Reading Questions
Helfield and Naiman 2001. Effects of salmon derived nitrogen on riparian forest growth and
implications for stream productivity.
1) The effect of marine-derived nitrogen was not observed on red alder trees. What
other important nitrogen cycling process do the authors attribute this observation
to? In your own words, briefly describe this process.
a. Nitrogen fixation
b. The conversion of relatively inert atmospheric N (N2) to bioavailable ammonia
(NH3). Certain prokaryotes/bacteria (diazotrophs) are capable of N-fixation, some
plants, and some animals (e.g. termites) who have formed mutualisms with gutdiazotophs.
2) Throughout the paper, the author mention the process of isotopic fractionation.
What is isotopic fractionation and what kind of ecological deductions can be
inferred from it?
a. During biological reactions, lighter isotopes react at a faster rate than heavier
isotopes. With respect to nitrogen, organisms process, digest and excrete (i.e.
metabolize) more of the lighter isotope (i.e. 14N). This results in enrichment of 15N
relative to their diet.
b. The trophic status of an organisms can be established by looking at the relative
enrichment of 15N compared to their diet. Where an organisms is feeding on a
food chain/web can therefore be established. This can be very useful for example
to determine how events of disturbance affect food webs.
3) The authors briefly discuss (in the Results/Discussion section) other hypotheses to
explain elevated foliar nitrogen and lower C: N ratios – other than being marine and
salmon-derived. Briefly describe these other hypotheses and how the authors
account for them as non-contributing mechanisms to increased foliar N.
a. Other studies have observed the enrichment of 15N in valley bottoms (i.e. the
bottom of the watershed) relative to upland sites. In this study however, there was
no correlation between 15N values and distance from the stream.
b. Nitrogen fixation via alder might influence how N is distributed into soils.
Authors account by this by stating that the total riparian forest was comprised of
relatively litter alder thus their effect (if any) would be small.
4) This paper is a correlative study and there could be other factors that influence
foliar N in addition to what the authors account for in question three. Briefly
describe how you would design a manipulative experiment (field or lab-based) to
test the effects of salmon-derived nitrogen on riparian.
a. Answers will vary. One example: greenhouse study to hold all other variables
consistent and amend with salmon carcasses and appropriate experimental
controls.
5) The authors end with a discussion of how marine-derived nitrogen can affect stream
ecosystem processes; however, they did not perform any experiments of provide any
empirical data. What are some of the mechanisms by which they propose that
stream productivity can be influenced by marine-derived salmon? Do you accept
these ideas based on the data that is provided?
a. Example: Foliar N is increased. Leaf litter entering streams (allochthonous input)
will potentially provide more N relieving any N limitation that might exist for
stream organisms.
b. Example: Bigger trees equals more shading keeping streams cooler.
c. Answers will vary regarding the student’s acceptance of author’s ideas.