Exploring water clarity with a DIY Secchi disk

In this mini lesson, learners will investigate water clarity by building a simple Secchi disk and conducting basic tests in different environments. This activity should help learners understand water clarity and its importance in assessing water quality, as well how different materials affect water clarity.

Recommended grades

This activity is recommended for grades 4 to 6. We’ve included tips and tricks to adapt the activity for a wide range of students throughout the lesson.

Learning outcomes

• Build a Secchi disk out of everyday materials.
• Demonstrate the use of a Secchi disk to measure water clarity in different types of environments.
• Explore the concept of water clarity and its influencing factors, including matter, particles, and minerals.
• Investigate the effects of natural processes, pollution, and depth on water clarity.

Lesson

Background

What is water clarity?

Baseline testing is important as each body of water will have unique factors contributing to water clarity. Some very healthy lakes will have very low readings if they are naturally muddy or coloured. Recording in the same place over time will give you a more accurate picture of what is normal for your body of water.

What affects water clarity?

Completely natural as well as man made materials can affect water clarity. Be sure to note if you’ve had a recent rainfall or if you’ve noticed unusual events near the test site.

Several factors can influence water clarity:

1. Suspended particles: Solid materials, such as rocks, minerals, soil, or other organic matter, can float in water, reducing its clarity. These particles can be natural, like soil erosion, or human-made, like pollutants.
2. Natural processes: Natural occurrences like erosion, runoff, weather events, and changes in currents or water flow can impact water clarity by introducing sediments or stirring up the water.
3. Pollution: Human activities introduce pollutants that can dissolve and affect clarity. These activities could be urban runoff, agricultural practices, industrial discharges, and improper waste disposal.
4. Algae and phytoplankton: Excessive growth due to nutrient pollution can cloud water, leading to algal blooms that reduce clarity and oxygen levels, affecting aquatic life.
5. Depth and sunlight: Clarity varies with depth and sunlight penetration, with deeper waters naturally having lower clarity due to light absorption. Sunlight affects visibility by illuminating and penetrating water.
6. Temperature and seasonal changes: Changes in water temperature and seasonal variations can impact the growth of algae, phytoplankton, and other aquatic life, affecting water clarity.

Understanding these factors helps in assessing and managing water quality to ensure healthier aquatic ecosystems and safe water for human use.

In the classroom: DIY Secchi disk

Duration: 45 minutes

Objective: To make a simple version of a scientific tool.

Groups: We recommend groups of 2-4 for this activity.

Materials needed

• Clear or white bottle cap 
• Black permanent marker 
• A coin or something to use as a small flat weight
• Hot glue or strong transparent tape 
• White or light-colored string 
• Ruler 
• Tooth pick
• Scissors 
• Pencil 
• 3 pieces of white paper  
• 3 large glasses  
• Liquid measuring cup (not absolutely necessary)
• 1 bottle of 2L dark colored soda, or make your own colored water with dies
• 2L of water

Steps

Pre-class preparation

You can prepare the glasses with the solutions before the activity, while the students are working on their Secchi tubes, or ask them to prepare:

1. On the white pieces of paper, write “Sample 1”, “Sample 2”, and “Sample 3”. Place a glass on top of each one, on the part that is still white.
2. Using water and your colored liquid, make three different samples. Try to have the same amount of liquid (or height) in each glass. You can use a measuring cup if available.
3. Fill the glasses as follows:
   • Sample 1: 3/4 water and 1/4 soda
   • Sample 2: 1/4 water and 3/4 soda
   • Sample 3: all soda

Making the Secchi disk

1. Draw 4 sections on the top of the bottle cap. You can show a real Secchi disk as an example or draw a simple design on the board for students to copy.
2. Fill in black two opposite pieces of the pie with a black permanent marker. 
3. Cut a 30-centimeter piece of string or hand out pieces of that length to students.
4. Instruct students to make a knot at one end of the string.
5. Use the black permanent marker and ruler to mark the string at every centimeter after the knot.
6. Attach the coin (weight) under the cap using glue or tape. This will prevent the disk from floating.
7. Attach the knot on the string to the middle of the disk using glue or tape. Press down with the toothpick firmly to secure the knot.

Testing the Secchi disk

Take your Secchi reaching in each glass:

1. Take your Secchi disk and lower the disk into the glass until it becomes invisible.
2. Bring it up slightly and lower it again to find the exact spot where it disappears.
3. Pinch the string at the top of the liquid and pull it out while still pinching it.
4. Count the number of black marks from the knot to where you are pinching the string. Write down that value next to the sample number on the piece of paper.
5. If you can lower the disk all the way to the bottom, record the water depth instead of the Secchi depth. This indicates very clear water.
6. Discuss the differences in clarity between the glasses and proceed to the discussion questions.

Reflection and discussion

• How did the depth at which the Secchi disk became invisible vary in the different glasses of water? What does this tell us about water clarity?
• How would different types of water (clear water, water with some sediment, and water with a high amount of sediment) affect the visibility of a Secchi disk?
• How do you think the clarity of water can affect the growth and survival of underwater plants and organisms?

During the activity, we saw that the Secchi disk disappeared at different depths in the glasses of water and soda mixtures. This tells us that the clarity of the water varied. Difference in water clarity can affect the types of plants and animals that can survive in the water body. Lots of aquatic organisms, like seagrass and phytoplankton, need sunlight for photosynthesis — the process of making food. If the water is murky or unclear, sunlight may not penetrate deep enough for them to thrive. On the other hand, clearer water increases in temperature faster, which affects lots of other parameters we measure. Understanding water clarity helps us understand the conditions that support different types of aquatic life!

Outdoors: Measuring water clarity

Duration: 30 minutes

Objective: To measure water clarity in an outdoor environment, using Secchi disks to observe how different materials or natural factors can affect water clarity.

Groups: We recommend groups of 2-4 for this activity.

Steps

How to use Water Rangers Secchi disk

If you are near a stream or pond and it’s safe, bring your class by the water, ideally to a dock with access to deeper water. Always follow our safety tips for waterfront activities. In case safe water access isn’t available, you can set up large buckets for the activity and consider the materials you want to add in ahead of time.

Using buckets

1. Get 3-5 large buckets and fill them with water using a hose.
2. To create different clarity environments, get creative with your students! Brainstorm items to put in the buckets or let them pick up items if you are in a wooded area.
   • Experiment with different materials like rocks, soil, twigs, branches, or household items (flour, food coloring, sand) to observe their impact on water clarity.
3. Add a time component: Mix everything in the buckets, take initial measurements, wait 5 minutes, and measure again. Observe the differences.
4. Students can use their homemade Secchi disks for this experiment.

By a water body

1. Share information about the water body where you are located. Ask students if they know any facts or what they would like to discover later.
2. Identify a safe spot for students to access the water. Demonstrate to students how to take a reading:
   • Lower the disk slowly into the water in a sunny area, dropping it straight down.
   • Raise the disk slightly until it becomes visible, then lower it back to the point of disappearance.
   • Use a rope to mark the spot and record readings in meters, rounding to the closest tenth (e.g., 1.3m).
3. Place students in groups of 2-4 and instruct them to take turns taking a reading.
4. If you are on a dock or with multiple access points, students can measure clarity at different locations, discussing any variations they find.
5. Discuss differences in measurements among team members, emphasizing that slight variations are expected due to individual vision and color perception.
6. Record water clarity at one location by calculating the average of students’ findings, and upload the data to our platform!

Reflection and discussion

1. Does a lower clarity imply that water quality is not as good? 
   • Not always! The clarity of water is influenced by various factors, and it is important to understand the normal conditions of the water body. For example, rivers flowing over hard rock formations tend to have clearer water. Additionally, ponds surrounded by leaves may have a brown but clear color due to components from decomposing leaves leaching into the water. Therefore, lower clarity does not always indicate poor water quality, but rather the natural conditions specific to the water body.
2. What would happen to plants if the water clarity suddenly decreased? 
   • If water clarity suddenly decreases, plants in the water may face challenges. Underwater plants rely on sunlight for photosynthesis, which is the process of making food. When the water is not clear, less sunlight can reach these plants, affecting their growth. They may struggle to produce enough food, resulting in stunted growth or even death. A sudden decrease in water clarity can make it harder for underwater plants to thrive and negatively impact their overall health.
3. During what season would you expect a lower clarity and why? 
   • Lower water clarity is typically expected during seasons with increased rainfall or snowmelt, such as spring. Heavy rainfall or melting snow can wash soil, debris, and pollutants into water bodies. This runoff carries particles and soil, reducing water clarity. The increased presence of these particles and dirt during seasons with more rain or snowmelt can cloud the water and result in lower clarity.

Curriculum links

Check out our curriculum connections to see how these activities fit in the curriculum of various provinces.

Glossary

Clarity: The measure of how clear or transparent a substance, such as water, is.

Photosynthesis: The process by which plants convert sunlight, carbon dioxide, and water into energy and oxygen.

Pollutants:  Harmful substances introduced into the environment that can cause damage to living organisms and ecosystems. Pollutants may be in the form of gas, liquid, solid or energy.

Sample: A small amount of liquid taken for testing or analysis.

Sediment: Solid particles that settle at the bottom of a liquid, such as sand, rocks, minerals, or silt.

Turbidity: The cloudiness or haziness of a fluid caused by lots of particles that are generally invisible to the naked eye.