What is a Secchi Disk?

Floating Classroom participant uses a Secchi Disk

A Secchi disk is a disk 8 inches in diameter designed with alternating black and white quadrants. The Secchi is used to determine water transparency. The disk is lowered into a body of water until it is no longer visible. The point at which the disk is no longer visible is known as the “Secchi depth”. That depth is measured in meters. The Secchi disk is a quick and easy way to measure lake water clarity and water quality. Transparency of lake water is affected by a variety of factors including but not limited to human activity, decaying plant matter, algal blooms, suspended sediments, and plant nutrients. Measurements of transparency can serve as early indicators of the impact of human activity on the land surrounding the lake and in the water.

The Secchi disk is named after Father Pietro Angelo Secchi. Father Secchi was the scientific advisor to the Pope in 1865 when he was asked by the head of the Papal Navy to test a new transparency instrument in the Mediterranean Sea. This instrument (Secchi Disk) now bears his name.
0-1.0 meters=Eutrophic status (very murky and high in nutrients. Like most ponds)
1.1-7.9 meters=Mesotrophic
8.0-64 meters= Oligotrophic (very clear, few nutrients) (Crater Lake in Oregon has the record for highest reading of a Secchi Disk in the US-39 meters)