Dissolved oxygen over three days

1
A comparison of methods for measuring aquatic
total system metabolism in six experimental
wetland cells Janet Ackerman, Kristin Braziunas,
Polly Gibson Systems Ecology (ENVS 316), Fall
2006, Oberlin College
BACKGROUND
METHODOLOGICAL ANALYSIS
Comparison of dissolved oxygen values from two
probes
We calculated net primary productivity (NPP) and
respiration (R) from both the in situ and hand
probe data. We compared these measurements of
TSM to determine the suitability of using dawn
and dusk to capture metabolic activity. We
also compared in situ and hand probe DO readings
to determine the reliability of the probes.
In light of the current emphasis on wetland
restoration and the growing understanding of the
ecological importance of wetlands, ecologists
need simple and accurate tools for assessing
wetland function. One important measure of
function is aquatic total system metabolism (TSM)
- the productivity (photosynthesis) and
respiration of aquatic organisms living in the
wetland. Since aerobic metabolism produces or
consumes oxygen, measuring the concentration of
dissolved oxygen (DO) in the water column is a
convenient way to assess wetland TSM. DO
generally increases over the course of a day as
photosynthesis produces oxygen and then decreases
overnight as respiration consumes it. The best
methodology for measuring TSM is unclear. There
are two common tools for measuring DO in aquatic
systems in situ probes, which automatically log
DO at specified intervals over a long period of
time, and hand-held probes, which are typically
used to measure DO at a much lower frequency. The
hand probe is cheaper and simpler to use, but the
more frequent measurements possible with the in
situ probe produce a more complete data set. This
project seeks to compare the data produced by
these two techniques so that future projects can
select the method best suited to their goals .
The high R² value indicates a strong linear
relationship between the two sets of
measurements. The slope is close to 1 and the
y-intercept is small, meaning that the two
probes gave us almost identical DO values.
RESULTS
Dissolved oxygen over three days
CONCLUSIONS

• Measurements of DO are consistent between the
  hand-held and the in situ DO probes.
• Rates of productivity and respiration within this
  wetland cell vary over the course of the day.
  This makes it difficult to use twice-daily
  measurements to calculate accurate rates.
• Dawn-dusk measurements with the hand-held probe
  provide a reasonable estimate of daily change in
  DO. However, taking two afternoon measurements
  of DO may be a better way to accurately determine
  a steady rate of productivity.
• FUTURE RESEARCH
• Our study was limited to a few short, fall-season
  days. Future research should collect data over a
  longer time period to get a more complete picture
  of patterns in DO fluctuations. Data collection
  during the summer is especially important, when
  long, sunny days foster high productivity.
• Other studies could quantify spatial variation
  within a single wetland cell or measure average
  daily productivity as a function of light.

QUESTIONS

• Are the hand-held and the in situ DO probes
  reliable (do they read consistent measurements)?
• How accurately do hand probe measurements at
  dawn and dusk capture wetland productivity in
  comparison with in situ readings? Are there
  better times to take measurements?

• We compared several methods for calculating rates
  of NPP and R
• Maximum rate regression of region of linear
  increase or decrease in continuous probe data
• Extrapolations from dawn and dusk values
  calculated based on afternoon DO increase
• Plateaus from peaks and troughs in continuous
  probe data
• Hand probe from dawn-dusk measurements with
  hand-held probe

METHODS
DATA COLLECTION
Our study took place in one of six ¼-acre
experimental wetland cells near Oberlin, OH in
late fall 2006.
We measured DO in the wetland over four
relatively sunny days with both an in situ probe
and a hand probe. All measurements were taken at
a marker in the northwest corner of the cell at a
depth of approximately 33 cm.
Calculated Metabolic rates (mg/L/hr)
ACKNOWLEDGEMENTS
We thank John Petersen for his dedication and
support throughout the course of this project and
the environmental studies department at Oberlin
College for providing us with equipment to
conduct our research. We also thank the New
Agrarian Center and the residents of the George
Jones Farm for sharing their space with us.
Finally, we thank our fellow classmates for their
valuable input, especially the Wetland Nutrients
group for sharing their research.
Janet and Kristin measure DO using the hand-held
probe