HOW IS IT MEASURED?

Transparency is measured by using a Secchi Disk and a viewing scope. (The Secchi disk is named after Fr. Pietro Angelo Secchi who invented the device in 1865 to measure the transparency of the Mediterranean Sea. Secchi was scientific advisor to the Pope and an accomplished astrophysicist.) A Secchi disk is a black and white disk, usually made of metal or plastic, attached to a measuring tape. A viewing scope is a tube that is used to make it easier to obtain more accurate Secchi depth readings by cutting out interference from water-surface glare and waves. The viewing scope has a foam mask at one end to place your face on to look down into the water column, and the other end is capped with plexiglass.

While looking down into the water through the viewing scope, the disk is lowered vertically down into the water. Once the disk disappears completely from view, it is slowly raised to the point where it can be seen again, and then lowered again until it completely disappears. The average between these two points is the Secchi depth.

Scientists from the University of Maine take a Secchi depth reading on a lake in Maine. The picture to the left shows how the viewing scope is used to look down through the water. the picture below shows the assistant slowly lowering the disk through the water.

The Secchi disk is probably the most inexpensive, easy-to-use water monitoring tool, and the results are fairly easy to interpret. There are several important considerations to this test, though. Because the ability to see the disk is affected by the amount of light there is shining down from the sky, the reading should be taken during the brightest hours of daylight. The Maine Department of Environmental Protection suggests taking the reading between 9 a.m. and 3 p.m. Also, the sky/cloud conditions during the reading should be recorded, as they affect the amount of light in the sky, and may affect the depth of the reading. Windy conditions on the lake may cause the disk to drift off vertical, so the wind conditions are recorded as well.

One other consideration when taking a Secchi depth reading is if there are other tests that you are conducting that my stir up sediments on the lake bottom, for example a sediment core sample or a benthic dredge. These tests may make the water cloudy or turbid, and can affect the accuracy of the reading. Conduct Secchi depth readings before you stir up sediments.

All photos this page courtesy of the Senator George J. Mitchell Center for Environmental and Watershed Research