Visibility disc according to Secchi

The visibility disc according to Secchi is a simple, means of determining the visibility depth. The visibility disc is lowered into the water with a wire, and when the disc is no longer visible in the water a reading of the depth is made by means of the marks on the rope.

The disc is then lowered another 0.5 metres and pulled up slowly. When the disc becomes visible, the second reading is done. The visibility depth is determined by taking the arithmetic mean of both measurements. The visibility disc is used according to an internationally used method. After use, you can immediately assess the condition of the water.

Transparency can be affected by the colour of the water, algae, and suspended sediments. Transparency decreases as colour, suspended sediments, or algal abundance increases. Water is often stained yellow or brown by decaying plant matter. In bogs and some lakes, the brown stain can even make the water the colour of strong tea. Algae are small, green aquatic plants whose abundance is related to the amount of plant nutrients, especially phosphorus and nitrogen. Transparency can therefore be affected by the amount of plant nutrients coming into the lake from sources such as sewage treatment plants, septic tanks, and lawn and agricultural fertiliser. Suspended sediments often come from sources such as resuspension from the lake bottom, construction sites, agricultural fields, and urban storm runoff. 

Transparency is an indicator of the impact of human activity on the land surrounding the lake. If transparency is measured through the season and from year to year, trends in transparency may be observed. Transparency can serve as an early warning that activities on the land are having an effect on a lake.