The Secchi disk is widely used as a basic indication of water clarity—it provides a convenient method for measuring light penetration below the surface of a body of water. Water clarity is important to the growth of submerged aquatic vegetation (SAV) and aquatic habitats. If water clarity is measured through the season and from year to year, trends may be observed. Improving water quality trends indicate that restoration efforts are working.
A Secchi disk is an eight-inch (20 cm) disk with alternating black and white quadrants. It works by lowering the disk into a body of water by hand using a graduated line. This enables the observer to determine how deep they can see into the water.
How to Monitor with a Secchi Disk
Download the Field Data Sheet to use while you are making your measurements in the field.
Without wearing sunglasses, take a position in a shaded area alongside the water around mid-day. The depth of the water at the sample location should not exceed six feet.
Lower the Secchi disk into the water on a graduated line.
Record the depth at which the Secchi disk is no longer visible as D1 (to nearest tenth of a meter).
Raise the Secchi disk and record the depth at which the disk reappears as D2.
The average of the D1 and D2 measurements is the Secchi depth (SD). Record Secchi depth on the field log.
Measure and record the other parameters.
Collect Other Parameters
While at the Secchi disk sampling location, please collect the following:
Use a thermometer to measure air temperature. First, set the thermometer mode to °C. Next, hold the thermometer in the air above the same location where the Secchi depth is measured. Now allow a few seconds for the thermometer to display the temperature reading. Lastly, record reading on field data sheet.
Use a thermometer to measure water temperature. First, set the thermometer mode to °C. Next, place the thermometer in the water in the same location where the Secchi depth is measured. Now allow a few seconds for the thermometer to display the temperature reading. Lastly, record reading on field data sheet.
Use your best judgment to approximate wind speed and wind direction. You may also refer to sources such as the Internet, television, newspaper, etc. Record approximate wind speed in mph or as none, slight, moderate, or strong and approximate wind direction using W, SW, S, SE, etc. on field data sheet. For weather observations, record on the field data sheet the current weather conditions using one of the following terms: sunny, cloudy, drizzle, or rain. Rain data should also be collected. On the field data sheet, record the number of days it has been since the last rainfall occurred at the sampling location.