Critical Depth Theory

Question 1

Mixed layers

Answer 1

Surface mixed layer of the ocean can be indicated by temperature structure
Phytoplankton cell may be able to photosynthesise at the surface, but as light decreases exponentially with depth, light can become too low in intensity and respiration uses up C and energy faster than it is produced.
Therefore no net C accumulation / no net NPP

Question 2

The compensation depth

Answer 2

Dc where photosynthesis of a cell equals respiration.
Ec = compensation light intensity
Above Dc and Ec: P > R
below R < R
R includes both phytoplankton and heterotrophic respiration.
But phytoplankton are mixed and said to experience an average irradiance

Question 3

Turbulent mixing

Answer 3

Turbulent mixes cells up and down through the water column and the associated exponential light gradient.
Provided mixing is fast enough, phyto might experience the mean irradiance within the mixed layer.

Question 4

The critical depth

Answer 4

Dcr = the depth above which the total integrated photosynthesis for the water column is equal to total integrated respiration of the PPs
- Σ Pw = Σ Rw units same e.g. mg O2 m-2 d-1 (NB area units)
At the critical depth the average irradiance for the water column will thus be equal to the compensation irradiance
Ewater column average = Ec

Question 5

Critical depth theory, finding Ec and Dc

Answer 5

Can relate Ec to Dc (recall E(z) = Eo e^-kz)
At Dc where Eavg = Ec
Ec = Eo x e^-kDc
Dc = ln(Eo) - ln(Ec) / k

Question 6

Critical depth theory, finding Eavg

Answer 6

If up and down mixing occurs useful to know average light in euphotic zone = Eavg

Eavg(z) = E0 x (1-e^-kz)/kz
Eo = surface irradiance
k = extinction coefficient
z = depth over which is averaged

Question 7

Critical depth theory, finding Dcr

Answer 7

Rearrange Eavg equation, substitute Ec for Eavg:
Dcr = Eo x (1-e^-kDcr) / kEc

Dcr = Eo / kEc

Critical depth corresponds to the depth at which total water column respiration = total water column photosynthesis
If cells are mixed below there will be no Pn
Therefore can use equation linking:
Surface irradiance Eo
Diffuse attenutation coefficient k
Known compensation irradiance Ec
to estimate when spring bloom starts in temperate latitudes.

Question 8

Sverdrops model

Answer 8

Assumes:
Phytoplankton uniformly distributed with depth in mixed layer.
Non limiting nutrients
Extinction coefficient in water column is constant
Production proportional to irradiance
Respiration is constant with depth.
Winter Zmix > Dcr
Spring bloom Zmix = Dcr
Summer Zmix <Dcr

Question 9

Beyond critical depth theory

Answer 9

Newer theories to explain:
1. Critical mixing hypotheses, mixing rather than mixed layer depth important.
2. Eddy restratification hypotheses, stratification from 3D stratification
3. Disturbance recovery hypothesis, balance between phytoplankton division and loss rates
Ultimately light levels have to be high enough such that NPP > loss for the bloom to occur