Our ecological stoichiometry course kicked off today. My initial training is in this area, which I called “ecosystem biogeochemistry”, but it was very much a geologically focused, big systems approach. It’s interesting to approach this material from a more organism- and population-centric perspective that most Davis ecologists have.
Today, Alison started us off with some first principles of nutrient constraints on ecology. Drawing a lot from Sterner and Elser (2002), but noting that a lot of these principles go all the way back to Lotka (1925). In essence:
- Organism growth is limited by nutrients, and they take up (and excrete) nutrients preferentially to get the ratios they need
- Nutrient requirements are driven by the elemental composition of biomolecules
- Different nutrients are limited by different processes. N is widely available in the atmosphere, but energetically costly to acquire. P is rarer and locked up in mineral deposits.
- Ecosystems vary widely in the nutrients available, and ecological structure is driven partly by these availabilities and how they propagate through physiology and trophic structure
Here is Alison’s presentation. Papers she mentioned are cited below:
Then we dove into a couple of papers:
Linking scales: Elser et al. (2000a)
This is a pretty great review. I didn’t learn about the RNA linkage when I first studied this stuff. Elser et. al. lay out a pretty convincing case that phosphorus limitation is driven by the need for ribosomal RNA for growth:
- Organisms with high nutrient compositions (High C:N/P ratios). are more vulnerable to poor food quality
- Most animals maintain relatively constant nutrient ratios, except for storage and structural materials. \(\mu'\) declines when foods have low nutrient content. (Lots of variation among taxa, though.)
- P is correlated with rRNA content, and in Daphnia and other species, differences in rRNA content are sufficient to explain differences in P.
- Organisms are in part characterized by their maximum temp-normalized max growth rate, \(\mu_m\). \(\mu'\) is their realized rate.
- Organisms with high \(\mu_m\) have high maximum rRNA levels.
- For organisms without P storage mechanisms, P requirements are driven by growth rate due to demands for P-rich rRNA in growing cells.
- For autotrophs, N:P increases with size, and \(\mu_m\) decreases. But there is just as much variation within one species of algae growing under different conditions, and such ratios can track nutrient supply for many autotrophs.
There’s also a really intriguing evolutionary component here. Elser et al. point out that, while ribosomal DNA sequences are highly conserved across taxa, there’s LOTS of variation in the intergene spacer regions between the genes for rRNA, and gene copy numbers, and these areas appear to have a lot of control over gene expression. These areas change under selection pressure on growth rate.
Experimental Manipulation: Elser et al. (2000a)
This is a classic whole-lake manipulation study performed at the Experimental Lakes Area in Canada (Save the ELA!). Elser et. al. take a look at the interaction of two major frameworks in lake ecology: trophic structure and nutrient fertilization. Trophic structure can modify how ecosystem compensation occurs in response to nutrient loading, e.g., previous studies showed that phytoplankton nutrient limitation switched from P to N depending on the species composition of zooplankton. Here they examine how a trophic manipulation - adding an apex predator - modifies nutrient cycling.
Hypotheses
Enhanced piscivory will result in:
- Decreased C:P and N:P ratios in zooplankton
- Zooplankton P becoming an important part of the total P pool
- Increased sedimentation loss of P
- Increased relative availability of inorganic N (compared with P)
- N fixation would decrease
Methods
Study Site: Experimental Lake L227
- 5 ha, 10-m max depth lake in the ELA
- Strong thermal stratification
- Artificially eutrophied with enhanced P since 1990
- High cyprinid (minnow) population
Manipulation
- 200 pike added in 1993-1994
Monitoring
- 1992-1996
- Minnow sampling
- C,N, and P assessed in organic and inorganic pools
- Sediment trapping for C, N and P
Results
- Minnow populations reduced by 4X at first, driven to extinction by 1996
- Zooplankton increased 4X in 1996, increasing in size, too, then crashing as invertebrate predators grew.
- Phytoplankton decline
- Seston C:P declines, bacteria decline, particulate C residence time declines, particulate P residence time declines
- Inorganic N:Dissolved P ratio increases
- N-fixing cyanobactria crash. Appear to only be present with TIN:TDP ratio is low enough. Need enough P for fixation to be energetically favorable
Discussion
- Zooplankton biomass became P-enriched via dominance by P-rich Daphnia species.
- Zooplankton went from 1% to 32% of the P pool, and locked up much of available P, and making N more relatively available.
- Food-web configuration modified sedimentation by changing particle residence time.
Thoughts:
- Given the Daphnia appearance, should we consider how the available species pool could affect each trophic level response?
- Alternative stable states: Wasn’t this just a long transient to a new equilibrium created by introducing the pike?
References
Cleveland, C. C., & Liptzin, D. (2007). C:N:P stoichiometry in soil: is there a “Redfield ratio” for the microbial biomass? Biogeochemistry, 85(3), 235–252. doi:10.1007/s10533-007-9132-0
Elser, J. J., Bracken, M. E. S., Cleland, E. E., Gruner, D. S., Harpole, W. S., Hillebrand, H., Ngai, J. T., et al. (2007). Global analysis of nitrogen and phosphorus limitation of primary producers in freshwater, marine and terrestrial ecosystems. Ecology Letters, 10(12), 1135–1142. doi:10.1111/j.1461-0248.2007.01113.x
Gruber, N., & Sarmiento, J. L. (1997). Global patterns of marine nitrogen fixation and denitrification. Global Biogeochemical Cycles, 11(2), 235–266.
McGroddy, M., Daufresne, T., & Hedin, L. O. (2010). Scaling of C:N:P stoichiometry in forests worldwide: implications of terrestrial Redfield-type ratios, 1–12.
Miller, T. E., Burns, J. H., Munguia, P., Walters, E. L., Kneitel, J. M., Richards, P. M., Mouquet, N., et al. (2005). A critical review of twenty years’ use of the resource-ratio theory. The American Naturalist, 165(4), 439–448. doi:10.1086/428681
Redfield, A. C. (1958). The biological control of chemical factors in the environment. American Scientist, 46(3). Reiners, W. A. (1986). Complementary models for ecosystems. American Naturalist, 59–73.
Vitousek, P. M., Porder, S., Houlton, B. Z., & Chadwick, O. A. (2010). Terrestrial phosphorus limitation: mechanisms, implications, and nitrogen-phosphorus interactions. Ecological Applications, 20(1), 5–15.
Walker, T. W., & Syers, J. K. (1976). The fate of phosphorus during pedogenesis. Geoderma, 15(1), 1–19.
Elser, J. J., R. W. Sterner, A. E. Galford, T. H. Chrzanowski, D. L. Findlay, K. H. Mills, M. J. Paterson, M. P. Stainton, and D. W. Schindler. 2000a. Pelagic C:N:P Stoichiometry in a Eutrophied Lake: Responses to a Whole-Lake Food-Web Manipulation. Ecosystems 3:293–307.
Elser, J. J., R. W. Sterner, E. Gorokhova, W. F. Fagan, T. a Markow, J. B. Cotner, J. F. Harrison, S. E. Hobbie, G. M. Odell, and L. W. Weider. 2000a. Biological stoichiometry from genes to ecosystems. Ecology Letters 3:540–550.
Lotka, A. J. 1925. Elements of physical biology. . Williams & Wilkins, Baltimore.
Sterner, R. W., and J. J. Elser. 2002. Ecological stoichiometry: the biology of elements from molecules to the biosphere. . Princeton University Press.