Secchi Disk

What is a Secchi disk?

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.

A portion of this graphic illustrates a Secchi disk up close. The disk is circular with alternating 2 black and 2 white pie shaped wedges. The next illustration demonstrates the use of the Secchi disk aboard a boat, the disk is lowered into the water body. The visibility of the Secchi disk decreases the deeper depths. The illustration show that this corresponds with the ability of sunlight to penetrate deeper depths.
Graphic Credit: Joel Davenport

Why do we measure Secchi depth?

Secchi depth is important to measure because the clarity of water impacts the amount of light penetration, which 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 water clarity of water can be an indicator of a human impact to an ecosystem.

Student submerging Secchi disk over the edge of dock and into lake water.
Photo Credit: Mellissa Kowalski

Secchi disks are weighted to ensure they go straight down. The deployment line can be marked at meter intervals for ease of measurement. 

What affects water clarity?

A variety of natural and human factors can influence water clarity. 

  • The color of water is sometimes affected by tannin-rich waters that flow through peaty areas.
  • An algal bloom can lead to reduced water clarity when the water becomes cloudy and discolored due to an excess of algal growth.
  • An increase in suspended sediments can be the result of urban and agricultural runoff.
  • Wave energy can also affect water clarity. 

Secchi Disk Factsheet

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