Description
Secchi Disk
Secchi Disk Transparency is a quick, simple and accurate method for determining lake water quality.
Transparency is a measurement of how deep into a lake the sunlight can penetrate. Factors that interfere with light penetration are algae, zooplankton, water color, and silt. Since algae is usually the most abundant item, by measuring transparency one also is measuring the algae population (see Figure 1.) Lake water quality can be defined as the amount of algae that a lake supports.
Follow these steps to obtain accurate Secchi Disk Transparency readings:
- Motor or paddle to the deepest part of the lake or pond. This will allow you the deepest possible reading. Lake depth maps with sampling locations are available through the VLMP.
- Always anchor your boat. If the boat moves while taking the reading, and the disk does not hang straight, the reading will not be accurate.
- Take the SDT reading on the shady side of the boat. This will also insure the deepest possible reading. The shade reduces glare and can improve the readings up to 3ft. (~1 meter.)
- Viewing the disk through the scope, slowly lower the Secchi disk until you can no longer see it (no white glow.) Then, raise it until it is visible again and lower it one more time, very slowly, stopping when it disappears.
- Hold the Secchi disk and tape steady, remove your face from the scope and mask and place the scope safely inside the boat. Pinch the meter tape where it meets the water and record that depth onto the data sheets.
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