Credits
Rich Axler, Glenn
Merrick, and Bruce
Munson developed this lesson.
Goals
Students
will investigate the effects of respiration on water chemistry and a
lake ecosystem.
Introduction
Respiration by
visible organisms (minnows or aquaria fish) and by "invisible" organisms
bacteria uses oxygen to oxidize organic materials and
releases energy, water, and carbon dioxide. In this activity students
will use laboratory microcosms and WOW data to explore the affects of
respiration on the water chemistry of lakes. In this lesson students
learn that the simple act of respiration can have profound effects on
an ecosystem.
Students
can meet the goals for this lesson by completing a directed study or
a student inquiry lesson.
In the directed
study lesson students set up a microcosm. Dissolved
oxygen (DO) and pH are used as indicators of the complex chemical
interactions that occur. Students complete a worksheet corresponding
to the laboratory experiment. They also analyze RUSS data for DO and
pH and complete a corresponding worksheet. They should print the worksheet
and microcosm set-up directions.
The directed
study lesson is found in the student section of WOW under the title:
"Studying Aquatic Respiration."
The student
inquiry lesson asks students to research water quality measures with
respect to aquatic
respiration. They choose to analyze one parameter
(measured by RUSS) that correlates to aquatic respiration. They analyze
that measure using a microcosm and RUSS data. The instructor specifies
a format for the final presentation: written paper, oral report, poster,
or multi-media presentation. Students need to print directions for this
lesson.
The student
inquiry lesson is found in the student section of WOW under the title:
"Investigating Aquatic
Respiration."
Outcomes
Students
will be able to:
- Provide
data collected from microcosms and RUSS monitored lakes as evidence
that respiration occurs and can be quantified in aquatic settings.
- Describe
the effects of respiration on a body of water.
- Design
an appropriate water sampling regimen for assessing pH and DO (or the
selected water quality measure in the student inquiry lesson) in the
hypolimnion of a lake.
- Explain
how organisms affect pH and DO (or the selected water quality measure
in the student inquiry lesson) in the hypolimnion of a lake.
Keywords
Respiration,
pH,
dissolved
oxygen, conductivity,
detritus,
hypolimnion
Materials/Resources/Software
Part I - Lab
Water Quality
Testing Equipment and Materials (per student group)
- Hach or Lamotte water quality testing materials or meters for each
group's workstation (for analyzing DO and pH). Each group will make
up to 5 pH and DO analyses with a meter or two analyses with chemical
reagent kits.
- 250 ml
bottle with a cap
- Each
group should receive at least one of the following three options below
to place in their jar:
- 4 - two-inch minnows (fathead minnows are readily available
and tolerant of low oxygen levels)
- 75 -100 ml of sandy sediment
- 75 -100 ml of organic/mucky sediment
- Respiration in a
Microcosm Worksheet for groups doing the directed study lesson
- Thermometers (if water temperature is the selected measure for the
student inquiry lesson)
- Conductivity pens (if conductivity is the selected measure for the
student inquiry lesson)
Part II
- Technology Application
- Aquatic Respiration
in a Lake Worksheet for groups doing the directed study lesson
- Access to WOW data on the Internet
Time
Required
The lesson
is completed in two parts. Generally, the lesson can be completed in
two class periods, but students will need some time to finish homework
before the final results are handed in. Part I requires one hour in
a lab setting. Part II also requires about one hour. Student will need
access to computers with Internet capabilities or a data file that has
been downloaded from the website in order to complete Part II.
Curriculum
Connections
Biology - respiration,
lake stratification,
detritus
Chemistry
-chemical equilibrium, pH,
dissolved
oxygen, carbon
cycle
WOW
Curriculum Links
Effect
of Photosynthesis and Respiration on Aquatic Respiration, Effect
of pH of Aquatic Organisms
Procedure
Part
I - Lab Microcosm Studies
Knowledge
Base
Discuss
students observations of events such as aquaria that have been
overfed or left untended for prolonged periods of time, or minnow buckets
that have been left without fresh water for a prolonged period. What
happens? Why do the animals often die in these situations? What water
quality measures might correlate to this situation? Why do students
think similar situations might, or might not, occur in lakes? What
water
quality measures might correlate to aquatic respiration?
Notes:
Respiration by visible organisms (minnows or aquaria fish) and by "invisible"
organisms bacteria uses oxygen to oxidize organic materials
and releases energy, water, and carbon
dioxide. In a system where respiration exceeds the input of oxygen
(through photosynthesis
or aeration), there will be a measurable decline in dissolved oxygen
and a corresponding decline in pH due to the addition of CO2
to the system. A lack of dissolved oxygen or a significant decline in
pH
can kill aquatic life.
Student
Inquiry
Ask students
to research the effects that respiration can have on water quality,
pH,
DO,
conductivity,
and turbidity
(see RUSS Unit section). Students
should use the WOW page and links. They should select the water quality
measure that they feel most strongly correlates to aquatic respiration.
Experimental
Design
Divide students
into lab groups of two to four and ask them to complete either the
directed study ("Studying Aquatic
Respiration") or student inquiry ("Investigating
Aquatic Respiration") lesson. Explain that when conducting
scientific research, scientists need to carefully design experiments
to answer the relevant questions. Students should reflect on the relationships
among the questions being asked in both parts of this lesson.
Suggest
students divide chemical testing work within their group so that everyone
gets hands-on experience doing the testing. Note that test kit instructions
need to be followed precisely. (A practice test with water of known
dissolved oxygen content and pH may help build students skills
and confidence.)
Data
Collection
Introduce
the microcosm study. Discuss procedural variables that could influence
the outcomes of students' measurements.
Directed
Study
Have one
third of the class work with minnows in their lab microcosms. One third
will work with sandy sediment, and the remaining third will work with
an organic sediment. Review the procedures for using the water test
kits or meters. Ask students to make predictions for DO and pH in each
of the three microcosms.
Test the
DO and pH concentrations in samples of the remaining aged water using
the water chemistry kits or meters. If using Hach or Lamotte kits, proceed
by testing the oxygen first in each bottle. They should perform the
tests rapidly to minimize the development time between microcosms. After
30 minutes, retest the DO and pH in each of the 3 microcosms. Have students
record their results on the worksheet.
Student
Inquiry
Students
need to choose a water quality measure to analyze. They should research
the effects that respiration can have on at least one other WOW parameter
(water temperature, pH, DO, and conductivity) using the WOW page and
links. They should choose a measure that they feel correlates strongly
to aquatic respiration. You may want to limit the number of variables
(sediments and fish) and amount of those variables the students can
use for their lab experiment.
Data
Management and Analysis
Directed
Study
Refer students
to this section of the "Respiration
in a Microcosm Worksheet."
Student
Inquiry
Students
should graph their data and summarize their results. Remind students
to title their graphs and label legends, axes, and units of measure.
Interpretation
of Results
Directed
Study
Refer students
to this section of the "Respiration
in a Microcosm Worksheet."
Student
Inquiry
Ask students
to share possible explanations for their experimental results.
Notes:
Organisms are respiring in all of the microcosms. The minnows are obvious.
In the other microcosms bacteria are respiring. In all cases organisms
are using up the available oxygen and giving off carbon dioxide. The
number of organisms available in each microcosm and the metabolism of
those organisms will affect the rate and amount of DO used, carbon dioxide
given off, and resulting changes in pH. The
buffering capacity of the
water will also affect the decline of pH in the microcosms.
Reporting
Results
Directed
Study
Students
should turn in their completed "Respiration
in a Microcosm Worksheet."
Student
Inquiry
Students
should develop a presentation using the format specified by the teacher:
written paper, oral presentation, poster, or multi-media presentation.
The final presentation should reflect both parts of the lesson.
Part
II - The Effects of Respiration in a Lake
Knowledge
Base
The WOW
data visualization tools can help illustrate changes in DO and pH (see
Figures 1 & 2). These changes could also be demonstrated by advancing
the Profile Plotter
through several sampling periods or by creating graphs in Excel (see
Figure 3). You may want to display these for the students. This could
be done either during your initial discussions for this lesson, or as
part of the discussion and closure for the lesson.
Discuss
students perspectives on the possible relationships between the
lab microcosms and what might be found in a lake.
Figure
1. DO Profiles for Ice Lake

Figure
2. pH Profiles for Ice Lake

Figure
3. Monthly Records for Ice Lake DO and pH at 10 m

Experimental
Design
Directed
Study
Direct students
to the Aquatic Respiration
in a Lake Worksheet. Students can begin work on Part II while completing
the microcosm study if they have time, but they will need additional
time and access to WOW data in order to complete Part II. (If Internet
access is difficult, you could provide students with data files downloaded
from the WOW web site.)
Student
Inquiry
Students
are placed in the role of consulting biologists trying to answer questions
from a lake association. The students should determine how they will
analyze WOW data for the water quality measure they selected for Part
I. A written plan should be developed that describes how the students
will answer the Lake Associations questions.
Data
Collection
Remind students
of the need to carefully collect and record their data in completing
Part II. Students also need to reflect on the quality of the data. There
is always the possibility that inaccurate data have been collected or
posted to the WOW web site. Scientists need to reflect on the quality
of any data that are used.
Directed
Study
Students
complete the table in the Aquatic
Respiration in a Lake Worksheet.
Student
Inquiry
Students
should create a table to record data for their selected water quality
measure. They should also write a description of their data collection
plan as a protocol that others could follow. Remind students they only
need to record measurements from the hypolimnion.
Data
Management and Analysis
Directed
Study
Students
will graph dissolved oxygen and pH for their sampling periods. They
should be reminded to label the axes and include proper titles, legends,
and units of measure.
Student
Inquiry
Students
should create a graph that illustrates changes in the selected water
quality measure.
Interpretation
of Results
Discuss
the students results. (Brief oral presentations by the student
lab groups can be useful in comparing group approaches and results).
Discuss
the following questions:
- Over the
course of the summer, did pH and DO continuously decline near the bottom
of the lake?
- What similarities
and differences are observed in comparing the group graphs and results?
- Why might
some lakes have sudden changes in DO and pH near the lake bottom during
the summer?
Notes:
Lakes that remain stratified
and are highly productive will usually demonstrate a continuing decline
in pH and DO during the summer months due to bacterial action in the
sediments. Some lakes may go anoxic
(lacking any DO in the depths). Other lakes may have sudden increases
in DO and pH if the water is mixed through events such as high winds
(this also depends on basin
shape and lake depth).
Directed
Study
Refer students
to this section of the Aquatic
Respiration in a Lake Worksheet.
Student
Inquiry
Students
should write a summary of the changes in the selected measure.
Reporting
Results
Directed
Study
Students
should turn in their worksheet and graphs.
Student
Inquiry
Students
should develop a presentation using the format specified by the teacher:
written paper, oral presentation, poster, or multi-media presentation.
The final presentation reflects both parts of the lesson.
Teacher
Notes
A steady "rain" of detritus
(dead stuff, mostly algae
and particulate material washed into the lake from the watershed)
falls to the bottom of lakes. 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 May-November depending on
the basin shape, lake depth and weather.) In the sediments at the bottom
of the lake (at the base of the hypolimnion),
the detritus is decomposed by bacteria through the process of respiration.
The bacterial processes of respiration release the potential energy
stored in the chemical bonds of the organic carbon compounds, consuming
oxygen in oxidizing the compounds, and releasing carbon dioxide (CO2).
This CO2 rapidly dissolves in water to form carbonic acid
(H2CO3), bicarbonate
ions (HCO3-) and carbonate
ions (CO3--). The relative amounts of these
depends on the pH of the water. The newly formed carbonic acid gradually
decreases the pH of the water. The ions produced in the water as CO2
dissolves increase the TDS
(total dissolved solids), and therefore, increase the electrical
conductivity (EC) in the water.
CO2
+ H20 <-----> H2CO3 <----->
H+ + HCO3-
Resources
- Behar, S., Dates, G., Byrne, J. (1996). Testing the Waters. River Watch Network: Montpelier, VT.
- Cole, G.C. (1988). Textbook of Limnology. Waveland Press: Prospect Heights,
IL.
- Munson, Bruce A. (1992). A Primer on Limnology (Second Edition).
Public Report Series #6. Water Resources Research Center, St. Paul,
MN.
Extensions
- Compare DO and pH of lakes during different seasons.
- Use a similar lesson approach to see if you can observe pH and DO
changes in a lake during a 24-hour period.
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