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The Secchi Dip-In |
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Contact the Dip-In at:
Web Site Updated: May 05, 2008 |
The method described below is that of the Wisconsin Department of
Natural Resources Self-Help Lake Monitoring Program, as modified
for use by CLAM, the Ohio Lake Management Society's Citizen's Lake
Assessment and Monitoring volunteer program. For more information on disk design and sampling procedures please visit our Secchi Disk Methods page. Warning: this procedure is presented for information only and is not a recommendation of the Dip-In. It should be considered for use only by new programs. Volunteers or coordinators should not change established procedures without careful consideration because of the potential of data loss.
Methods Used in other Volunteer Monitoring Programs (edited from Carlson and Simpson 1995) Below are methods used
by some monitoring programs to measure Secchi depth.
These methods are given without comment or recommendation as to
whether the procedures are appropriate or correct.
The reader should use this section to gain insight on how other
programs are using the technique, and then combine those insights with
the other discussions in the chapter do decide on an appropriate method
for your program.
Volunteer Lake Monitoring: A
Methods Manual
The EPA Volunteer Lake
Monitoring manual (Simpson, 1991) suggests the use of a black and white
Secchi disk and a marked line. The
disk is lowered on shady side of boat and observed until it disappears. The disk is then moved up and down until the single point of
vanishing and reappearing is found.
A clothespin is attached to the line at this point, and the
distance measured. Repeated
measures by the volunteer is suggested as a quality control check. The average of two measurements is recorded.
Florida
Volunteers in the Florida
Lakewatch program use a black and white disk lowered on the downwind
side of the boat. The
volunteers are urged to use their own shadow to minimize glare.
They should not wear sunglasses.
The vanishing point of the disk is found by lowering the disk out
of sight and then raising it slowly until it can be faintly seen.
Florida has made a comparison
between measurements made by the volunteers and made by professional
working on the same lakes. The
nature of the sampler (volunteer or professional) accounted for less
than 1% of the variation in the relationship.
More important sources of variation were lake to lake differences
(62%) and seasonal differences (20%), and station differences (16%)
(Canfield, 1991).
Illinois
The Illinois volunteer
program uses a black and white disk.
Volunteers are urged to take readings as close to mid-day as
possible or at least to measure transparency at the same time of day.
Volunteers average
the depths of the disappearance and reappearance of the disk. The readings are taken on the shady side of the boat with
sunglasses removed.
Indiana
The Illinois volunteer
program uses a black and white PVC plastic disk made at Indiana
University. Measurements
are made on bright, calm days, between the hours of 10:00 AM and 4:00 PM
at a single lake location. The
same lake site is used for all measurements on a given lake to promote
consistency. Some volunteers also monitored their lakes at other locations
as well, but these measurements were not reported (Jones, et al.,
1992). The method for
taking the Secchi depth reading is that described in the Wisconsin
Self-Help Lake Monitoring program (see below).
Minnesota
The Minnesota Citizen
Lake-Monitoring Program uses the all-white Secchi disk described by
Shapiro et al. (1975). The
volunteer is asked to make readings between 10 a.m. and 3 p.m. on
bright, calm days. The disk
is lowered over the shady side of the boat and the volunteer is asked to
remove sunglasses. The
depth of disappearance and reappearance of the disk is averaged, and the
depth is measured to the nearest 1/2 foot.
The line is marked. Dacron,
or other polyester material, is recommended for the line because nylon
has a tendency to stretch when wet, as well as being adversely affected
by repeated exposure to sunlight.
New York
The Secchi disk is lowered
into the water on the shaded side of the boat until it just disappears
from sight. This depth is
recorded to either the nearest 1/4 meter or to the nearest 0.1 meter or
1.0 foot on measured tape lines. The
depth is then lowered an additional meter and raised until it reappears.
These two values are then recorded separately.
Ohio (NEFCO)
This program uses a black and
white disk. The volunteer
is directed to remove any sunglasses and to lower the disk over the
shady side of the boat. This
program uses the methods first used in the Wisconsin Self-Help Lake
Monitoring program (see below). Unlike
the Wisconsin program, the Secchi depth is measured on the line with a
yardstick. The line is marked, but there are only 5 foot markings in the
range where Secchi depths would normally be found: the markings are only used to find the depth of the lake at
the sampling site.
Ohio (CLAM) The
Ohio Lake Management Society's Citizen Lake Assessment and Monitoring
program uses the same type of disk and procedure as the Ohio NEFCO
program, except the volunteers are given a 12 inch ruler rather than a
yardstick to measure the depth.
Wisconsin The Wisconsin
Self-Help Lake Monitoring program requests that the volunteer remove
sunglasses and lower the disk over the shady side of the boat.
To obtain the Secchi depth, the volunteers mark the line with a
clothespin at the depth of disappearance, then lower the disk well below
this depth, then slowly raise the disk until it reappears. The Secchi depth is the mid-point between the depth of
disappearance and the depth of reappearance.
The clothespin is then moved to this mid-point.
The Secchi depth is read to 1/4 of a foot.
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