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Monitoring Water Transparency with Secchi Disk and Transparency Tube

Learn how to measure water transparency using a Secchi disk or transparency tube to evaluate suspended solids affecting light passage and aquatic plant growth.

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Monitoring Water Transparency with Secchi Disk and Transparency Tube

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  1. Transparency • Suspended solids scatter light passing through the water and cause the water to become “cloudy”. • The transparency of water determines the depth to which aquatic plants can grow. • Transparency is measured by using a Secchi disk or a transparency tube. • The Secchi disk is used in deep, still waters • The transparency tube is used in shallow or flowing waters

  2. Secchi Disk Reading • Record the cloud cover on the Hydrology Investigation Data Sheet (see Cloud Cover Protocol) • Stand so that the Secchi disk will be shaded or use an umbrella or piece of cardboard to shade the area where the measurement will be made. • Lower the disk slowly into the water until it just disappears. • Mark the rope with a clothespin at the water surface. If you cannot easily reach the water surface (for example, if you are standing on a dock or bridge), mark the rope at the edge of the dock or bridge.

  3. Secchi Disk Reading (continued) • Lower the disk another 10 cm into the water, then raise the disk until it reappears. • Mark the rope with a clothespin at the water surface (or edge of the dock or bridge). • There should now be two points marked on the rope. Record the length of the rope between the marks and the Secchi disk on your Hydrology Investigation Data Sheet to the nearest cm. If the depths differ by more than 10 cm, repeat the measurement and record the new measurements on your Data Sheet.

  4. Secchi Disk Reading (continued) • If you marked the rope at the water surface, record “0” as the distance between the observer and the water surface. • If you marked the rope at the edge of a bridge or dock, lower the disk until it is at the surface of the water and mark the rope again at the edge of a bridge or dock. Record the length of the rope between the mark and the Secchi disk as the distance between the edge of the dock or bridge and the water surface. • Repeat the measurement two more times with different students observing.

  5. Transparency Tube Reading • Pour the 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 to the image. • Rotate the tube while looking down through it to see if the black and white areas of the decal are distinguishable. • Record this depth of water on the Hydrology Investigation Data Sheet to the nearest 1 cm.

  6. Transparency Tube Reading (continued) • If the tube is filled to the top and you can still see the image, simply record the length of the Tube and put a greater than (>) symbol in front of it. • The tube reading should be made by three different student groups and each measurement must be reported separately.

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