Pretty raw notes this week from a discussion about how food webs respond to nutrient starvation and excess.

Whalen on consumption in Food Webs

  • Boersma and Elser (2006) - fitness cost of excessive P. Implications for models - but what about competition as a more likely limiting factor?
  • Not sure about the “food left on the table” argument.
  • What about the metabolic costs? Not mentioned in the paper.
  • Nonlinearity in P-growth relationship could be coexistence mechanism
  • Raubenheimer and Simpson (2004) - Fitness and body composition can be affected by food quality. Can’t maintain homeostasis. With locusts, fitness maximized at protein/carbohydrate balance, but body composition changes
  • Plath and Boersma (2001): Genralists may change intake behaviorally in response to P enrichment
    • Dynamic energy models of Daphinia S. Kooijman.
  • Raubenheimer and Simpson (2004) - organisms reach different compositions on monoculture diets, but diverse on polyculture diets (They promote geometric framework)
  • What are the evolutionary adaptations to food quality (gut architecture, consumption physiology, out-of-body digestion, feeding at multiple trophic levels/guilds (Some evidence of N-limited predators), seasonal issues). Cannibalism is optimal, but other costs? (Disease, etc.)
  • Apple et al. (2009) - High density of plants leads to low-P plants, making food quality low and reduction in predator abundance. Note that the plant (lupines) are N-fixers. However, there are other potential explanations (predation + plant cover)
  • Hawlena and Schmitz (2010): Predation causes more energy use in grasshoppers, increasing C consumption and then reducing fecal elemental composition. Higher C:N. Then changes the composition of plants, which are fertilized by grasshoppers.
  • What’s the correct scale? For higher trophic levels, is it more about macromolecules? Ecosystem v. 2-species interaction.

John Haggerty on the microbial loop

  • Moore et al. (2001) Diatoms are iron and silicate limited. Different organisms limited in different areas. But this model doesn’t incorporate light limitation, nor secondary limitation. Moore et al.
    1. Found high P content in autotrophic communities, low in heterotrophs
  • Most cyanobacteria are N fixers, but not those in Bertilsson et al.
  • Berman-Frank and Cullen (2001) - N fixation is iron lmited, results in loss of photosynthetic
  • Kustka and Sanudo-Wilhelmy (2003) - Swotch between Fe and P limitation at distinct N:P ratio
  • White et al. (2006) - differences between lab and field. Organisms are flexible, but co-limitation limits extreme variation
  • How to reconcile Frigstad and Andersen (2011) with Bertilsson et al. (2003)? Filtration artifact?
  • Gruber et al. (2009) Ciliates prefer older E.Coli with higher levels of carbon.

Notes on Boersma and Elser (2006)

Central idea: There’s a cost to consuming foods with excess nutrients and needing to excrete the extra.

(Given that human babies can die from excess nitrate in the water, I must agree)

Many lakes are P-limited, but it has been assumed that variation in P:C ratios above levels that limit growth - the threshold element ratio (TER) - is irrelevant to animal performance

Many of these studies come from aquaculture. Several taxa (Fig. 1) have a peak of growth at some fraction of dietary P. Ecological studies are rarer, typically using only 2 levels of treatment.

Possible mechanisms:

  • In plants, excess P “might have changed other variables in the plants, such as carbohydrate availability”. What?
  • Since P enrichment is rare, organisms don’t adjust well to being carbon limited, reducing growth.
  • Experimental limitations, but also organisms that store P, have prevented this observation in ecological systems

Context

  • Basal consumers in food webs are generally nutrient-deprived and more more likely not to have mechanisms to deal with excess nutrients. Similarly slow-growing and other low P-content organisms.
  • There can be considerable heterogeneity in nutrient content, leading to localized excesses in nutrients. This may be why we see what appear to be obvious food items under-utilized.
  • What about energetic cost of excretion?

Notes on Bertilsson et al. (2003)

Cyanobacterium Prochlorococcus is the numerically dominant autotroph in the oceans, mostly present in warm waters. Synechococcus is important, too, and bigger and more widely distributed, making them equivalent in global biomass.

Ocean POM stays close to the Redfield ratio (106:16:1), but there is considerable spatial and seasonal variability, in part due to organismal constraints.

Here they report on the plasticity of these organisms’ C:N:P ratio. This requires pure straints, only recently available.

The organisms differ in pigment size and motility.

When growth plateaued, P was added to verify P limitation.

Cutures reached stationary phase quicker in P-limited culture, and growth rates were lower in P-limited media, though not significantly.

P-limited cells had greater C content (by about 33%), as well as volume. Possibly because of need for more P in order to divide

Change in N content was variable. Only one species showed a difference, being N-enriched when P-enriched. Mostly C:N was tightly constrained.

But P-replete species gained 4X more P. (N:P ratios were 16 and 800, 50X difference)

Overall, all species had high C:N ratios, appear to have been P limited. P enrichment lower C:P significantly. These organisms have low P requirements, but can make use of more.

It’s possible these organisms are missed due to filtration techniques (less likely), or higher trophic levels are P-enriched (more likely).

P-starved level is likely the minimum, due to requirements just for maintaining a copy of DNA. (50% of measured amount)

DNA/RNA ratio must be 1:1, while in most organisms RNA is much greater.

Low P particles may be less appealing to predators (but don’t they have to eat more then?)

Low P-content may be a strategy to reduce virus-driven mortality.

These organisms may limit growth if there were more N in the ocean.

References

Apple, J. L., M. Wink, S. E. Wills, and J. G. Bishop. 2009. Successional change in phosphorus stoichiometry explains the inverse relationship between herbivory and lupin density on Mount St. Helens. PloS one.

Berman-Frank, I., and J. T. Cullen. 2001. Iron availability, cellular iron quotas, and nitrogen fixation in Trichodesmium. Limnology and ….

Bertilsson, S., O. Berglund, D. M. Karl, and S. W. Chisholm. 2003. Elemental composition of marine Prochlorococcus and Synechococcus: Implications for the ecological stoichiometry of the sea. Limnology and … 48:1721–1731.

Boersma, M., and J. J. Elser. 2006. Too much of a good thing: on stoichiometrically balanced diets and maximal growth. Ecology 87:1325–1330.

Frigstad, H., and T. Andersen. 2011. Seasonal variation in marine C: N: P stoichiometry: can the composition of seston explain stable Redfield ratios?. ….

Gruber, D. F., S. Tuorto, and G. L. Taghon. 2009. Growth phase and elemental stoichiometry of bacterial prey influences ciliate grazing selectivity.. The Journal of eukaryotic microbiology 56:466–71.

Hawlena, D., and O. J. Schmitz. 2010. Herbivore physiological response to predation risk and implications for ecosystem nutrient dynamics. Proceedings of the National ….

Kustka, A. B., and S. A. Sanudo-Wilhelmy. 2003. Iron requirements for dinitrogen-and ammonium-supported growth in cultures of Trichodesmium (IMS 101): Comparison with nitrogen fixation rates and iron:. Limnology and ….

Moore, J. K., S. C. Doney, D. M. Glover, and I. Y. Fung. 2001. Iron cycling and nutrient-limitation patterns in surface waters of the World Ocean. Deep Sea Research Part II: ….

Moore, S. a, T. J. Wallington, R. J. Hobbs, P. R. Ehrlich, C. S. Holling, S. Levin, D. Lindenmayer, C. Pahl-Wostl, H. Possingham, M. G. Turner, and M. Westoby. 2009. Diversity in current ecological thinking: implications for environmental management.. Environmental management 43:17–27.

Plath, K., and M. Boersma. 2001. Mineral limitation of zooplankton: stoichiometric constraints and optimal foraging. Ecology.

Raubenheimer, D., and S. J. Simpson. 2004. Organismal stoichiometry: quantifying non-independence among food components. Ecology.

White, A. E., Y. H. Spitz, D. M. Karl, and R. M. Letelier. 2006. Flexible elemental stoichiometry in Trichodesmium spp. and its ecological implications. Limnology and oceanography.