What does the depth at which a sechi disk is no longer visible measure?

The depth at which a Secchi disk is no longer visible measures the transparency of water. The Secchi disk is a simple tool used in aquatic studies, consisting of a painted disk that is lowered into the water until it is no longer visible to an observer. The depth at which this occurs provides a visual indication of how light penetrates the water column.

Transparency is influenced by various factors, including suspended particles, algae, and dissolved organic matter, all of which can affect water clarity. By measuring how far down the disk can be seen, researchers can assess the water's ability to allow light to pass through, which is crucial for understanding aquatic ecosystems and the health of the water body.