Secchi Disk

Secchi Disk

What is it?

This is a separate instrument from the Hydrolab in the Pelican case. A secchi disk contains alternating black and white quadrants and is attached to a line. This line is used to lower the disk into a body of water with the purpose of measuring the clarity of the water. The depth at which the disk can no longer be seen is called the secchi depth and is the measurement recorded.
An image that demonstrates the use of a secchi disk. The image is broken into two parts. On the left is an illustration of the disk itself—a flat disk with alternating black and white quadrants. The disk hangs from a rope connected by a ring in the center of the disk. The right side of the image shows an individual in a boat lowering the disk until such a time as they can no longer see the disk.

 Why do we measure it?

Secchi depth is important to measure because the clarity of water impacts the amount of light penetration and in turn can affect photosynthesis and the distribution of organisms. While people often focus on the negative aspect of losing clarity, completely clear water is usually not desirable either because that means the water is devoid of needed food like plankton.

Secchi disk readings are useful in comparing bodies of water or looking for changes to a specific body of water over time. Changes in clarity of water can be an indicator of a human threat to an ecosystem.


What affects it?

Clarity will decrease as color, abundance of algae or suspended sediments increase. The color of water is sometimes caused by staining, due to decay of plant material. Excess algae growth can occur where there is additional input of nutrients like phosphorus and nitrogen from agriculture and/or sewage treatment or septic system waste. An increase in suspended sediments can be the result of urban, agricultural or storm runoff.

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 Login | This program is part of a cooperative agreement between the U.S. EPA Great Lakes National Program Office and National Oceanic Atmospheric Administration (through Illinois-Indiana Sea Grant) and funded by a Great Lakes Restoration Initiative grant: DW92329201. This website was developed by Illinois-Indiana Sea Grant and designed by Gabriel Horton. For questions or comments, please contact Kristin TePas at or 312-886-6224.