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Monitoring Methods
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Web Site Updated:
January 04, 2007
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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"
Design for a Secchi Disk?
The
Secchi disk is and should be the cornerstone of volunteer lake
monitoring programs. It is inexpensive and provides useful data.
However, it does have a number of technical problems that need to
be addressed. Many of the
problems can be minimized by standardizing the equipment and carefully
training the volunteers.
Disk
Construction
It
is amazing the number of objects that have been used as disks,
apparently with success. I
have used a white Frisbee in an emergency.
Jones and Bachmann (1978) used plastic dinner plates in Iowa.
The dinner plates gave the same reading as a standard disk and
were far less expensive. A
painted 15 cm (6 in) margarine container lid
(McCauley, 1990) and a large coffee can lid
(Schelske, Personal communication) have also been used as disks. The volunteer program in Minnesota uses an all-white disk
with two opposing notches, which provide a convenient spot to wind the
rope.
Disk
Size
The size of the disk has also varied.
Marine disks are 40 or 50 cm (16 to 20 in) in diameter, but, in
lakes the standard has become 20 cm (8 in).
According to theory, a larger disk should be seen deeper than a
smaller one, but, there are studies that have found no difference
between large and small disks. Some
studies found that, in very clear waters, the disk disappeared, not
because of loss of contrast, but because the disk became too small to
see. In these instances,
scientists have used disks with diameters of up to 1.2 m (3.9 ft).
At the opposite end of the size spectrum, disks as small as 2.5
to 5 cm (1 to 2 in) have been used in small puddles (Richard Hill,
Personal communication).
Reflectance
It should be possible to standardize the reflectance of the disk.
Since the observer is viewing reflected light, theory suggests
that the more light reflected back up to the observer, the more intense
the contrast between the disk and its background.
The greater the contrast, the deeper the disk will be visible.
If reflectance were the only consideration, then why not use a
mirror for a disk? The
problem with a mirror is that it reflects all the light at an angle
equal that of the light striking the disk; therefore its amount of light
reflected directly upward should be highly dependent on the angle of the
sun. The best disk would
reflect light equally in all directions, whatever the angle of the sun.
A glossy finish may accentuate problems related to the angle of
the sun and of the time of day, but this has not been investigated.
A matte disk should eliminate or lessen these effects.
The same
paints should be used as more disks are made, so that the reflectance
remains constant. Volunteers should also be reminded to keep the disk clean and
free of scratches.
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| Figure 1. Disks should be designed to have a
matte finish rather than a glossy finish, which may act like a
mirror. |
Measuring Line
A number of programs use a marked line. Volunteers simply read
the Secchi depth based on the markings on the line. The advantage
of this method is simplicity, but there are several disadvantages to
marking the line.
Shrinkage could introduce a
significant error in the measurement.
A number of programs report shrinkage in the line.
The measuring marks used
(1/4 to 1 foot in some programs) does not allow for the accuracy that is
possible with the Secchi depth. Measuring
Secchi depth at intervals of even 1/4 foot can introduce significant
errors in low transparency waters (Fig. 2).
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Figure 2. The
figure to the right uses the Florida data of Canfield and Hodgson
(1983) to illustrate the problem of using larger intervals on the
measuring line. In this case, the Secchi readings were rounded
off to 10 cm (3.9 inches).
A plot of the inverse Secchi depth versus chlorophyll
reveal ever increasing gaps with no data as the Secchi depth falls
below 1 meter (1/SD > 1.0).
This effect could have been minimized if a smaller measurement
interval had been used. |
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The possible psychological effect
of having markers may be a third, as yet undocumented, error.
If the volunteer has a priori knowledge about the Secchi
depth based on values obtained during their last visit or if they take
replicate samples, this knowledge, combined with the visual clues of
depth given by the markings, could bias their readings.
Wilson (Personal communication) also suggests that marking lines
is very expensive.
We recommend the use of an unmarked
line and the use of a
yardstick or meter stick to measure depth.
This technique requires the additional equipment of a measuring
device and added time of measurement.
However, the advantages of not having to worry about shrinkage,
the reduced cost, and the elimination of statistical or possible
psychological errors far outweigh the additional equipment and effort.
The alternative would be a highly accurate marked line, such as a nylon
measuring tape, together with a viewscope. The tape gives the
reading the precision possible and the viewscope keeps the observer from
seeing the results until after the measurement has been made.
The
Observer
The human factor must also be considered. The
observer must accurately see and record the Secchi depth. The
human eye does not respond exactly to the spectrum of light being
reflected by the disk, and, therefore, the disk reading would be
different in waters of different color (Preisendorfer,
1986).
To correct for this ,the observer should view the disk through a
Wratten #61 green filter to standardize the color of light entering the
eye
(Williams,
1970).
However,
apparently no one has adopted standardized filter glasses.
What is the "Proper" Design
In the opinion of this author (R. Carlson)
the disk design should be as follows:
A flat 20 cm disk with alternating black and
white quadrants.
The surface should have a matte (flat) finish.
The lowering line should be unmarked or, if a
viewscope is used, a fiberglass measuring tape should be used for the
line.
In very clear waters a larger, 40 cm disk, again
with black and white quadrants, should be used.
This disk should, according to theory, work best
under all possible light climates, have the least interference with sun
angle, and minimize the errors involved with measuring the depth. However,
these recommendations are only guidelines for new programs.
If a program is already using a disk of a different design they should
be very cautious of changing designs, especially if the change would
invalidate comparisons with past year's data. Such changes
might include the size, reflectivity, or color of the disk.
Changing to a viewscope would also change the nature of the data being
gathered. If time trend analysis is at all important to the
program, be very cautious about making changes.
It is also
important to consider the possible effects of ordering new disks.
Will they be the same as the old models? If you are making your
own, are you using the same type of paint, thus insuring the same
reflectance? As the disks get older, are you insuring that the
surfaces of the disk are still clean? I have seen too many disks
are seen that are scratched, chipped, or rusted to deny that care of the
disk surface is not considered to be that important. Perhaps
someone should invent a disk holder that will protect the disk when it
is not being used.
Some Useful References
Canfield,
D.E. Jr. and L.M. Hodgson.
1983.
Prediction of Secchi disc depths in Florida Lakes:
impact of algal biomass and organic color.
Hydrobiologia.
99: 51-60.
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. 1986. 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.
Williams,
J.
1970. Optical Properties of the Sea.
U.S. Naval Institute.
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