Finding the Secchi Depth

Often marine scientists as well as fisherman want to test the murkiness (or turbidity) of the water. How can you do this quickly and accurately? Well, first you’ll need a Secchi Disk. With the cheap, easy-to-make Secchi Disk you can test water quality like a pro!

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The Secchi disk was invented in 1865 by Pietro Angelo Secchi, and it’s basically a circular disk hung from a rope and lowered slowly in the water until you can’t see the pattern on the disk anymore. Scientists call this depth the Secchi depth and is related to water transparency.

As you probably figured out, Secchi disk readings are not an exact measure of transparency, because it depends not only on a person’s eyesight but also the brightness of the day. But for most folks, the measurement is quick, inexpensive and relatively accurate for their needs.

The results are best when the disk is used between 10:00 am and 2:00 pm on the shady side of the boat or dock and the reading should be done by the same person. Both Minnesota and Indiana field scientists use this technique when assessing the quality of water in their lakes.

Here’s what you’ll need:

• A white plastic disk with an 8 inch diameter. A paint or bucket lid works well.
• A permanent marker.
• A broom handle (unless you’re measuring very clear water, and in that case use string).
• Graph paper.

Here’s what you do:

1. Divide the disk into four sections with the permanent marker. Color in two of the sections (such that if moving clockwise the sections alternate between being filled in and not filled in).
2. Attach the lid to the broom handle or string (if using the string, attach a weight to the other side of the lid).
3. Lower the disk into the water until you can’t distinguish between the dark and light sections. Mark that depth.
4. Pull the disk up and measure the first depth. Note that on the paper.
5. Lower the disk past the first mark and then pull it up until you can just make out the difference between the dark and light sections of the disk.
6. Pull the disk up, measure the second distance, and note it.
7. The average between the two depths is the Secchi depth.