Topic 3

A Secchi disc can be used to investigate the depth that light is able to penetrate in a body of water such as a lake, estuary or in the sea.

To use a Secchi disk we hold the line the disc is attached to and lower the disc into the water, observing the depth shown by the markings as the disc is lowered…

…when the disc just disappears, stop lowering the line and determine the depth shown on the line where the line enters the water. Record this result. Waves will create changes in the water height so it is appropriate to record the depth to the nearest 10 cm or 5 cm (half way between marked points).

Lower the disc by about a metre. Then slowly raise the disc again...

…when the disc just re-appears, stop raising the line and determine the depth shown on the line where the line enters the water. Record this result.

Calculate a mean for the run by adding the two depths recorded when the Secchi disc went down and back up, and dividing this value by 2. Repeat this process at least 2 more times to collect 3 sets of data.

Finally calculate an overall mean by adding the three means for each run and dividing the total by three.

A Secchi disc can be used to compare the penetration of light at different places on the (lake/sea/estuary) or the body of water where you are recording your results. This allows you to compare how clear the water is in different places.

If you are able to visit the same location at different times of the year you can also investigate the changes in light penetration over time.

Changes in results over time may be caused by changes in rainfall moving different amounts of sediment into the water, or changes in the amount of phytoplankton in the water.