Turbidity is a measure of water clarity, specifically how much suspended matter reduces the capacity of light to penetrate the water. Materials that become mixed and suspended, reduce water clarity. During periods of heavy runoff after spring melting of snow and ice, or heavy down pours of rain, silt-laden water can be seen running into rivers and estuaries. One tool used to measure water clarity is the secchi disk. This activity is used to introduce students to the idea of turbidity and show them a simple way to measure how turbid water is. 

Teacher Note:  All engage activities may be conducted in one class period or presented as the content is covered in the web resource. The water clarity activity should take approximately 10 minutes if using 5 gallon buckets with three turbid conditions. If you use an alternative activity, plan accordingly.

Objectives

• Understand how turbidity interferes with light penetration
• Learn how to measure turbidity with a Secchi disk
• Relate turbidity to the impact on striped bass larval populations

Materials

• Chart Paper
• Index Cards
• Graduated Cylinders/measuring cups
• 5-gallon buckets
• Dirt
• Purchased or home made secchi disks (see Lesson Resources)
• Water

Procedure

• Ask students what makes water cloudy.
• Using 4 corners of the room have 4 charts with the following headings; Pool water, Chesapeake Bay Water, Atlantic Ocean Water and Patapsco River Water (or enter bodies of water in your local area with which your students are familiar).
• Give each student group a stack of index cards (4) with the same headings as the poster.
• Ask students to brainstorm for 30 seconds about the four bodies of water.   Unpack their thinking by having them write anything they know about the four bodies of water.  Students should come up with ideas about differences in water clarity size, location etc.
• When given the signal, students write their response on the index card and pass each card to the left and the next student adds ideas to the topic card just received.  
• Cards circulate within the group until each student has written on each card.  
• A team reporter reads one contribution from their team for the selected chart on the wall, in a round-robin fashion.  
• When one chart is completed, focus moves to the next chart. (Strategy:All Hands on Deck)  
• Following the discussion, students will be introduced to a tool called a secchi disk that is used to determine water clarity.
• Directions for making a secchi  disk can be found below in the Lesson Resources Section.  
• You can purchase the disks through your local science supply house. Use 5 gallon buckets with varying amounts of suspended dirt to demonstrate water clarity.  
• In the 1^st bucket, make the water clear with no sediments. Students lower the secchi disk, noting that it does not disappear in the bucket.
• Have students take a secchi disk measurement. In the 2^nd bucket, suspend 2 cups of dirt in the five gallon bucket and be sure to stir the contents in the bucket before taking the secchi disk measurement. Record the results.
• In the 3^rd bucket, suspend 4 cups of dirt in the five gallon bucket making sure to stir the contents before making the secchi disk measurement.  Record the results
• Ask students which bucket would they want to live in if they were a juvenile rockfish and why.  
• Answers will vary, but the hope is that they will choose the cloudiest water that will help fish to hide from predators.

National Science Education Standards

9-12 A: Science as Inquiry

• Abilities necessary to do scientific inquiry
• Understandings about scientific inquiry

9-12 B: Physical Science

• Structure and properties of matter
• Interactions of energy and matter

9-12 C: Life Science

• Matter, energy, and organization in living systems
• Behavior of organisms 

9-12 E: Science and Technology

• Understandings about science and technology 

9-12 F: Science in Personal and Social Perspectives

• Environmental quality 

Lesson Resources

Info on secchi disks:

Secchi disks can be ordered from scientific supply stores and can be found for less than $30.  If you choose to make your own, below are some basic instructions.

Obtain a white Frisbee, plastic lid, white plastic plate or laminated white piece of paper (circle). Cut out a circle with a diameter of 20cm. Punch a hole in the center.The color of the secchi disk will be split up into alternating black and white sections (2 white, 2 black, see photograph above). You can use a black permanent marker or black acrylic paint to color in the black portion of the disk. Tie a knot at one end of about 5 ft of rope and place string through the hole.  Attach some weights to the bottom of the disk so it easily sinks to the bottom of the bucket. Metal washers work well for this purpose. You may wish to mark the rope every cm for ease of taking measurements. Your new, homemade secchi disk is ready for use!