Natural Selection in a Model Ocean Mick Follows, Scott Grant, Stephanie Dutkiewicz, Penny Chisholm
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Transcript of Natural Selection in a Model Ocean Mick Follows, Scott Grant, Stephanie Dutkiewicz, Penny Chisholm
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Natural Selection in a Model Ocean
Mick Follows, Scott Grant, Stephanie Dutkiewicz, Penny Chisholm
MIT
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Ocean productivity regulates distribution and storage of nutrients and carbon: biological pumps
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Composition and functional characteristics of pelagic ecosystem vary in space and time...
coccolithophores – CaCO3 structural material
diazotrophs – fix nitrogen picoplankton
diatoms – Si structural material
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...affecting efficiency/quality of export:e.g. recycling “microbial loop” vs. exporting diatom blooms
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Biogeography:
What are the dynamics underlying provinces?
(Longhurst)
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Johnson et al., (2006)
Prochlorococcus ecotypes along AMT section
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Models of the Marine Ecosystem
Volterra (1928), Cushing (1935)
Riley (1946)
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Nutrient conservation
NPZ models... e.g. Fasham et al. (1990)
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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)
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Functional group characteristics imposed by parameter values
Multiple functional groups of phytoplankton
simplified example...
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Prochlorococcus ecotypes (Johnson et al., 2006)
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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
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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
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Multiple functional groups:
generalized system...
Parameter values assigned with some randomnessSuccessful functional groups determined by competition
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“Random” assignment of functional properties (trade-offs?)
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phyto (log scale) temp & PAR nutrients
sub-tropical1-dimensionalmodel
seasonal cycle
initially100 functionalgroups
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phyto nutrients
Ensembleaverages
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max growth rate Kpo4 Kno3 Kpar
Kinhib Npref Topt
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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)
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● 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?
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Johnson et al., (2006)
Prochlorococcus ecotypes along AMT section
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● 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
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Single ensemble member (Iseed 5007)
annual mean surface phyto (uM P) after 5 yrs
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annual mean phyto (P), 0-120m (Iseed 5007)
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annual mean nutrients, 0-120m (Iseed 5007)
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Prochlorococcus Synechococcus
obs(log)
model(log)
model(linear)
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Observed Modeled
NO3
NH4
NO2
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Johnson et al., (2006)
observed modeled
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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?
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Single ensemble member (Iseed 17656)
annual mean surface phyto (uM P) after 5 yrs
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annual mean phyto(P), 0-120m (Iseed 17656)
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annual mean nutrients, 0-120m (Iseed 17656)
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Prochlorococcus
Diversity within species...
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Productivity of the oceans controlled by
● Availability of nutrients (light, phosphorus, nitrogen iron...)● Significant role for wind-driven, upper ocean circulation
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... and quality of sinking particulate material
association of organic carbon with CaCO3 and opal, >2000mKlaas and Archer (2002)