Reading Questions
McDowell and Fisher 1976. Autumnal processing of dissolved organic matter in a small
woodland stream ecosystem.
1. Briefly describe in your own words, the scale that this research in occurring at. Do
you think it is a small or large scale? How might results be scaled to a larger scale
(e.g. ecosystem)?
a. Answers will vary.
b. Authors provide the important first step of how this research can be scaled by
estimating the amount of litter entering the stream as a mass per area value.
2. Dissolved organic matter (DOM) is important to stream ecosystems. The authors
calculate that 42% of the total DOM is derived from the leaching of leaf litter. What
are other possible sources for the remaining 58% of the DOM?
a. Mostly from the terrestrial landscape and DOM that is produced in the soils
including the leaching of leaf litter on the forest floor.
b. Other sources could include: root exudates, and DOM produced my aquatic
macrophytes through photosynthesis, and cellular lysis from bacterial and fungal
cells.
3. What are possible mechanisms by which DOM is removed from the stream water?
That is: what are some pathways that would lower the concentration of DOM in a
stream?
a. Biotic uptake: DOM can be incorporated as biomass while some DOM may be
used metabolically to produce ATP (cellular energy) and lost via respiration as
CO2. Some DOM that becomes biomass can also subsequently leave the stream
(e.g. emerging insects that return to the forest). DOM can also move downstream
and can be stored in sediments.
b. Concentration could also be lowered by increased water volumes (e.g. rain event).
4. Examine Figure 5. Which of the three leaf litter types (beech, birch, and maple) has
the fast rate of decomposition? Based on these data develop a hypothesis for the
quantity and quality (i.e. bioavailability) of DOM leached among the three species.
a. Maple has the fastest rate of decomposition, birch is intermediate and beech is the
slowest.
b. Answers will vary. Example: Maple will leach the most DOM which will also
support the highest rates of respiration (i.e. Maple DOM is highly labile).
5. Why do the authors think that DOM that enters the stream through the forest floor
is different than DOM entering the stream directly from the leaching of leaf litter?
How might this affect the fate of DOM and thus rates of stream respiration?
a. The authors think that DOM that begins in the soil will be initially decomposed
and processed (and re-processed) by soil microbes increasing the recalcitrance of
the DOM. Thus when this pool of DOM enters the stream it is less bioavailabile
compared to fresh DOM that enters when new leaf litter directly enters the stream
channel. More recalcitrant DOM may experience less processing via aquatic
microbial processes and be associated with lower rates of respiration.
6. Based on the data presented here, does the species composition of a riparian forest,
with respect to presence and abundance, affect the way carbon (e.g. DOM) is cycled
within a stream ecosystem? How might these data have been different had the study
stream been adjacent to a monoculture of beech trees?
a. According to these data the species composition of a riparian forest can influence
the cycling of DOM within a stream ecosystem via variation in the bioavailability
of plant-derived nutrients (e.g. DOM).
b. A monoculture of beech, for example, would contribute more recalcitrant leaf
litter to the stream and less total DOM. This DOM is also potentially more
recalcitrant thus providing less DOM-derived C and N for aquatic organisms.