How to Use a Secchi Disk
To use a Secchi disk, lower the weighted, black-and-white disk into the water until it disappears, and record that depth. Slowly raise it until it just reappears, and record that depth too. The average of these two measurements is your final water clarity reading, which is called the Secchi depth.
Preparation:
- Choose the Right Time: For the most consistent and accurate readings, take your measurements between 10:00 a.m. and 2:00 p.m.
- Remove Sunglasses: Take off any polarized or tinted sunglasses to prevent artificial darkening of the water.
- Safety and Positioning: If you are on a boat, drop anchor in the deepest, safest spot available. Make sure to take your measurement on the shady side of the boat to reduce sun glare on the water surface.
Taking the Measurement:
- Lower the Disk: Slowly lower the Secchi disk into the water, keeping the calibrated rope completely vertical. Keep lowering until the black-and-white quadrants (or triangles) are no longer distinguishable.
- Record Depth 1: Note the exact depth reading on the rope at the water's surface when the disk vanishes.
- Raise the Disk: Lower the disk a bit further, then slowly pull it back up until the black-and-white pattern just barely becomes visible again.
- Record Depth 2: Note the exact depth reading on the rope at the water's surface as it reappears.
Calculating the Secchi Depth:
Calculate the Secchi depth (SD) using the two recorded depths: SD = (Depth1 + Depth2) / 2
- Shallow Water Tip: If your disk sits perfectly on the bottom but you can still see it clearly, record the depth at the bottom and note "visible on bottom" on your data sheet.
