Natural Selection in a Model Ocean

Mick Follows, Scott Grant, Stephanie Dutkiewicz, Penny Chisholm

MIT

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

Composition and functional characteristics of pelagic ecosystem vary in space and time...

coccolithophores – CaCO3 structural material

diazotrophs – fix nitrogen picoplankton

diatoms – Si structural material

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

Biogeography:

What are the dynamics underlying provinces?

(Longhurst)

Johnson et al., (2006)

Prochlorococcus ecotypes along AMT section

Models of the Marine Ecosystem

Volterra (1928), Cushing (1935)

Riley (1946)

Nutrient conservation

NPZ models... e.g. Fasham et al. (1990)

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)

Functional group characteristics imposed by parameter values

Multiple functional groups of phytoplankton

simplified example...

Prochlorococcus ecotypes (Johnson et al., 2006)

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

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

Multiple functional groups:

generalized system...

Parameter values assigned with some randomnessSuccessful functional groups determined by competition

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

phyto (log scale) temp & PAR nutrients

sub-tropical1-dimensionalmodel

seasonal cycle

initially100 functionalgroups

phyto nutrients

Ensembleaverages

max growth rate Kpo4 Kno3 Kpar

Kinhib Npref Topt

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)

● 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?

Johnson et al., (2006)

Prochlorococcus ecotypes along AMT section

● 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

Single ensemble member (Iseed 5007)

annual mean surface phyto (uM P) after 5 yrs

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

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

Prochlorococcus Synechococcus

obs(log)

model(log)

model(linear)

Observed Modeled

NO3

NH4

NO2

Johnson et al., (2006)

observed modeled

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?

Single ensemble member (Iseed 17656)

annual mean surface phyto (uM P) after 5 yrs

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

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

Prochlorococcus

Diversity within species...

Productivity of the oceans controlled by

● Availability of nutrients (light, phosphorus, nitrogen iron...)● Significant role for wind-driven, upper ocean circulation

... and quality of sinking particulate material

association of organic carbon with CaCO3 and opal, >2000mKlaas and Archer (2002)