The portions of the following has been excerpted from from copyrighted material.  Material in this section should not be reproduced without specific permission of the North American Lake Management Society (www.NALMS.org).  The reader is encouraged to read the original material for more details on Secchi disk theory and methods.  These are:

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.  (7/20/97)

 


Is There a "Proper" Procedure For Dipping Your Disk?    

 

Proper Secchi Disk Procedure

There is probably no single procedure to use a Secchi disk that is agreed upon by all that use the disk.  If the term, proper, refers to a procedure that comes closest to that constructed from disk theory (see section on Disk Design), then the procedure given below might be considered proper.  You might recognize elements of the procedure you use in the method, but some of the recommendations might seem strange.

The following  Secchi disk method was proposed by Davies-Colley and others in 1993

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.  A 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 it 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.

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

Disk Size
The idea of varying the size of the disk might seem awkward, but it is theoretically correct.  According to theory the size of the disk does affect its visibility.  The authors are suggesting that by changing the size of the disk, the apparent size of the disk when it disappears would remain approximately the same.

Glare
Theory suggests that anything that makes the disk more difficult to see, or that changes the degree of contrast between the disk and its background, should cause measurement errors.  For example, the sun's glare off the surface of the water makes it more difficult to see the disk.  In many volunteer programs, this glare is eliminated by viewing the disk on the shady side of the boat.  The trouble is that viewing the disk in the shade of the shadow of the boat or even viewing the disk through the shadow of the boat affects the contrast between the disk and its background.  This may cause even more error than viewing the disk on the sunny side (Fig. 4).

A solution to glare may be to view the disk on the sunny side of the boat but to use a "viewscope."  A viewscope is a tube, with or without a sealed transparent lens at the bottom that allows the observer to view the disk without surface glare.

Sunglasses
Some volunteer programs instruct the volunteers to remove sunglasses before taking a reading, presumably because the sunglasses cut down on the light.  This seems to be at odds with disk theory because the disk is a contrast instrument, and the absolute amount of light reaching the eye should not be important.  Sunglasses, especially polarized ones, should cut down surface glare and make the disk easier to see.  However, in the author's observations, sunglasses do appear to decrease the Secchi depth and there should be a note of standardized procedure (sunglasses on or off) in any monitoring procedure.

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).  The lower the sun is on the horizon, the shallower will be the reading.  Most programs limit readings to a period surrounding solar noon (approximately 1 PM Daylight Savings Time) to minimize this effect.  Verschuur (1997) has constructed equations that will correct values of any time and latitude to solar noon.  It may also be that the more glare reflected from the disk may also enhance the solar altitude problem.

Discussion

There is Secchi disk theory in abundance, but little agreement about a proper procedure.  Being theoretically correct may be important, but other factors may be even more important.  As a monitoring program accumulates data, it becomes increasingly difficult to change the procedure.  Using an "improper" procedure may be better than abandoning all the past data.  It is also not clear whether being theoretically correct adds sufficient  accuracy or removes sufficient variability to make change worthwhile.  Using a viewscope or different sized disks may be theoretically correct, but they add cost and complexity to a monitoring program.  Few published studies establish the relative importance of theoretically correct procedures.

 Program coordinators may debate whether the disk should be lowered on the sunny or shady side of the boat or whether sunglasses should be worn. Once a procedure is agreed upon, it is important that everyone follow the procedures exactly, otherwise the consistency and therefore, the reliability of the procedure will be lost.  Curiously, my experience is that it is not the volunteer monitor, but the "professional" that is most negligent about following procedure.  Professionals more become complacent because of familiarity and experience with the disk, or it may be that some professionals have the attitude that it is "only" a Secchi disk reading.  

In the next section, we discuss procedures that are actually being used in the United States.  These procedures vary considerably and serve to illustrate the problem of lack of standardization of Secchi disk design and procedure.

Some Useful References

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.  J. Lake and Reserv. Manage.  13(2):  142-153.


For more information on the Great American Secchi Dip-In, contact us at: DipIn@kent.edu or write

Great North American Secchi Dip-In
Department of Biological Sciences
Kent State University
Kent OH 44242