Ecological stoichiometry examines the balance of elements like carbon, nitrogen, and phosphorus in ecological interactions and processes. A key study found that removing planktivorous fish from an eutrophic lake and allowing zooplankton populations like Daphnia to increase altered nutrient cycling. This increased the relative availability of nitrogen over phosphorus, countering the original low nitrogen to phosphorus input ratio and decreasing cyanobacteria dominance. The results supported theories linking nutrient ratios, growth rates, and food web structure based on organisms' nucleic acid and ribosomal RNA composition.
2. Why is stoichiometry important?
• Conservation of mass and energy
• Growth is limited by nutrients, that are required
in fairly strict ratios
• Ecosystems have a finite amount of elements and
inputs/outputs
• Without limiting nutrients, energy, or
space, theoretical population dynamics may give
infinite growth (implicitly included in carrying
capacity)
3. • Humans have
much more C,N,P
as a fraction of
total mass than
occurs in the earth
as a whole
• Must be
preferentially
accumulating
these elements
Sterner and Elser, 2002
5. N vs. P
• P available in rock form and decreases as
ecosystem age
• N can be fixed by organisms from the atmosphere
to inorganic bio-available forms (but this is
energetically expensive and requires lots of P as
ATP)
• Aquatic ecosystems often thought to be P limited
because N can be brought in via fixation. Also
affected by anthropogenic N inputs (runoff, dep)
Walker and Syers 1976; Vitousek et al. 2010
6. N, P, and co- limitation
Large purple bars
suggests co-
limitation of N
and P. This could
be supported by
re-allocation of
one nutrient to
get another to
optimize growth
Elser et al., Ecology Letters, 2008
8. Background info: Redfield Ratios
• Phytomass displays an average C:N:P of
106:16:1
• This is similar to the C:N:P of dissolved
matter in the ocean
Redfield, American Naturalist, 1958
9. NO3- from PO43-
Oceaninferred and CNP deviations
N* - processes
Mean Phytoplankton
106:16:1
1:16 P:N line
N* = NO3- – 16PO43- + 2.9 Gruber and Sarmiento 199
10. N/P
Tropical Temperate
Leaf litter (reg) 43:1 12:1
Leaf: 43:1 25:1
Leaf litter: 63:1 27:1
All forest microbes 9:1
Fungi 15:1
Bacteria 7:1
Enzymes 1:1
McGroddy et al. 2004; Townsend
et al. ; Cleveland et al. 2007;
Reiners 1986; Sinsabaugh et al.;
12. Pelagic CNP in eutrophic lake with food
web manipulation
• Q: How do tropic dynamics and biogeochemistry
interact in regulating lake ecosystem dynamics
during a whole-lake food-web manipulation?
• HYP: elimination of planktivorous fishes would
result in a pelagic food web in which P-rich
zooplankton (for example, Daphnia) would have a
greatly enhanced role in regulating internal
nutrient availability and would differentially
increase the availability of N relative to P.
Elser et al. 2000 Ecosystems
13. Responses to Pike
• + Pike
• 3 yrs later:
- minnows
• 4 yrs later:
+ cladoceran
Daphnia
- zooplankton N:P
- seston C:P
+ DON and DOP
Elser et al. 2000 Ecosystems
14. N fixation
• Low external N/P ratio
• Internal processes driven
by food-web changes
fixed enough N relative to
P in the early season to
allow phytoplankton to
grow similarly to 25 years
previously
• Then cyanobacteria
crashed
• Suggests threshold N/P
ratio for N fixation to be
energetically favorable
Elser et al. 2000 Ecosystems
15. 5 aspects of stoichiometric effects
• Zooplankton became more P rich (lower C:P and N:P ratio)
• The importance of zooplankton as a nutrient pool in the
water column greatly increased
• Increased zooplankton biomass increased overal dissolved
nutrient availability (more for N than P). This caused shift
away from N-fixing cyanobacteria
• Seston C:P and N:P ratios were low, indicating relatively
rapid groth rates of remaining phytoplankton biomass.
Decreased phytoplankton bioass reflected less of the
limiting nutrient P
• Sedimentation appears to have been altered by food web
manipulation
Elser et al. 2000 Ecosystems
16. Conclusions
• Consumer-driven nutrient cycling processes
appeared to have increased N:P ratio in the
available nutrient supply.
• This should result in decreased dominance of
cyanobacteria in phytoplankton community
• Introduction of piscivorous pike and elimination
of planktivorous fish generated low N:P sink
(Daphnia zooplankton community) counteracted
the low N:P source of nutrients entering the
lake, drastically altering the response of the lake
Elser et al. 2000 Ecosystems
17. Modeling implications
• Eutrophic lakes are characterized by alternative stable
states
• These dynamics are consistent with stoichiometric models
of grazer-algae interactions
• These models predict the existence of intrinsic high grazer
and grazer-free stable states under eutrophic conditions
• Nutrient loading ,tropic cascades and stoichiometric
theories provide a fundamental understanding of eutrophic
lake dynamics
• Our ability to make specific predictions of the occurrence
and intensity of cyanobacteria biomass may be limited by
the nonlinear mechanisms underpinning the nutrient-
phytoplankton-zooplankton systems
Elser et al. 2000 Ecosystems
18. Biological stoichiometry
• Biological stoichiometry: coupling the first
laws of thermodynamics; evolution by natural
selection; and central dogma of molecular
biology
• Roots: optimal foaging; resource ratio
competition theory; Redfield ratio; nutrient
use efficiency
Elser et al. 2000 Ecology Letters
19. Biological stoichiometry from genes to
ecosystems
• Q: What determines the C:N:P of living
biomass?
• HYP: a connection between growth rate and
C:N:P stoichiometry based on rRNA allocation
and the organization of ribosomal genes in
diverse biota
Elser et al. 2000 Ecology Letters
20. Autotroph N:P rules of thumb
• Biomass N:P tracks N:P of the nutrient supply
• At fixed supply rate of nutrient X, biomass C:X
increases as light intensity and/or pCO2
increase
• Under concentrations of X-limited
growth, biomass C:X increases steeply as
realized specific growth rate declines
• High variation of C:N:P in base of food web
Elser et al. 2000 Ecology Letters
21. Growth rate and P relationships
• Organisms with high max specific growth rate
have high [RNA]
• RNA makes up 50-60% of the ribosome, which
promotes cell growth
• RNA is 10% P by weight
• P-rich, low N:P is a signature of rapid growth
and is a cellular necessity
• Most variation occurs in chromosomal rDNA
copy number
Elser et al. 2000 Ecology Letters
22. Growth Rate Hypothesis
• rRNA is needed for
protein synthesis;
rRNA is ~80% of all
RNA in organisms
• RNA has a relatively
low N/P
• Thus, growth rate is
limited by P and
N/P variation is
largely driven by
investment in rRNA
Sterner and Elser, 2002
23. Molecular genetics of food web
dynamics hypothesis
• Goal: generate functionally realistic model of
ecological dynamics informed by modern
genetic understanding
• Evolution of growth rate related to RNA
allocation and organism P content/CNP stoich
• HYP: variation in the relative abundance of
high growth rate, low C:P and N:P consumers
with high rDNA should be higher in systems
with good quality (low C:N and C:P food).
Elser et al. 2000 Ecology Letters
24. Resource
Ratio
Theory
Miller, American Naturalist 2005
25. Questions and discussion:
1. How might results differ if the lake were not eutrophic?
2. How do terrestrial and lake ecosystems differ, and what
are the problems?
3. In what situations is it worth incorporating nutrient
dynamics and stoichiometry, and when might it be
unneccessarily complicating the model?
4. Does Elser’s RNA hypotheses make sense when comparing
across global scales, like tropics vs. temperate?
Elser et al. 2000 Ecology Letters
