Secchi disk transparency
Secchi disk transparency
Source: Michigan Lake & Stream Associations, Inc
Background
The Secchi disk is used to measure how deep a person can see into the water. Even though the Secchi disk measurement of water clarity is an approximate evaluation of the transparency of water, it is used primarily for its simplicity. A more accurate measurement of underwater irradiance can be made by the use of a photometer. Vertical light penetration, as measured by Secchi disk, may also serve as a proxy for the phytoplankton biomass in a water body.
The greatest value of the Secchi disk measurements occurs when readings at each station are recorded and compared from week to week, month to month and season to season. Several factors are involved in depth determination, including the eyesight of the viewer, the time of day the readings are taken (midday- between 10:00 and 14:00 is preferred), the reflectance of the disk, the color of the water, clay particles or other materials suspended in the water, etc.
Equipment
Secchi disk - 8 inch diameter metal disk painted in alternate black and white quadrants (see below) with waterproof tape or rope marked to 20cm resolution.
Procedure
The disk is lowered into the water by unwinding a waterproof tape or marked line (20cm resolution) to which it is attached and until the observer loses sight of it. The disk is then raised until it reappears. The depth of the water where the disk vanishes and reappears is the Secchi disk reading. The depth level reading on the tape at the surface level of the water is recorded.
In order that the Secchi Disk measurement be done to provide the greatest accuracy, the following conditions should be met:
- The same person should perform all readings since sharpness of vision varies from person to person.
- The reading should be taken on the same day of the week, if taking weekly readings or at least not more than one day before or after the same day of the week.
- It is preferable that the measurement be taken between 10:00 a.m. and 2:00 p.m. so that the light rays from the sky are at a similar angle each time the reading is taken.
- Avoid taking the measurement when the water is choppy or rough.
- Readings should be taken on the shady side of the boat.
- The reading should be taken at the same location each week.
Record:
a. Cloud cover (see Cloud Cover Field Guide below).
b. Depth of Secchi disk disappearance and reappearance, time of day, tide,
current condition (strong, mild, or none), wind/wave condition (ripples, small waves, whitecaps), viewer’s initials and any other significant notes.
