Provided by the SeaWiFS Project, NASA/Goddard Space Flight Center and ORBIMAGE

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Measurements and Models of Primary Productivity
John J. Cullen Department of Oceanography,
Dalhousie University Halifax, Nova Scotia, Canada
B3H 4J1 Microbial Oceanography Genomes to
Biomes University of Hawaii June 28, 2007
Provided by the SeaWiFS Project, NASA/Goddard
Space Flight Center and ORBIMAGE
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Outline

• What is marine primary productivity?
• Ecological and biogeochemical significance
• Scales of variability
• Overview of measurements
• The need for models
• The nature of models
• Their reliance on measurements
• Hazards of forgetting the limitations of models

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What is marine primary productivity?
Net Primary Productivity (Production) Net rate
of synthesis of organic material from inorganic
compounds such as CO2 and water Chemosynthesis
chemical reducing power comes from reduced
inorganic compounds such as H2S and
NH3 Photosynthesis reducing power comes from
light energy Photosynthetic primary production
is usually measured and considered to dominate.
g C m-3 h-1 g C m-2 d-1
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Oxygenic Photosynthesis
This process can be quantified by measuring the
increase of oxygen, the decrease of CO2, or the
increase of organic carbon. For practical
reasons, oceanographers measure the incorporation
of radioactive 14C into organic compounds some
label water with 18O (stable isotope) and measure
its appearance in oxygen. (Inferences can be
made from 17O)
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Ecological and biogeochemical significance
marine.rutgers.edu/opp/
This is estimated primary productivity
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The ocean accounts for half the photosynthesis on
earth!
marine.rutgers.edu/opp/
This is estimated primary productivity
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and the point is???
From Falkowski and Raven 1997 Table 1.1
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The Growth of Phytoplankton
DaughterCell
Doubled Biomass
Photosynthesis
Single Cell
Daughter Cell
Nutrient Uptake
Cell Division
Result More suspended particulate organic
matter (food) Less dissolved inorganic
nutrients (N, P, Si) Less dissolved inorganic
carbon (CO2) (Oxygen is produced)
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What happens to the new growth?
Fates Accumulate (Bloom) Be eaten Sink Lysis
(blow up) Viruses Apoptosis
DaughterCell
Daughter Cell
Cell death site www.uwm.edu/berges/celldie/cldet
h.htm
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Consumption and Decomposition(deep ocean)
Microbial Decomposition
Organic Matter
DEEP-SEA LIFE

Nutrients CO2
Consumption Respiration Excretion
Result Less suspended particulate organic
matter More dissolved inorganic nutrients (N,
P, Si) Supersaturated dissolved inorganic
carbon (CO2) Oxygen is consumed
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CO2 is elevated in the deep ocean because
nutrients are depleted at the surface and
regenerated at depth
from a slide by Dave Karl
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Ocean Cycle of Life and Death At the Balance
Point
CO2
CO2 Nutrients ? Organic Matter
Primary production
sinking particles
upwelling and mixing
CO2 Nutrients ? Organic Matter
Decomposition
Bottom
Organic C
Cullen et al. 2007 Oceanography Mag.
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Variability and disequilibrium structure food
webs and biogeochemical cycles
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Scales of variabilityThe co-occurrence of light
and nutrients explainspatterns of primary
productivity in the sea
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Typical Structure of Chl and PP
Sikes - MS 320 - Rutgers
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Starting Point The 14C method for measuring
primary productivity
HOT website
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The 14C method for measuring primary productivity
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An incredibly useful tool for time series and
process studies
HOT website
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Interaction of physiology and vertical mixing is
a big twist
Denman, K. L., and A. E. Gargett (1983), Time and
space scales of vertical mixing and advection of
phytoplankton in the upper ocean, Limnol.
Oceanogr., 28, 801-815. Lewis, M. R., et al.
(1984), Relationships between vertical mixing and
photoadaptation of phytoplankton similarity
criteria, Mar. Ecol. Prog. Ser., 15,
141-149. Cullen, J. J., and M. R. Lewis (1988),
The kinetics of algal photoadaptation in the
context of vertical mixing, J. Plankton Res., 10,
1039-1063. Franks, P. J. S., and J. Marra
(1994), A simple new formulation for
phytoplankton photoresponse and an application in
a wind-driven mixed-layer model, Marine Ecology
Progress Series, 111, 145-153.
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Vertical mixing and temporal scales of
measurements
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Ideally, the 14C method measures net primary
productivity
Sometimes, it does
12 - 24 h incubations
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The measurement is subject toartifacts and biases
12 - 24 h incubations
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The measurement is subject toartifacts and biases
12 - 24 h incubations
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The measurement is subject toartifacts and biases
12 - 24 h incubations
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The measurement is subject toartifacts and biases
12 - 24 h incubations
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The measurement is subject toartifacts and biases
12 - 24 h incubations
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The measurement is subject toartifacts and biases
12 - 24 h incubations
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and thats not all
Toxicity Relief of iron limitation Exposure to
bright light Disruption of fragile cells for
Simulated in situ Poor match of
irradiance Inappropriate temperature Possible
diel bias
12 - 24 h incubations
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Productivity normalized to Chl is biased by
Changes in Chl during incubations Inadequate
extraction of Chl by 90 acetone Interference
from Chl b(fluorometric acid-ratio method)
12 - 24 h incubations
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and in general
Irradiance is not controlled Respiration is not
accurately measured
12 - 24 h incubations
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The measurement has its limitations
Net production? Gross production? Overestimate?
Underestimate?
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But we use it routinely
Water column integral is something like net
primary production (photosynthesis less losses to
respiration)
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and it has served us very well
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Process studies
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Direct Measurements will Never Provide Synoptic
Estimates of Productivity
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Models are required for many applications
Productivity from ocean color Ecological
prediction Biogeochemical models Climate change
scenarios
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Many share the same basic form
see Talling, Ryther and Yentsch, Rhode, etc.
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Differences are relatively minor, butthey can
matter
Dependence on Irradiance
Behrenfeld and Falkowski 1997a LO
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One approach model the measurements
Optical Depth
Behrenfeld and Falkowski 1997a LO
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Simplified functions describe major patterns
Behrenfeld and Falkowski 1997a LO
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Is the measured/modeled pattern real?
PB (mg C mg Chl-1 d-1)
Measured near-surface inhibition
Depth (m)
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Is the measured/modeled pattern real?
PB (mg C mg Chl-1 d-1)
or an artifact of fixed-depth incubation?
Depth (m)
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Measurements must be made on a shorter time scale
Photosynthetron Controlled laboratory
incubation (Lewis and Smith 1983)
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A comprehensive approach
Cullen, J. J., M. R. Lewis, C. O. Davis, and R.
T. Barber. 1992. Photosynthetic characteristics
and estimated growth rates indicate grazing is
the proximate control of primary production in
the equatorial Pacific. Journal of Geophysical
Research 97 639-654.
Approach introduced by Jitts, H. R., A. Morel,
and Y. Saijo. 1976. The relation of oceanic
primary production to available photosynthetic
irradiance. Aust. J. Mar. Freshwater Res. 27
441-454.
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Short-term measurements can detect near surface
inhibition
Chlorophyll a (mg m-3)
PBm . Chl (mg C m-3 h-1)
Depth (m)
Depth (m)
0630 h Chlorophyll and Pmax are nearly uniform
in the 50-m mixed layer
MORNING
Principles described by Marra, Harris, Yentsch --
all prior to 1980
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Is measured near-surface inhibition
real?Assessment with short-term P vs E
Chlorophyll a (mg m-3)
PBm . Chl (mg C m-3 h-1)
Depth (m)
Depth (m)
1215 h Chlorophyll and Potential
Productivity are still nearly uniform in the 50-m
mixed layer
MIDDAY
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Surface incubation led to artifact
Chlorophyll a (mg m-3)
PBm . Chl (mg C m-3 h-1)
Depth (m)
Depth (m)
1530 h Fixed-depth incubations at the
surface are fried an artifact of sustained
exposure 35 underestimation of ML productivity
INCUBATED
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Conventional 14CNear-surface inhibition is
overestimated
PB (mg C mg Chl-1 d-1)
Artifact
Depth (m)
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Consequences for a model minor except for
irradiance dependence of ?P at higher daily
irradiance
Behrenfeld and Falkowski Consumers Guide LO
1997
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But rememberNear-surface inhibition is also
underestimated
Polycarbonate bottles exclude UV-B
Thats a different talk
(this refers to 14C incubations of 12 - 24 hours)
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Many models calculate P from measured
relationships
From PB vs E and E vs depth and
Chl to daily water column net
primary productivity
P (mg C m-3 d-1)
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Results can be generalized
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Maximum PB is still a key parameter for most
models
Even the spectral ones even the quantum-yield
models
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Compared to other measures, PBopt is relatively
insensitive to artifact
PB (mg C mg Chl-1 d-1)
Depth (m)
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Global assessment of primary productivity
requires global assessment of maximum PB
Behrenfeld and Falkowski 1997b LO
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PBopt modelled as a function of temperature
Behrenfeld and Falkowski 1997b LO
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Two commonly used functions
Behrenfeld and Falkowski 1997b LO
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You may have seen the figure
Behrenfeld and Falkowski 1997b LO
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Here are the measurements
Rutgers OPP Study Data Base
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Compared with more measurements
Texas Shelf
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and more measurements!
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General functions of T cannot capture major
causes of variability in water column productivity
2x error
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Conclusion General functions of T cannot capture
major causes of variability in water column
productivity
Relative Error
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We must appreciate the implicationsof this
unexplained variability
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Estimates like these (NPP) do not yetaccount for
variable physiologybeyond central tendencies
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The need to compare models with measurements
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HOT data
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Comparison with measurements is instructive
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It is also useful to remember the
distinction between models and observations
Follows et al. Model
VGPM model
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Many models cannot be directly verified
No one is immune!
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Conclusions

• Models of primary productivity are a fundamental
  requirement for describing and explaining
  ecosystem dynamics and biogeochemical cycling in
  the sea
• The models are based on measurements
• The measurements are not perfect
• The models are not perfect
• Capabilities and limitations of models should
  always be considered when they are applied
• Effects of underlying assumptions
• Comparison with real measurements when available
• Know your model!