1. Bio-optical Controls on
Primary Production
Shubha Sathyendranath and Trevor Platt
Plymouth Marine Laboratory, UK

2. Pelagic Ecosystem in Thermodynamic Context:
Fundamental Role of Phytoplankton Pigments
Pelagic Ecosystem is an open, dissipative system sustained only through daily input of solar energy
Phytoplankton pigments provide the transducer to couple solar input with trophic chain
Thermal dissipation is principal fate of energy absorbed by pigments, with corresponding effect on heat budget of ocean’s upper layer

3. Dual Role for Light Absorbed by Phytoplankton
Photosynthesis
Heating of photic layer

4. Focus: on K the attenuation coefficient for downwelling irradiance
The rate of decrease of downwelling light flux with depth in the ocean is determined by K = -dI/(Idz)
Three principal characteristics of K:
It is a function of phytoplankton pigment biomass, B:
K = Kw + Bkc + …
K, Kw and kc are all strongly dependent on wavelength λ
The specific attenuation coefficient kc is largely determined by the specific absorption coefficient of phytoplankton a*B
Approach: illustration of bio-physical interactions through simple models

5. Dual Role for Light Absorbed by Phytoplankton
If Net Photosynthesis
≠ 0
Rate of Heating of
Photic Layer
Change in K
Change in Biomass

6. Diffuse attenuation coefficient K and mixed-layer depth
High K
Low K
Sea surface
Sea surface
Shallow photic layer
Favours shallow mixed layer =
Deep photic layer
Favours deep mixed layer

7. Dual Role for Light Absorbed by Phytoplankton
Shallower Mixed Layer
Net Photosynthesis > 0
Rate of Heating of layer
Increase in Biomass
Change in K

9. Daily Growth Rate (g) in the Mixed Layer
Dimensionless Group
Platt et al. 1993

10. Net Photosynthesis > 0
Biological Feedback Loop
Higher growth rate
Shallower Mixed Layer
Net Photosynthesis > 0
Heating of photic layer
Change in K
Change in Biomass

11. Specification of Vertical Attenuation Coefficient K for
Visible Light in Mixed-Layer Models
Infinite McCreary & Kundu (1989)
Global constant Niiler & Kraus (1977)
According to Jerlov water types Gaspar (1988)
Tunable Denman (1973)
Derived from pigment field Sathyendranath et al. (1991)
From pigment via partially-coupled model Simonot et al. (1998)
From pigment via fully-coupled model Sathyendranath & Platt (1994)

12. Biologically-induced temperature differences in the ocean
Wu et al. 2007, DSR II

13. Difference in SST due to biologically- induced change in K
Wu et al. 2007, DSR II

14. Low chl
High chl
Edwards et al (JGR)

15. See also works by, for example:
Siegel et al Heating of Equatorial Pacific
Nakamoto et al OGCM
Murtugudde et al Tropical ocean circulation
Shell et al Atmospheric response
Ohlman Climate model
Strutton and Chavez Heating of Equatorial Pacific
Aumont et al HNLC regimes
Manizza et al Ice Cover
Le Queré et al Dynamic Green Ocean Models
Ballabrera-Poy et al Interannunal variability

16. Biological feedback is a positive feedback loop
So why is it that runaway blooms are not more common?
What are the brakes on the system (other than depletion of nutrients)?

17. Two Important Depth Horizons
The Sverdrup (1953) theory introduces two important depth horizons into considerations of phytoplankton dynamics
Mixed-layer Depth: Determined by the balance between stratification due to solar heating and destratification due to turbulence
Critical depth: Determined by mass balance of generation and loss of phytoplankton
Both mixed layer depth and critical depth are influenced by the optical properties of the medium, in particular by the diffuse attenuation coefficient, K

20. Role of K in Development of a Bloom
Effect on Critical Depth:
Accumulation of biomass in the water increases K, which would in turn lead to a decrease in critical depth Zc.
Unless there were a simultaneous change in mixed-layer depth Zm, then Zc<Zm, such that phytoplankton accumulation in the mixed layer would be self-regulating.
Effect on Mixed layer:
An increase in K would localise the solar heating nearer to the surface, so that the change in mised-layer deoth would be in a favourable sense for maintenance of bloom.
How do the mised-layer depth and the critical depth change relative to each other, when there is a change in K?

21. Zc / Zm
Sathyendranath and Platt 1999

22. Convergence of Biomass to the Fixed Point
B (t +1) = B (t )
B (t +1) B*
B (t )
Platt et al. 2003

23. This is a bio-optical limit on phytoplankton growth in the mixed layer.
Platt et al. 2003

24. Relation to Critical Depth

25. Is this very typical after all?

26. Platt et al. PRSLA 2003

29. Scatterometer Data

30. See also Margalef 1978
Platt et al. MEPS 2003

32. High kc associated with small cells will imply lower production, and lower values of equilibrium biomass for the same set of physical conditions than for large cells.

33. Maximum concentration of Chl a
Theoretical curve from Platt et al. 2003
The mixed layer depth : an upper limit (Light)
The simulated maximum concentration is often lower:
Other limiting nutrients: Si, N (Equatorial Pacific, North of the Polar Front)
Grazing by zooplankton (South of the Polar Front, Eastern equatorial Pacific)
From Olivier Aumont and Laurent Bopp 2006

34. Spectral Quality of Light Underwater:
Relevance for Biological – Physical Interactions
Do the underwater light field and the bio-optical-physical controls on biomass influence the distribution of phytoplankton taxa?

38. Or, (z,t) = m ∫ I(l,z,t) aB (l,z) dl

40. i is quantum yield (wavelength independent); class-dependent
Computation:
Primary Production in the Photic Zone using photosynthesis-irradiance formalism:
Bi () = i aBi ()
where:
Bi is initial slope of phytoplankton class i;
aBi is initial slope of phytoplankton class i;
i is quantum yield (wavelength independent); class-dependent
 is wavelength;
Bi indicates normalisation to biomass Bi of phytoplankton class i = 1,5

41. Computation
Mixed-layer depth tracks photic depth (1% light level). All biomass increase in the layer redistributed evenly throughout the layer.
Carbon growth rates for each phytoplankton class transformed to chlorophyll growth rates using class-specific carbon-to-chlorophyll ratios
The assimilation number (PmB, mg C mg Chl-3 h-1), is also allowed to vary with class
Computations are carried out for a range of chlorophyll concentrations
Goal: To isolate the impact of spectral quality of underwater light field on growth rates (in chlorophyll units).

43. B

44. PmB in mg C mg Chl-3 h-1

47. Taxa-specific growth rates
Optical Homeostasis?
Biomass
Mixed Layer
Spectral Quality of
Underwater Light
Heating of photic layer
Taxa-specific growth rates
Change in spectral K
Bases of community structure & species succession?

48. Conclusions
Critical depth is attracted to the mixed-layer depth, in such a way as to set a bio-optical limit to maximum phytoplankton concentration in the layer
Spectral quality of underwater light field can modulate specific rate of change of chlorophyll-a, even when the input parameter set remains unchanged.
This spectral effect does not impact different phytoplankton types in the same manner, leading to relative changes in their growth
This could be one of many mechanisms responsible for changes in species composition, species succession and biodiversity in phytoplankton
Potential applications in remote sensing and modelling of phytoplankton functional types

49. Acknowledgements:
All our colleagues, especially Carla Caverhill, George White, Venetia Stuart, Heidi Maass, Yongsheng Wu, Charles Tang, Andrew Edwards and Olivier Aumont