Secchi depth
The Secchi disk, as created in 1865 by Angelo Secchi, is a plain white, circular disk 30 cm (12 in) in diameter used to measure water transparency or turbidity in bodies of water. The disc is mounted on a pole or line, and lowered slowly down in the water. The depth at which the disk is no longer visible is taken as a measure of the transparency of the water. This measure is known as the Secchi depth and is related to water turbidity. Since its invention, the disk has also been used in a modified, smaller 20 cm (8 in) diameter, black and white design to measure freshwater transparency.
The original Secchi disk was a plain white disk and it was used in the Mediterranean Sea. Today, a plain white, 30 cm diameter Secchi disk remains the standard design used in marine studies. In 1899 George C. Whipple modified the original all-white Secchi disk to “…a disc about 8 inches in diameter, divided into quadrants painted alternately black and white like the target of a level-rod…”. This modified black and white Secchi disk is the standard disk now used in limnology (freshwater) investigations.
The Secchi depth is reached when the reflectance equals the intensity of light backscattered from the water. This depth in metres divided into 1.7 yields an attenuation coefficient (also called an extinction coefficient), for the available light averaged over the Secchi disk depth. While used as a variable, the extinction coefficient is also used as a variable for turbidity. The light attenuation coefficient, k, can then be used in a form of the Beer–Lambert law,
to estimate Iz, the intensity of light at depth z from I0, the intensity of light at the ocean surface.
The Secchi disk readings do not provide an exact measure of transparency, as there can be errors because of the sun's glare on the water, or one person may see the disk at one depth, but another person with better eyesight may see it at a greater depth. However, it is an inexpensive and straightforward method of measuring water clarity. Because of the potential for variation between users, methods should be standardized as much as possible.
A Secchi disk measurement should always be taken off the shady side of a boat or dock between 9:00 and 15:00 The period for best results is between 10:00 and 14:00. The same observer should take Secchi depth measurements in the same manner every time. One can approach the measurement by lowering the disk beyond a point of disappearance, then raising it and lowering it slightly to set the Secchi depth. Another method is to record the depth at which the disk disappears, lower another few feet, then record the depth at which the disk reappears as it is slowly brought up. The Secchi depth is taken as the average of the two values.
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