A Citizen's Guide to Understanding and Monitoring Lakes and Streams
Chapter 4 - From the Field to the Lab
Measuring Secchi Disk Depth in Lakes
Field Sampling Considerations
Excessive waves, wind, or sunlight may jeopardize Secchi Disk readings. To minimize
these effects, take readings during calm days that are partly cloudy to sunny. Anchor the
boat at the sampling station to avoid boat drift, and lower the Secchi disk off the shady
side of the boat. If the Secchi disk drifts too fast for an accurate reading – that
is, the line is not vertical in the water – try weighting the bottom of the disk to
make it sink faster or taking the measurement on the downwind side of the boat. If none of
these techniques work, and you do not think you can obtain an accurate reading, DO NOT
make the measurement because it will not be a good representation of lake conditions on
that day.
Be sure to note weather conditions along with the Secchi disk reading.
NOTE: Secchi depth readings are rarely taken in streams because of the inaccuracies associated with flowing water, disk movement, and shallow depths.
Measurement Methods
- Slowly lower the disk into the water to the point where it just disappears.
- Place a clothespin on the line where it meets the water surface, or mark the point on the line in some other way.
- Continue lowering the disk a few more inches, and then slowly raise it until it just becomes visible again. Mark this spot with another clothespin or hold the rope here between your fingers.
- The spot halfway between the two marks represents the average Secchi disk reading. Mark the spot by moving the clothespin or other marker to the spot.
- Carefully measure or count the distance from the disk to the marked spot. Record the distance to the nearest tenth of a foot or meter.
QA/QC Considerations
The Secchi disk reading is subjective because of differences in people’s vision and weather conditions. There is no QA/QC check that can be used to "calibrate" the different readings. The slight differences in vision generally are considered insignificant. Some of the error caused by the subjectivity of this measurement can be reduced by having the same person make the measurement each time.
The next section discusses measuring nutrient concentrations in lakes and streams.
Measuring Temperature | Measuring Dissolved Oxygen | Measuring pH | Measuring Secchi Disk Depth | Measuring Nutrient Concentrations | Measuring TSS | Measuring Chlorophyll a | Measuring Fecal Bacteria