Secchi disk
The Secchi disk is a limnological aid device developed by the Italian polymath and Jesuit father Angelo Secchi in 1865 and is still in use today for the quick and easy determination of the depth of view in a body of water.
The Secchi disc is usually a circular sheet metal disc with a diameter of approx. 20–50 cm. It is alternately painted black and white in four sectors. A weight is attached to the back under the center point. A rope with a length marking is attached to the center at the top. On this rope, this disc is lowered into the water in a horizontal position and observed until it visually disappears. The depth of the disappearance is read from the graduation of the rope and registered as "depth of view" or "Secchi depth" z s .
This depth of view is u. a. Used to estimate the depth of the trophogenic zone in a lake or generally to characterize the turbidity and color of a body of water, in particular by plankton organisms or by mineral cloudiness. The limit of the positive photosynthesis balance (assimilation> dissimilation) is often reached at about twice the viewing depth.