Using a Secchi Disk (Procedural Considerations)

Demonstration Videos

The videos (below) are intended to be used only as generic visual demonstrations. Please, adhere to all specified instructions and methodology provided by the volunteer monitoring organization for which you are sampling!

If preferred, the Maine Volunteer Monitoring Program provides access to simple, written instructions.

*Note: To ensure accurate data, always double-check the units (e.g., feet, meters, etc) used to mark your Secchi disk and the distance between the rope’s marking intervals (e.g., every meter, every half-meter, etc).

Secchi Disk Methodology & Theory

Portions of the text below have been excerpted from the following NALMS publications:

Carlson, R.E. 1995. The Secchi disk and the volunteer monitor. LakeLine. 15(1): 28-29, 35-37.

Carlson, R.E. and J. Simpson. 1996. A Coordinator’s Guide to Volunteer Lake Monitoring Methods. North American Lake Management Society. 96 pp.

Carlson, R.E. 1997. The Secchi disk in black and white. LakeLine. 17: 14-15, 58-59.

Proper Procedure

No single procedure to use a Secchi disk is agreed upon by all that use the disk. An example procedure, constructed with consideration of disk theory (see section on Disk Design), is provided below.

Secchi disk method proposed by Davies-Colley et al. 1993
Davies-Colley, R.J, W.N. Vant, and D.G. Smith. 1993. Colour and Clarity of Natural Waters. Ellis Horwood.

  1. Use a disk of the appropriate size for the clarity range (20 mm for 0.15-0.5 m, 60 mm for 0.5-1.5 m, 200 mm for 1.5-5 m, 600 mm for 5-15 m), painted matte white or in black and white quadrants. Use a graduated line, and attach a weight to hold the line vertical.
  2. Lower the disk on the sunny side of the boat. An underwater viewer (viewscope) is desirable.
  3. Allow sufficient time (preferably 2 min) when looking at the disk near its extinction point for the eyes to adapt completely to the prevailing luminance level.
  4. Record the depth at which the disk disappears. Slowly raise the disk and record the depth of reappearance. The Secchi depth is the average of the depth of disappearance and reappearance.
  5. The readings should be made as near to mid-day as possible.
  6. The water depth should be at least 50% greater than the Secchi depth so that the disk is viewed against the water background, not bottom-reflected light.

 

Factors Which Influence Secchi Depth Measurements

Disk Size

Theoretically, the size (diameter) of the Secchi disk affects its visibility. Therefore, the authors suggest that changing the size of the disk helps standardize the apparent size of the disk as it disappears in the water.

 

Sun Glare

Theoretically, measurement errors may occur due to:

  • Anything that makes the disk more difficult to see.
  • Anything which changes the degree of contrast between the disk and its background.

Example #1

Problem: The sun’s glare off the surface of the water makes it more difficult to see the disk.

Solution(s):

View the disk on the shady side of the boat.

[NOTE: Shade affects the contrast between the disk and its background, potentially causing even more error than viewing the disk on the sunny side (Fig. 4).]

View the disk on the sunny side of the boat through a viewscope.

Viewscope  –  a tube (with or without a sealed transparent lens at the bottom) which eliminates surface glare for an individual observing a Secchi disk.

Example #2

Problem: Wearing sunglasses decreases surface glare, but also decreases Secchi disk measurements.

Solution:

Identify a standardized method (sunglasses allowed or not allowed) in any monitoring procedure.

If sunglasses are allowed, distinguish between polarized and non-polarized.

 

Time of Day

The angle of the sun relative to the position of the observer affects the depth of the Secchi disk disappearance (Verschuur, 1997).

Example #3

Problem: Sun closer to horizon = shallower Secchi depth reading.

Solution(s):

Minimize the aforementioned affect by perform Secchi disk measurements within a period surrounding solar noon (approximately 1 PM Daylight Savings Time)

Use constructed equations (e.g., Verschuur 1997) to correct values of any time and latitude to solar noon.

 

Discussion

There is Secchi disk theory in abundance, but little agreement about a proper procedure.

  • Accuracy and variability  –  Few published studies establish the relative importance of theoretically correct procedures, making it unclear whether being theoretically correct adds sufficient accuracy or removes sufficient variability to make change worthwhile.
  • Proper Procedure  –  Altering procedures becomes increasingly difficult and costly as a monitoring program accumulates data. Using an “improper” procedure may be preferred by a program over abandoning all historical data.
  • Methodological Integrity  –  To ensure consistency and reliability, it is critical that everyone follow standardized procedures with precision. The seasoned sampler must actively guard against becoming complacent and negligent about precise adherence to routine procedures.

Secchi disk procedures currently being used in the United States vary considerably and serve to illustrate the problem of lack of standardization of Secchi disk design and procedure. For additional detail, access the resources listed below.

 

Useful Resources

Davies-Colley, R.J. 1988. Measuring water clarity with a black disk. Limnol. and Oceanogr. 33: 616-623.

Davies-Colley, R.J, W.N. Vant, and D.G. Smith. 1993. Colour and Clarity of Natural Waters. Ellis Horwood.

Hutchinson, G.E. 1957. A Treatise on Limnology. Vol. 1. Geography, Physics, and Chemistry. John Wiley & Sons.

Preisendorfer, R.W. 1986a. Eyeball optic of natural waters: Secchi disk science. NOAA Tech. Memo. ERL PMEL 67. 90 p. NTIS PB86 224060/AS.

Tyler, F.E. 1968. The Secchi disc. Limnol. Oceanogr. 13: 1-6.

Verschuur, G.L. 1997. Transparency measurements in Garner lake, Tennessee; the relationship between Secchi depth and solar altitude and a suggestion for normalization of Secchi depth data. Lake and Reserv. Manage. 3(2): 142-153.