Secchi Disk Transparency

**For the Mass. Department of Environmental Protection-approved SOP, get the pdf file here**

Equipment:

  • Secchi Disk
  • Calibrated line
  • View scope
  • Two spring clip clothespins
  • Field data sheet and pencils

Build your own equipment

Sampling Method

The Secchi disk is used to measure the depth that a person can see into the water (transparency). The Secchi disk transparency is somewhat subjective and also affected by light and waves. To minimize these individual differences, MassWWP requires that you use a viewscope. Your measurements should be taken between 10 a.m. and 2 p.m.

If possible, a team of two should perform the Secchi disk measurement. One team member should lower the Secchi disk on the sunny side of the boat. The other team member should submerse the end of the viewscope and watch the lowering of the Secchi disk. When the disk just disappears, a clothespin should be clipped on the calibrated line at the water's surface. The disk should be lowered another one or two feet and then slowly brought up toward the surface until it can just be seen again. Another clothespin should be clipped on the line at the water's surface. The Secchi disk depth is the average of these two measurements to the nearest tenth of a meter.

Repeat the measurement with the team members reversing roles. Record the Secchi disk reading of each viewscope operator. Average the two measurements and record on the field data sheet .

Maintenance

At the beginning of each sampling season, the calibrated line should be checked for shrinkage/stretching. Attach a cloth tape measure to the bottom of the calibrated line. Lower the Secchi Disk as deep as you can without hitting lake bottom, feeding the tape measure so it is taut.
Compare the reading on the tape measure with the calibrated line reading. If they don't agree within 10%, you should adjust your Secchi Disk readings accordingly. Alternatively, use a fiberglass tape (see equipment list) or carry a meter-stick (yardstick!) with you and measure the distance between the disk and the clothespin on the uncalibrated rope.

Build Your own equipment

Secchi Disk

image of sechhi components

View Scope

image of viewscope components


you can buy a ready-made viewscope here