Also: sighting depth
An Italian invention, eponymously named for Fr. Pietro Angelo Secchi, who, at the behest of a Papal naval officer, Commander Ciardi, devised the method. In 1865, the papal yacht L'Immacolata Concepzion systematically used the technique to measure sighting depth while on patrol in the Mediterranean
Secchi disk depth is a measure of the clarity of sea or lake water. The technique consists of lowering a Secchi disk (a circular plate, sometimes all-white, sometimes quartered black-and-white) on a cord into the water. When the disk ceases to be visible from the surface, the depth (marked on the cord) is noted, and the disk retrieved.
The higher the Secchi disk depth, the deeper light penetrates into the water. As a common rule of thumb, enough light to permit photosynthesis usually penetrates to about 1.7 times the Secchi disk depth.
Secchi disk readings show (unsurprisingly) seasonal variations, as a result of changing presence of algae over the year. It can also vary with the presence of particulate matter (for instance, as a result of landslides or erosion). Since both of these factors are important in determining the "health" of a lake, fjord, or other enclosed or semi-enclosed body of water, they are extremely important in environmental monitoring of such bodies of water. They are also useful in overall environmental monitoring of the open ocean.
Secchi disk depths can vary immensely. In polluted lakes, the sighting depth can be as little as 10-15 cm, whereas a clean lake might easily have a sighting depth of 8 m. In the Memorabilia nonnulla lacus Veteri from 1705, the Swedish lake Vättern is reported to have had a sighting depth of 30 m, but this is exceptional.