zone creates a large
perturbation in the ACC The flow via
Shackleton Gap onto the Shackleton Shelf
allows ACC water to interact with the shelf water
and the mixture is then transported into the deep
ocean Shelf water enriches the off shore region
with Fe and enhances phytoplankton biomass and
primary production Complex variability in
photosynthetic and bio-optical parameters are
related to iron limitation and must be considered
in photosynthesis models for Southern Ocean The
relative role of off shelf transport, open ocean
upwelling and atmospheric input needs to be
better understood
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How does the strength of the Shackleton Jet
vary? How does topography influence this system?
Does wind and Eckman flow play a role? What is
the inter-annual variability of the system?  How
representative is the circulation observed in
2004? How does the magnitude of seasonal sea ice,
wind, and the strength of the Weddell Sea outflow
influence the system? What is the source of
off shore green water to the north of our study
region ? Is it from the shelf or localized
offshore upwelling? What is the relative role
of sediment conditioning of winter water, surface
run off from islands, and hydrothermal vents for
introducing iron into the surface waters
northeast of the Antarctic Peninsula? What is
the light, iron and temperature co-limitation
matrix for key bloom species? Does light limit
productivity? What is the role enhanced
productivity by iron enrichment for higher
trophic levels including sea birds and marine
mammals?  
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