Group Names:
_______________________________________________
Date: _______________________
Introduction
A steady
"rain" of detritus falls to the bottom of lakes. Detritus
is dead stuff, mostly algae
and particulate material washed into the
lake from the
watershed.
This "rain" of detritus is greatest during the most productive time of year.
This productivity coincides with the period when lakes are thermally
stratified for long periods of time. In Minnesota this might be from
May to November depending on the basin shape, lake depth, and weather.
The bacterial processes of
respiration
decompose detritus at the bottom of lakes (at the base of the
hypolimnion)
and affect the amount of
pH
and dissolved oxygen in the hypolimnion.
Experimental
Design Select one
of the lakes that offers RUSS derived data via the Internet. Review
the general information available about this lake. Picture yourselves
as lakeshore owners concerned about DO and pH levels in the depths of
your lake. The research question you want to resolve is, "How does
the process of respiration change the pH and DO in the hypolimnion of
your lake during the summer?"
1. Which
lake will you investigate?
Your lakeshore
association has decided it can afford to have six water quality analyses
done on the lake during the summer (between May 1 and November 1). Your
group needs to select one date and time per month to conduct those analyses.
Record your groups' choices of analysis dates and times in the
table below.
2. Choose
one date and time per month (between May 1 and November 1) to analyze
pH and DO in the hypolimnion. Enter those dates and times in the table
below.
3. Write
a brief explanation and be prepared to orally present your choices to
the rest of the class. (Which dates would you choose and why?)
4. Based
on your experience with your class microcosms, predict what your group
thinks would happen to the pH and DO in the bottom of the lake (in the
hypolimnion) during the summer. Why do you think this would happen?
Data
Collection 5. Complete
the following table.
For the
purposes of this study, you will record only measurements closest to
the bottom of the lake in the table.
Lake Name:
___________________________
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Time
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Depth
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DO
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pH
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Comments
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Data
Management and Analysis 6. Graph
dissolved oxygen and pH by date over your sampling period. This may
be done manually or using a spreadsheet such as Excel (see instructions
for graphing with the WOW Excel template.) Be sure you have labeled
axes and included proper titles, legends, and units of measure on your
graph before it is turned in or presented in class.
Interpretation
of Results 7. Summarize
the changes in pH and DO measurements in the hypolimnion of your lake
over the summer.
8. Why might
some lakes have sudden changes in DO and pH in the hypolimnion during
the summer?
9. How might
pH and DO vary during different seasons?
10. How
might pH and DO vary during a 24 hour period?
Reporting
Results
Turn in
your worksheet, including the data table, graph, and answers to all
of the questions.
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