1. Natural Selection in a Model Ocean
Mick Follows, Scott Grant, Stephanie Dutkiewicz, Penny Chisholm
MIT

2. Ocean productivity regulates distribution and storage of nutrients and carbon: biological pumps

3. Composition and functional characteristics of pelagic
ecosystem vary in space and time...
coccolithophores
– CaCO3 structural material
diatoms – Si structural material
diazotrophs – fix nitrogen
picoplankton

4. ...affecting efficiency/quality of export:
e.g. recycling “microbial loop” vs. exporting diatom blooms

5. Biogeography:
What are the dynamics underlying provinces?
(Longhurst)

6. Johnson et al., (2006)
Prochlorococcus ecotypes along AMT section

7. Models of the Marine Ecosystem
Volterra (1928), Cushing (1935)
Riley (1946)

8. Nutrient conservation
NPZ models... e.g. Fasham et al. (1990)

9. recent biogeochemical models begin to represent functional diversity in the ecosystem
(e.g. Moore et al., 2002; Gregg et al., 2002; Chai et al.; 2002; Dutkiewicz et al., 2005)

10. Multiple functional groups of phytoplankton
simplified example...
Functional group characteristics imposed by parameter values

11. Prochlorococcus ecotypes (Johnson et al., 2006)

12. AMT observations
Johnson et al. (2006)
From modeling point of view, reveals...
More complexity: functional diversity within species
More simplicity: well defined functional differences between otherwise very closely related organisms

13. Simplify modeling approach by introducing explicit natural selection:
Many possible functional groups (10's – 100's)
Nutrient conservation (physical principle)
Natural selection (ecological principle)
Generic phytoplankton
assign “functions” randomly
choose sensitivities randomly within prescribed ranges

14. Multiple functional groups:
generalized system...
Parameter values assigned with some randomness
Successful functional groups determined by competition

15. “Random” assignment of functional properties (trade-offs?)

16. sub-tropical
1-dimensional
model
seasonal
cycle
initially
100 functional
groups
phyto (log scale) temp & PAR nutrients

17. Ensemble
averages
phyto nutrients

18. max growth rate Kpo4 Kno3 Kpar
Kinhib Npref Topt

22. Why do only a handful of functional groups persist in each case?
Reflects number of potentially limiting resources (Tilman, 1977)
Also sensitive to physical environment, e.g. scales of turbulent variation (Tozzi et al., 2004)
Tilman (1977)

23. Applying principle of competition simplifies model construction
Level of diversity emerges, not imposed
Self-selects “functional groups” according to physical conditions and nutrient availability
Do plausible biological regimes and “ecotypes” emerge?

24. Johnson et al., (2006)
Prochlorococcus ecotypes along AMT section

25. global circulation model
30 functional groups of phytoplankton
2 grazers
nutrients NO3, NH4, NO2, PO4, Si, Fe
phytoplankton functions and parameter values set by random process
ensemble approach

26. Single ensemble member (Iseed 5007)
annual mean surface phyto (uM P) after 5 yrs

27. annual mean phyto (P), 0-120m (Iseed 5007)

28. annual mean nutrients, 0-120m (Iseed 5007)

29. Prochlorococcus Synechococcus
obs
(log)
model
(log)
(linear)

30. Observed Modeled
NO3
NH4
NO2

31. Johnson et al., (2006)
observed modeled

32. Outlook
Natural selection approach appropriate for modeling ocean ecosystems and biogeochemical cycles
Enables focus on underlying dynamics of model, not tuning of parameter values
Dynamic ecosystem approach can adapt to different climate/nutrient environments
Ensemble approach provides statistical viewpoint
(c.f. adaptive approach?)
Prochlorococcus ecotype observations provide well defined system – can model help interpret the observed structures?

34. Single ensemble member (Iseed 17656)
annual mean surface phyto (uM P) after 5 yrs

35. annual mean phyto(P), 0-120m (Iseed 17656)

36. annual mean nutrients, 0-120m (Iseed 17656)

37. Prochlorococcus
Diversity within species...

38. Productivity of the oceans controlled by
Availability of nutrients (light, phosphorus, nitrogen iron...)
Significant role for wind-driven, upper ocean circulation

39. ... and quality of sinking particulate material
association of organic carbon with CaCO3 and opal, >2000m
Klaas and Archer (2002)