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5th Grade and Up
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The purpose of the Water Transparency Protocol is to discover how deep light can penetrate into a body of water. Light is essential for the growth of green plants and sunlight provides the energy for photosynthesis, the process by which plants grow. Thus, the penetration of sunlight into a body of water determines the depth and quantity of algae and other underwater plants. Water transparency decreases as color, suspended sediments and algae increase. Most natural bodies of water have transparency ranging from 1 meter to a few meters. Extremely clear lakes and areas around clear coral reefs can have a transparency of up to 30 – 40 meters. Transparency of water can be measured by two methods: by either the use of a Secchi disk or a turbidity tube. The Secchi disk is used in deeper, stiller waters and the turbidity tube is more often used with either still or flowing waters.
Secchi disk
A Secchi disk is a 20 cm black and white disk that disappears when lowered into water and again when raised. Before using the Secchi disk, students should find a place in the shade to take the measurement of transparency of the water sample. Students should then lower the Secchi disk into the water until it just disappears. At this point a student should make a mark on the rope. Then students can raise the Secchi disk until it just reappears into view and again a student should mark this point on the rope. Students can then measure the distance between the two points on the rope. Since different people see the Secchi disk disappear at different depths this observation should be made by three different students.
Turbidity tube
A Turbidity tube is a tube with a pattern similar to that of the Secchi disk on the bottom. Students should again find a place in the shade before taking the measurement of transparency of the water sample. Students can then pour sample water into the tube until the image at the bottom of the tube is no longer visible when looking directly through the water column at the image. Students can then record the depth of water in the tube when the pattern disappears.
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Rope
Secchi disk
Meter stick
Or
Turbidity tube
Meter stick
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None.
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1) Students seek out a shady spot in which to measure the transparency of their water sample.
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2) Students fill the turbidity tube with the water sample.
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3) Students record the depth of water at which the image at the bottom of the tube is no longer visible.
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Click here to view QuickTime movie of protocol!
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![Protocols [Image: Blue-Eyed Grass]](images/protocols_h.jpg)
