The Secchi disk is used as a measure of water transparency in large bodies of water. It is a relatively simple device that was developed by Father Pietra Angelo Secchi in Italy.
The Secchi disk is:
(1) about 8 inches (20 cm) in diameter
(2) divided into quadrants
(3) quadrants painted with alternating white and black
(4) central opening suspending from a rope or string
(1) The disk is dropped into the water.
(2) The depth below the water surface that the disk disappears is recorded.
(3) The disk is then raised until the disk reappears and that depth below the water surface is recorded.
Secchi disk reading =
= (depth where reappears) + ((depth where disappeared) - (depth where reappears)) / 2
• The Secchi disk reading is the distance midway between the point of disappearance and the point of reappearane.
• The higher the Sechhi disk reading the less turbid the water.
• A standard disk should be used for measurements. Several measurements should be taken and the results averaged.
• There can be considerable observer variability based on experience, eyesight, accuracy, etc.
• The readings are affected by the amount of light available.
• The readings may fluctuate during the year based on water temperature, rain run-off, algae, plankton and other factors.
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