The Secchi Disk – A Classroom Simulation

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Indiana Expeditions Show: Life Science

Overview

Students will explore how a Secchi disk is used to evaluate the transparency of water.  Using graduated cylinders filled with different concentrations of powdered milk solution, students will measure the depth at which a miniature Secchi disk is visible.

Background Information

Secchi disks have been used for over 100 years to evaluate the transparency of water in lakes and other large bodies of water.  The disk is lowered into the water on a rope or tape until it can no longer be seen and is then slowly raised until is it just visible.  The depth level reading on the rope or tape at the surface of the water is then recorded. 

Tracking water transparency from week to week using a Secchi disk can provide a great deal of information about the physical, chemical, and biological processes taking place in the water.  Changes in water transparency, as indicated by different depth readings over time, are often related to the seasonal changes within bodies of water. This can alert investigators to look for possible explanations for what is causing the change. 

For example, if the depth at which the Secchi disk is visible decreases, it might indicate an increase in:

·  a seasonal change in the rate of photosynthesis;

·  nutrients available to support plankton growth;

·  soil erosion from surrounding areas; or

·  turbidity (cloudiness created by suspended particles) from recreational boating activities.

Connections to the Indiana Academic Standards for Science

            3.2.4, 3.2.7, 3.5.1, 3.5.2, 3.5.3, 3.6.3, 4.1.2, 4.1.5, 4.2.1, 4.2.4, 5.5.1, 5.6.4, 6.1.2,

            6.2.5, 6.2.6, 6.2.8, 6.5.2, 6.5.4, 7.1.4, 7.4.14

            If alternatives and/or extension activities are used: 3.2.2, 3.2.5, 5.1.6

Science Process Skills

·  Communicating

·  Constructing and interpreting data tables and graphs

·  Developing hypotheses

·  Measuring

·  Observing

·  Predicting

Estimated Time Requirements

Two 45- minute sessions

Materials

·  five 500mL graduated cylinders

·  five rulers

·  five miniature Secchi disks (see “Constructing Miniature Secchi Disks” directions)

·  water

·  powdered milk

·  balance

·  stirring rod

·  five small, flat-bottomed buckets (optional)

·  one “Secchi Disk Data Table” handout for each student

·  one “Sample Secchi Disk Measurements” handout for each student

Objectives

Students will be able to

1.      Demonstrate and explain how to use the Secchi disk.

2.      Generate possible explanations for variations in Secchi disk measurements.

Procedure

Preparation prior to the lesson: 

·  Construct the miniature Secchi disks as described in “Constructing Miniature Secchi Disks”. 

·  Prepare the milk solutions as follows:

o     Label each graduated cylinder with one of the following labels: 0 grams, 0.2 grams, 0.4 grams, 0.6 grams, and 0.8 grams.

o     Add the corresponding amount of powdered milk to each of the graduated cylinders.

o     Add approximately 100mL of water to each graduated cylinder.

o     Gently swirl and shake the graduated cylinders to dissolve the powdered milk.

o     When the milk is completely dissolved, add enough water to the graduated cylinder to fill it to within 2cm of the top of the graduated cylinder.

o     Use a Secchi disk to help stir the solution by raising and lowering the disk in the solution.

Note:  As an alternative, students can measure and prepare the milk solutions prior to beginning the activity.

·  Place the graduated cylinders in five different locations throughout the classroom.  Place each graduated cylinder in a flat-bottomed bucket to prevent drips and spills, if desired.

·  Make a copy of the “Secchi Disk Data Table” and “Sample Secchi Disk Measurements” for each student.

Anticipatory set:

·  View the Life Science Indiana Expeditions segment.

·  Ask the students about their experience with observing lakes or other bodies of water.  When they look into a lake can they see the bottom?  Explore the students’ thinking about how far down they can see.

Lesson sequence:

·  Explain to the students that they will be exploring how far down they can see in some simulated lakes.  (i.e. The graduated cylinders with different concentrations of powdered milk solution.)

·  Ask students to predict which solution, when they look straight down into the graduated cylinder, they think will be the easiest to see through to the bottom.

·  Show students the miniature Secchi disk and elicit from them how to use the disk to make measurements to either support or refute their predictions.

·  Establish a common procedure for measuring Secchi disk depth.  (Note: It is critical to look at the Secchi disk directly from the top of the graduated cylinder, not from the side!)

·  Divide the students into five teams.  Each team will start at one of the graduated cylinders and record their data for the depth at which they can see the Secchi disk in that graduated cylinder.

·  Teams rotate through the five stations until all teams have had a chance to record data for each station.

·  Graph the results.  (Class data can be collated and averaged or students can work with their own data.)

·  Discuss the results and their meaning.

·  Make the connection between how investigators use Secchi disks in the field (see Background Information) and how the students used the Secchi disk in the classroom. Include the implications of the measurements and what those might indicate to an investigator.

Closure:                      

Ask students to generate some possible explanations for why they can see the bottom of a body of water and why, at other times, they cannot to see the bottom of the body of water. 

Suggested Student Assessment

Objective #1:

            Provide students with other liquids and a Secchi disk and ask them to demonstrate and/or explain how they would use the Secchi disk to measure the transparency of        the solution.

Objective #2:

            Provide students with the “Sample Secchi Disk Measurements” handout and ask them to generate possible explanations for the changes in the Secchi disk

            measurements over time. 

Extending the Lesson

·  Let students investigate Secchi disk design and history and then design and construct their own Secchi disk from scrap materials.

·  Place water from a pond in several graduated cylinders.  Take a Secchi disk measurement for the water.  Add different amounts of fertilizer to the various graduated cylinders and allow the water to sit in a sunny spot for two to four weeks.  Take Secchi disk measurements for the various containers.  Graph and discuss the results.

Source of Lesson

Deb Sachs
Director, Office of Professional Development
School of Education
University of Indianapolis

Constructing Miniature Secchi Disks

(for use with 500mL graduated cylinders)

Materials required:

Hex nuts – size: 8-32

Fender washers – size: 3/16 x 1 ½

Nylon string – size: #36

Sandpaper – fine or medium grade

White spray paint – satin or flat

Black permanent marker

Hot glue gun and glue sticks

Flame (matches or candle to melt cut ends of string)

Directions:

1.      Use the sandpaper to roughen one of the flat surfaces of the washers.

2.      Spray paint the sanded surface and the sides of the washers using the white spray paint

3.      Cut the string into 90cm lengths.  Burn one end of the strings to keep them from unraveling.  Tie a knot in the other end of the strings.

4.      Use the black permanent marker to color the Secchi disk as illustrated in figure 1.

	Figure 1 – the Secchi disk

5.      Thread one nut, then one Secchi disk washer (with the colored side toward the knot in the string), then a second nut, onto the string.  Tie a knot at the burned end of the string and slide the nuts and Secchi disk washer to that end. 

6.      Use the hot glue gun to glue both nuts (and the string inside the nuts) to the Secchi disk washer.  The nuts and washers should be glued at the knotted, burned end of the string.

Secchi Disk Data Table

Name________________________________________________________

The Effects of Different Amounts of Dissolved Powdered Milk on the Depth at Which a Secchi Disk Remains Visible

Sample Secchi Disk Measurements

The following Secchi disk measurements were taken from April through October at a local lake.  Generate some possible explanations for the changes in the Secchi disk measurements over time.  For each possible explanation, be prepared to explain what additional information would be helpful to begin to provide more support for your idea.