Transcript puddle.mit.edu

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
diatoms – Si structural material
diazotrophs – fix nitrogen
picoplankton
...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)
Multiple functional groups of phytoplankton
simplified example...
Functional group characteristics imposed by parameter values
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 randomness
Successful functional groups determined by competition
“Random” assignment of functional properties
(trade-offs?)
sub-tropical
1-dimensional
model
seasonal
cycle
initially
100 functional
groups
phyto (log scale)
temp & PAR
nutrients
Ensemble
averages
phyto
nutrients
max growth rate
Kinhib
Kpo4
Npref
Kno3
Topt
Kpar
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
NO3
NH4
NO2
Modeled
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
Klaas and Archer (2002)
association of organic carbon with CaCO3 and opal, >2000m