Secchi Disk
A
Secchi disk is a weighted circular plate, 8 inches in diameter with
opposing black and white quarters painted on the surface. It is used to determine the clarity or transparency
of the water. It is attached to a line that is marked in meters and
is lowered until it disappears from sight. The Secchi depth is the
distance from the surface of the lake to the point the disk
disappears.
All Secchi disk
readings are in meters, but are sometimes later converted into feet.
Note:
[1 meter (m) = 3.28 feet]
Factors which
reduce clarity are algae, zooplankton, water color and silt. Since algae
are the most abundant item, measuring transparency indirectly measures
the algal productivity. Secchi disk readings can be used to track
changes in water quality over time.
Transparency values
in Maine vary from 0.4m (1.5ft) to 20.0m (67 ft), with the average being
4.9m (16.2 ft). Unless a lake is highly colored (see explanation of
color), a transparency of 2m (6.5 ft) or less indicates a water quality
problem that has resulted in an algal bloom. In Maine, the mean
(average) Secchi disk readings are related to algal productivity using
the following guidelines:
- 4m
(13 ft) or less = Productive
- 4-7m (13-23 ft)
= Moderately Productive
- 7m (23 ft) or
greater = Unproductive
See a
list of Maine Lakes with recorded algal blooms
Care to Compare?
The data used to create this graph were provided
to PEARL by the Maine Department of Environmental Protection
(DEP). DEP summary data were
used, which can be viewed by clicking "Browse Data Sets" then
the "Water Quality" button from PEARL's homepage. The
distribution of mean
(or average) Secchi disk depth recorded at 946 sampling sites are shown.
Need data? You can search for a lake or pond
from the PEARL homepage.
Out of
946 sample stations:
|
Secchi reading in meters |
Number of sample
sites with readings in this range |
Percentage of
sample sites with readings in this range |
|
0 - 1.9 |
56 |
6.0 |
|
2 - 3.9 |
251 |
26.5 |
|
4 - 5.9 |
371 |
39.2 |
|
6 - 7.9 |
180 |
19.0 |
| 8 -
9.9 |
72 |
7.6 |
|
10 - 11.9 |
11 |
1.2 |
|
12 -13.9 |
4 |
0.4 |
|
14 - 15.9 |
1 |
0.1 |
-
56, or 6.0%, had Secchi readings of 0
- 1.9 meters
-
251, or 26.5%, had Secchi readings of
2 - 3.9 meters
-
371, or 39.2%, had Secchi readings of
4 - 5.9 meters
-
180, or 19.0%, had Secchi readings of
6 - 7.9 meters
-
72, or 7.6%, had Secchi readings of 8
- 9.9 meters
-
11, or 1.2%, had Secchi readings of
10 - 11.9 meters
-
4, or 0.4%, had Secchi readings of 12
- 13.9 meters
-
1, or 0.1%, had Secchi readings of 14
- 15.9 meters
|
