Teaching

Sample problem

There is great interest in understanding the role of ecosystem processes in regulating the balance of CO2 in earth’s atmosphere. Terrestrial ecosystems act as sinks for atmospheric CO2 when organic carbon, produced through photosynthesis, accumulates in soils and woody tissues. Aquatic ecosystems are sources of CO2 because decomposition of organic matter results in respiration rates exceeding photosynthesis. Catchments are units of the landscape that integrate runoff and are comprised of terrestrial and aquatic habitats. The net CO2 balance of a catchment will depend on the proportion of terrestrial and aquatic areas and relative rates of photosynthesis and respiration. Your task is to determine whether the Mirror Lake watershed is a net source or sink for atmospheric C.

To solve this problem, you will need to derive estimates of net primary production and respiration for stream, lake and forest habitats comprising the Mirror Lake watershed. Mirror Lake is the site of long-term investigations carried out by Gene Likens and colleagues participating in the Hubbard Brook ecosystem study. Using the data provided below, can you determine whether the watershed (as a whole) is a net source or sink for atmospheric carbon?

Photo of Mirror Lake from Hubbard Brook Web site.

Geographic data:

• Watershed area = 103 ha (includes lake and streams)
• Lake area = 15 ha
• Stream lengths = 1070, 1314 and 741 m
• Stream width (average) = 1.22 m
  

Net primary production:

• Forest = 704 g C m-2 y-1
• Lake = 62 g C m-2 y-1
• Stream = 1.7 g C m-2 y-1

Respiration:

• Forest = 650 g C m-2 y-1
• Lake = 67 g C m-2 y-1
• Stream = 101 g C m-2 y-1

E-mail me to get your answer or to get a solution hint.