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Measuring lake health: Secchi disk how-to

With the following steps, you’ll learn how to make your own Secchi disk at home, take water clarity measurements on a lake or pond, and make your data available for scientists and others to view and use. 

Aquatic ecologist Jonathan Stetler explains the importance of studying water clarity as an indicator of lake health, and demonstrates how you can measure water quality by making your own Secchi disk.

Learn how to take a Secchi depth reading and contribute your lake’s data to active research projects


How do you measure water clarity?

Have you ever wondered about the health of your favorite lake? This is something scientists think about too. One indicator of lake health is water clarity. This is a growing concern around the world as lakes are becoming less clear due to the effects of climate change, development, and even recovery from acid rain. 

Water clarity is important because it controls how far light penetrates into the water column. This determines where and how well aquatic plants can grow, with implications for animals that depend on those plants for oxygen – like fish.

Scientists use various tools to measure water clarity. One method involves using high-frequency sensors that measure the amount of light at different depths of a lake. An example is an instrument called a submersible radiometer, which measures light through the water column every second and feeds that data to a computer. 

Scientists can also use satellites to measure water clarity; this is called remote sensing. A team at Rensselaer Polytechnic Institute is working to estimate water clarity for every lake in the United States using satellite observations.

Sensors and satellites help scientists collect lots of water quality data over space and time, but these aren’t the only tools that get the job done. A reliable, low-tech way to measure water clarity is with a Secchi disk – a tool that lake researchers have used for over a century. 

With the following steps, you’ll learn how to make your own Secchi disk at home, take water clarity measurements on a lake or pond, and make your data available for scientists and others to view and use. 

Materials

secchi disk materials
  • Plastic lid, about 6 inches in diameter
    • This could be a lid from a coffee or peanut can, an oatmeal container, or similar – many lids could work
  • Rope or string at least 10 feet long, (longer is better, especially if your lake is very clear)
  • Several large washers (or other small weighted object, like a binder clip – anything with enough weight to help your disk sink through the water column)
    • Washers work well as they are inexpensive and can be found at any hardware store
  • Black (or dark) permanent marker
  • Black waterproof electrical tape
  • White waterproof electrical tape
  • Ruler
  • Scissors
  • Something to puncture the plastic lid (scissors will often work, a nail or drill bit will also do)
    • Children should ask an adult for help puncturing the lid

How to make a Secchi disk

1. Draw two lines on the plastic lid to make a crosshair.

secchi disk step 1

 

2. Next, puncture a hole in the center of the lid where the two lines intersect using scissors. A nail or drill bit can be used if the container is too hard to puncture with scissors. (Children should ask an adult for help with this step.)

hw to make a secchi disk step 2

3. Use the black electrical tape to cover two opposite quarters of the lid. Use the white electrical tape to cover the other two quarters of the lid. Make sure that the tape did not cover the hole that you punctured earlier. If it did, use your scissors to carefully re-open the hole.

secchi disk step 3
secchi disk step 4

4. Slide the rope through the hole in your Secchi disk and tie a simple knot on the bottom of the disk so that the rope will not come through the hole.

secchi disk step 4

5. Place washers or another weighted object on the rope at the bottom of your Secchi disk. Secure with another knot. This will help the disk sink through the water column.

secchi disk

6. To make sure your washers or other weight(s) do not slip off, secure with plenty of electrical tape, even after knotting. 

secchi disk

7. Now it's time to mark your Secchi disk. Use your ruler to mark 1-ft increments on the rope with permanent marker, starting where the rope first pokes out on the non-weighted side of the Secchi disk. Mark at least 10 ft, and more if your rope is longer. 

secci disk

 

Your Secchi disk is now ready for use!

How to take water clarity measurements

Head out to a nearby lake or pond. If you have access to a boat or kayak, find an area of the water body where you can’t see the bottom. Be sure to wear a life jacket. If a boat isn’t an option, you can also take a Secchi reading off a dock or pier. Again, it’s best if you can find a place where you can’t see the bottom.

measuring water clarity wih a secchi disk

Sunny, calm days are the best for Secchi measurements. 

If you are wearing a hat or polarized sunglasses, be sure to remove them before taking a Secchi disk reading.

  1. Slowly lower the disk into the water; as you do, keep track of how deep the disk goes by counting the 1-ft increments on your rope.
  2. When you can no longer see the disk, record the depth. If you are between 1-ft marks, you can estimate the inches. (If you bring your ruler with you, you can measure to be more accurate.) This reading is your M1.
  3. Next, slowly raise the disk until you can see it again. Record this depth; this is your M2.
  4. To calculate your Secchi depth measurement, add M1 + M2, then divide that answer by 2. The equation looks like this: Secchi Depth = (M1 + M2) ÷ 2
  5. Record your Secchi depth in a notebook to start a log of measurements.

For research purposes, scientists will typically measure the Secchi depth once or twice a month in a given lake. You can measure as often as you'd like.

When creating a log, try to take your measurement at the same place and at the same time of day. As your dataset builds, you can begin to look for patterns in your lake. For example, you might notice that the Secchi depth is shallower after a rainstorm when the water is cloudier.

There are great resources online where you can contribute your Secchi depth data to support ongoing research projects. The Lake Observer website (and mobile app) allows anyone to submit their data to an online repository where both scientists and others can view and use it.

When you record your measurements in Lake Observer, be sure to mention in the comments along with your measurements that you built your own Secchi disk and include its diameter. You can even participate in the annual Secchi Dip-in coordinated each summer by the North American Lake Management Society.

Happy dipping! 



Jonathan Stetler is an NSF intern working with Cary’s Chris Solomon, and a PhD candidate at Rensselaer Polytechnic Institute.