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  Aquatic Respiration in a Lake
 

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: ___________________________

Date

Time

Depth

DO

pH

Comments

           
           
           
           
           
           

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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date last updated: Saturday March 06 2004