Use a Secchi disk to quantify the transparency of your local body of water.
- Secchi disk
- Water transparency
The Secchi disk is named after Father Pietro Angelo Secchi (1818–1878), an astrophysicist and scientific advisor to the pope. Father Secchi created the first device of this type when he was asked to measure the transparency in the Mediterranean Sea in 1865. Secchi disks are commonly used to measure the water transparency in open waters of lakes, bays and oceans. The Secchi disk is a 7⅞" diameter circle with four quadrants (alternating two white and two black) with an attached line. The transparency of the water is measured by lowering the Secchi disk into the water until the white and black quadrants can no longer be distinguished. The depth of the water when the Secchi disk is at this level in the water is known as the Secchi index or Secchi depth.
Secchi disk measurements do not provide an exact reading of transparency due to possible errors such as the Sun’s glare on the water, differences in vision between readers, etc. It is, however, a straightforward and inexpensive way to obtain rough values of water transparency. Some variables that may lead to increased or decreased Secchi index values are as follows:
- Increased Secchi Index
Reduced nutrient input
Increased algae consumption by zooplankton
Seasonal algae levels
Reduced soil erosion into the body of water
Reduced zooplankton levels
- Decreased Secchi Index
Increased levels of algae
Erosion of shoreline
Circulation of bottom sediments from motor boat activity
Line, braided, 48 ft*
Markers, two different colors, permanent
*Materials included in kit.
Follow all normal field safety guidelines.
- Tie one end of the line to the eyebolt attached to the Secchi disk. Make sure that the knot is secure.
- Using a color marker and a meter stick, mark the line every meter away from the disk.
- Use another color marker to make ½-meter markings.
- Travel to sampling site location.
- Slowly begin to lower the Secchi disk into the water. Count the meter marks on the line as the disk is lowered into the water.
- Continue to lower the disk into the water until the black and white quadrants of the disk can no longer be distinguished. Note the Secchi index in meters at this point.
- Continue to lower the Secchi disk until it can no longer be seen.
- Raise the Secchi disk until the black and white quadrants can once again be seen. At this point begin to count the number of meters on the line until the Secchi disk reaches the surface of the water. Note the Secchi index in meters at this point once again.
- Repeat steps 2–5 two more times to obtain a total of six readings.
- Add the six readings together and divide by six. This will be the average Secchi index.
- One complete Secchi disk setup is given in this kit. The Secchi disk may be reused as many times as desired.
- For the best results, take Secchi disk readings in shallow water in a shaded location.
- The same person should take all readings since the sharpness of vision varies from person to person.
- It is preferable that all measurements take place between 10 am and 4 pm.
- Avoid taking measurements if the body of water is choppy or rough.
- If readings are to be taken on different days, be sure that the readings are taken in the same location at approximately the same time of day.
- The Secchi disk may also be used to measure the overall water depth of a location by lowering the disk until the bottom is reached.
- The actual turbidity level, which describes the cloudiness of water caused by suspended particulates, may be measured using a turbidity sensor, Flinn Scientific catalog number TC1528.
Correlation to Next Generation Science Standards (NGSS)†
Science & Engineering Practices
Planning and carrying out investigations
Analyzing and interpreting data
Using mathematics and computational thinking
Disciplinary Core Ideas
HS-PS1.A: Structure and Properties of Matter
HS-ESS2.A: Earth’s Materials and Systems
Cause and effect
Scale, proportion, and quantity
HS-PS1-1: Use the periodic table as a model to predict the relative properties of elements based on the patterns of electrons in the outermost energy level of atoms.
HS-PS1-2: Construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties.