Abandoning Sverdrup

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Abandoning Sverdrup
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June Chlorophyll 1998 - 2006 (mg m-3)
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1953
Google citation count 561
Net phytoplankton growth occurs when area acd gt
abdf
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• Sverdrups 1953 paper was a formalization of the
  critical depth
• concept originally proposed by Gran and
  Braarud in 1935
• The critical depth hypothesis attempts to
  explain what initiates a
• vernal bloom, not what controls the magnitude
  of a bloom
• A bloom is an increase in biomass, not
  photosynthetic rate
• The hypothesis states that a bloom begins when
  the mixed layer shoals to a depth
• above the critical depth horizon where
  production (P) gt respiration (R)
• R grazing sinking phytoplankton
  respiration all other losses
• R is assumed constant
• Inverse of Sverdrup prior to crossing the
  critical depth criterion, net growth is
  negligible
• or negative

Gran Braarud. 1935. J. Biol. Board Can. 1
(5), 279-467
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• Sverdrup net growth can be independent of gross
  production
• under heavy grazing
• Sverdrup the bloom observed 2 days after the
  depth of the mixed
• layer was for the first time smaller than the
  critical depth likely
• reflected advection not rapid local growth
• Sverdrup the first increase in biomass occurred
  before stratification
• Sverdrup It is therefore not advisable to
  place too great emphasis on the agreement between
  theory and the Weather Ship M observations
• (occurrence of blooms in the absence
  of stratification is not uncommon)

e.g., Townsend et al. 1992. Nature 360, 59-62
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Abandoning Sverdrup
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Chlorophyll (mg m-3)
1989 NABE 2008 NAB focus in later
slides

• SeaWiFS data 1998 2006
• 8-day resolution
• 12 central NA bins, minimize advect.
• Chlsat OC4-V4
• Cphyto GSM / Westberry et al 2008

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NA-5 - Latitude range 45o 50oN

• North Atlantic seasonal cycles are dominated by
  changes in biomass
• Thus, Cphyto Chlsat
• Differences between Cphyto and Chlsat consistent
  with photoacclimation
• All analyses have been completed with both C and
  Chl
• Results to follow are the same irrespective of C
  or Chl
• Remaining slides focus on C
• within-bin standard deviations shown above

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NA-5 - Latitude range 45o 50oN

• peak biomass occurs in spring
• coincident with rising PAR and shoaling MLD
• also associated with rapid rise in primary
  production
• Conclusion phytoplankton in the North Atlantic
  exhibit a repeated vernal bloom caused by
  increased primary production and growth
  associated with rising light and shallowing mixed
  layers aka, Sverdrup

unfortunately, biomass can be a terribly
misleading thing. correlation is not
causation
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Bloom in a Bottle

• To understand what causes a bloom, it is
  necessary to first identify when a bloom started
• The start of a bloom can not be defined by
  biomass - e.g., when biomass X mg m-3 or Y above
  annual median
• Using biomass can lead to the wrong start date
  and association of bloom initiation with the
  wrong environmental forcing
• Bloom initiation implies a change in the rate of
  growth for Sverdrup it was the beginning of
  positive net growth
• An easy way to get a first-order sense of rate
  changes is to plot biomass on a logarithmic scale

5 gt mean
Net growth rate r ln(C1/C0) slope of log
plot t1 t0
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Abandoning Sverdrup

• The North Atlantic bloom does not begin in the
  spring
• Net exponential growth begins mid-winter
• Shift from negative to positive biomass changes
  coincides
• with the cessation of mixed layer deepening
• Net growth rates are, on average, comparable
  from winter
• through spring
• Net growth rates do not reflect changes in
  incident light,
• photosynthetic rate, or gross growth rate (µ)
• The critical depth hypothesis can be
  dismissed

NA-5 - Latitude range 45o 50oN
Cphyto
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Bloom in a Bottle

• Population specific net growth rates (r) can be
  calculated from changes in phytoplankton
  concentration (m-3) so long as the mixed layer is
  either shoaling or not deepening
• However, one must consider the influence of
  dilution when the mixed layer is deepening
• A dilution correction should be considered when
  assessing growth rates during mixed layer
  deepening

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NA-5 - Latitude range 45o 50oN
Deep
Shallow

• Population net specific growth rate (r) for the
  active water column becomes positive in
  late-autumn / early winter and remains positive
  through the spring until nutrients are depleted
• Growth-phase maxima in r can occur during MLD
  deepening, MLD maximum, or MLD shoaling
• Overall, r is inversely related to PAR and µ
• In 100 of available complete annual cycles, r
  becomes positive before PAR begins to increase

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NA-5 - Latitude range 45o 50oN
Shallow
Deep
Starting now in July

• The vernal bloom appears to be an event
  initiated in late fall
• Triggering of the bloom appears to be associated
  with mixed layer
• deepening (not shoaling)
• How is this possible? Why the mid-winter
  decrease in r?

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How is this possible?
µ NPP / CZ
NA-5 - Latitude range 45o 50oN

• A net specific growth rate of 0.02 implies
  approximately 1 division per month
• Typical winter C 4 8 mg m-3, Typical spring
  C peak 25 70 mg m-3
• NA bloom requires 2 4 doublings over 3 - 4
  months, or average r of 0.009 to 0.03 d-1

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• Positive r through winter is allowed because
  losses co-vary with µ (Sverdrup assumed this
  respiration to be a constant) r µ
  0 as PAR 0 at very high latitudes in
  mid-winter (no light) a critical depth can
  never be reached
• The increase in r during winter implies that the
  fraction of µ that escapes predation and other
  losses (i.e., rµ) must increase in the winter

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The mid-winter decrease
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Diluted Digression
Landry Hassett 1982 Mar. Biol. 67,
283-288 Landry et al. 1995 Mar. Ecol. Prog. Ser.
120, 53-63

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The Grand Dilution Hypothesis

• As a replacement for the Critical Depth
  Hypothesis, it is proposed that the north
  Atlantic bloom is a consequence of a massive
  scale dilution experiment
• Mixed layer deepening causes a slight decoupling
  between phytoplankton growth and losses (grazing,
  mostly)
• The decoupling increases so long as the mixed
  layer continues to deepen
• Mixed layer shoaling drives a re-coupling of
  phytoplankton growth and losses (grazing)
• Thus, while spring shoaling and increasing light
  favor enhanced photosynthesis and growth, they
  also favor heavier grazing losses

Landry Hassett 1982 Mar. Biol. 67,
283-288 Landry et al. 1995 Mar. Ecol. Prog. Ser.
120, 53-63

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Modeling the Grand Dilution

• As an initial attempt, a simple model was
  developed and compared to average annual cycles
  of the rµ ratio and r for each of the 12 bins
• Model input was the value of rµ during the
  first week in July (-0.01) and MLD and Zeu (from
  Chlsat)
• The 3 model conditions were as follows

• Mixed layer deepening within the euphotic zone,
  entrains phytoplankton and grazers
• NO CHANGE in rµ

MLD0
Zeu
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Modeling the Grand Dilution

• As an initial attempt, a simple model was
  developed and compared to average annual cycles
  of the rµ ratio and r for each of the 12 bins
• Model input was the value of rµ during the
  first week in July (-0.01) and MLD and Zeu (from
  Chlsat)
• The 3 model conditions were as follows

• Mixed layer deepening below euphotic zone,
  entrains phytoplankton free water
• Dilutes predators Prey
• rµ CHANGES IN PROPORTION TO DILUTION

Zeu
MLD0
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Modeling the Grand Dilution

• As an initial attempt, a simple model was
  developed and compared to average annual cycles
  of the rµ ratio and r for each of the 12 bins
• Model input was the value of rµ during the
  first week in July (-0.01) and MLD and Zeu (from
  Chlsat)
• The 3 model conditions were as follows

• Mixed layer shoaling cuts off the lower
  population of phytoplankton, has no direct effect
  on phytoplankton concentration, but concentrates
  mobile grazers
• Shoaling concentrates predators but not prey
• rµ CHANGES IN PROPORTION TO MLD CHANGE BUT AT A
  SLOWER RATE THAN DEEPENING EFFECT

Zeu
MLD0
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Modeling the Grand Dilution
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Final Comments

• Temporal coverage of the satellite record
  provides a unique opportunity to re-evaluate
  bloom dynamics
• The critical depth hypothesis is found wanting
  (actually, it fails miserably)
• A Grand Dilution Hypothesis is suggested, but is
  not the only potential explanation (aggregation,
  temperature effects, sinking.?)
• Dilution Hypothesis accommodates blooms w/o
  stratification
• Climate change effects on North Atlantic (and
  other) blooms may be very much different for a
  Critical Depth concept of blooms and a
  Dilution concept of blooms
• Revisiting bloom experimentation in North
  Atlantic?

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Winter Chlz Lat trends Mixing velocity Feb C
max Lat r µ Cphyt vs Csat
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