OC211OA211 Phytoplankton

1
OC211(OA211) Phytoplankton Primary Production

• Dr Purdie SOC (566/18) email DAP1_at_soc.soton.ac.u
  k

LECTURE 6 Week 6 (i) Photosynthesis Light (ii)
Critical Depth Theory
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The Photosynthesis Light Curve

• Instantaneous rates of photosynthesis are
  controlled by external factors
• Temperature
• Irradiance
• CO2 and O2 concentrations
• The Photosynthesis Irradiance response (P vs E)
  can be divided into three regions.(i) light
  limited region(ii) light saturated region(ii)
  photoinhibited region

Fig7.2 Falkowski Raven
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The Photosynthesis Light Curve

• (i) At low light levels photosynthesis rates are
  linearly proportional to irradiance ie double
  light level doubles photosynthesis rate.
• In this part of curve (light limited part of
  curve) rate of photon absorption determines rate
  of steady state electron transport from water to
  CO2.
• If measure photosynthesis using O2 changes then
  at low irradiance the rate of O2 consumption
  will be greater than O2 production hence net O2
  evolution is negative.
• The light level where photosynthetic production
  of O2 balances consumption of O2 by respiration
  is the Compensation Irradiance (Ec). (in natural
  populations of microalgae 5-30 mmol/m2/s)

Fig7.2 Falkowski Raven
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The Photosynthesis Light Curve

• The initial slope of the P vs E curve is
  proportional to
• the maximum quantum yield.
• It is often given symbol a
• This is a function of the Light Reactions of
  photosynthesis.
• Usually photosynthesis is normalised to
  chlorophyll biomass (ie rate of production
  divided by chlorophyll) and if so a superscript B
  is added to indicate this normalization aB

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The Photosynthesis Light Curve

• (ii) As irradiance increases photosynthetic rates
  become increasingly non linear and rise to a
  saturation level or Pmax .
• When the photosynthesis rate is chlorophyll
  normalized (ie divided by chlorophyll
  concentration express as PBmax or Assimilation
  number
• units mgC.mgchl-1h-1
• At saturation the rate of photon absorption
  exceeds the rate of steady state electron
  transport from water to CO2.
• Pmax is thus a function of the Dark Reactions
  or Light Independent reactions of
  photosynthesis and is thus influenced by
  environmental conditions influencing enzyme
  reactions eg temperature.

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The Photosynthesis Light Curve

• The intersection of a and Pmax is given the
  symbol Ek or
• light saturation parameter. It represents an
  optimum on the P vs E curve.
• Ek Pmax/a
• Ek is independent of whether Pmax or a are
  normalized to chlorophyll, cell volume, cell
  carbon etc.
• This does not allow for photoinhibition and more
  complex mathematical formulations used to include
  inhibition effects.

Fig7.2 Falkowski Raven
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The Photosynthesis Light Curve

• (iii) Further increases beyond light saturation
  can lead to a reduction in photosynthesis rate
  from maximum saturation level. This is
  photoinhibition and is dependant on intensity of
  light and duration of exposure.
• A number of models mathematical formulations
  used to fit data to a line. The hyperbolic
  tangent function often used
• P Pmax .tanh(a.E/Pmax)
• This does not allow for photoinhibition and more
  complex mathematical formulations are used to
  include inhibition effects.

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The Photosynthesis Light Curve

• Pvs E curves are usually measured over a few
  hours under artificial light conditions.
• This provides an indication of the physiological
  adaptive state of the phytoplankton population
• The P vs E response can indicate light stress at
  high light levels if the population is shade
  adapted ie sampled from deep chlorophyll
  maximum.

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The Critical Depth Theory

• Photosynthesis is driven by light and therefore
  restricted to upper parts of ocean
• Photosynthesis rates decrease with depth.
• Calculation of production per unit area mgC m-2
  h-1
• note volume units must be in m-3
• 
  mgCm-3h-1
• 
  (5 x 15) ((22-15) x 5)/2)
• 
  (7 x 15) ((15-11) x 7)/2)
• (5 x
  15) ((11-5) x 5)/2)
• (5 x
  15) ((5-0) x 5)/2)
• Total
  389 mgC m-2 h-1

22
0m
5m
15
12m
11
17m
5
22m
0
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The Critical Depth Theory

• The surface mixed layer of the ocean indicated by
  temperature structure
• A single cell can photosynthesise in surface
  water but if mixed down the light is too low in
  intensity and respiration uses up carbon and
  energy . Therefore no carbon accumulation or no
  net positive primary production.
• This gave rise to the Critical Depth concept.
• Gran and Braarud (1935) first suggested.
• Sverdrup (1953) developed a mathematical model
  

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The Critical Depth Theory

• COMPENSATION DEPTH (Dc) where photosynthesis of a
  cell equals its respiration PcRc
• Compensation irradiance (Ec) (5-10 mmols
  m-2s-1)
• Therefore above Ec Pc gt Rc
• below Ec Pc lt Rc (therefore a net loss)
• Phytoplankton are continually mixed above and
  below the compensation depth therefore experience
  an average irradiance. The euphotic zone is the
  portion of the water column supporting net
  primary production and the base of the euphotic
  zone is the compensation depth.
• Above the compensation depth net daily
  photosynthesis is positive, below this depth it
  is negative.

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Figure 9.3 Falkowski Raven


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The Critical Depth Theory

• CRITICAL DEPTH (Dcr) defined as "the depth to
  which phytoplankton can be mixed and at which the
  total photosynthesis for the water column is
  equal to total respiration (of the primary
  producers)"
• At the Critical Depth, Photosynthesis throughout
  the water column will equal respiration
• Pw Rw units same mgC m-2 h-1 (NB area
  units)
• Also at the critical depth the average irradiance
  for the water column equals the compensation
  irradiance.
• E Ec

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The Critical Depth Theory
Fig 41. Parsons, Takahashi Hargrave


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The Critical Depth Theory

• The model relates Ec to Dc
• Ec Eoe-kDc
• Dc ln(Eo)- ln (Ec)
• k
• Phytoplankton are mixed up and down in the
  surface layers of the water column, it is useful
  to know the average amount of light (ED) in the
  euphotic zone. This is given by the expression
• ED Eo. (1 - e-kD)
• kD
• where Eo surface irradiance
• k extinction coefficient
• D depth over which irradiance
  averaged

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The Critical Depth Theory

• How far down can a population of phytoplankton be
  mixed to balance photosynthetic production and
  respiration consumption of carbon? i.e. Critical
  Depth
• Rearrange above equation and substitute Ec for ED
  we get the following expression to calculate the
  critical depth
• Dcr Eo. (1 - e-kDcr)
• k.Ec
• If k.Dcr is large then this is simplified to
• Dcr Eo
• k.Ec

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The Critical Depth Theory

• Where the amount of phytoplankton carbon respired
  is matched by area of carbon gained by
  photosynthesis then
• this is Critical Depth.
• If cells are mixed downward below this depth
  there will be no net
• photosynthesis.
• If depth of mixing is above critical depth
  positive net photosynthesis can occur.
• Therefore we can use a simple equation linking
• 1) surface irradiance,
• 2) diffuse attenuation coefficient and
• 3) a known compensation irradiance
• To estimate when the spring bloom starts in
  temperate latitudes. Model assumes
• (i) plants uniformly distributed with depth in
  mixed layer
• (ii) plant nutrients non limiting
• (iii) extinction coefficient in water column is
  constant
• (iv) production of plant material is
  proportional to amount of radiation
• (v) respiration is constant with depth

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The Critical Depth Theory
Fig 3.11 Mann Lazier