WHAT IS IT
The Secchi Disk is an 8-inch (20 cm) disk with a weight on the bottom and alternating black and white quadrants on top which are used to measure how far an observer can see into the water.
HOW IS IT USED
It is simply lowered into the lake using a measured length of waterproof line to determine the depth at which it disappears from view.
This depth is called the "Secchi Depth" and is a measure of the turbidity , or cloudiness, of the water.
The depth is recorded in the log book for future reference.
WHY IS IT USED
Since turbidity is affected by the amount of plant nutrients coming into the lake from sources such as sewage treatment plants, septic tanks, and lawn and agricultural fertilizer, it is a good indicator of the impact of human activity on the land surrounding the lake.
If it is measured through the season and from year to year, trends in transparency may be observed. Turbidity can serve as an early-warning that activities on the land are having an effect on a lake.