It has been shown that the extinction coefficient is related closely to Secchi disk visibility in meters: K = the extinction coefficient, and z = depth in meters. Where e = base of the natural logarithm (2.303), Light at depth z = Incident light x e -kz

The passage of light through a column of water is described by the equation: Secchi disk visibility usually is reported in centimeters, and in natural waters, the value may vary from a few centimeters to several meters. It is attached to a calibrated line and fitted with a weight so it will sink rapidly. The standard Secchi disk is a 20-cm-diameter disk with alternate black and white quadrants. The Secchi disk since has become a popular limnological instrument for determining the clarity of water. The Secchi disk apparently was named after an Italian sea captain who in the early 1800s used the degree of disappearance of a white disk as an index of water clarity. Still, this test is often executed incorrectly, and the limitations of its interpretation are often not fully understood. It would be difficult to find a pond aquaculture worker who has not measured Secchi disk visibility or at least seen someone measure it. Secchi disk visibility testing likely is the most frequent measurement of water quality made in pond aquaculture. Exposed black dirt and eroded shorelines should be stabilized by natural vegetation (or riprap if the slope is steep) to prevent impact on lake water quality (see Best Management Practices).Limnological instrument determines water clarity Although simple in nature, accurate secchi disk visibility testing results can be improved by following a few guidelines. This dirt makes the water cloudy in itself, but it also contains nutrients ( phosphorus and nitrogen) that can cause algal blooms. The exposed dirt from digging gets blown into the lake by wind and washed into the lake by rain. When the water transparency is low and cloudy, the Secchi depth is low (Figure 2) ( more information on the relationship of Secchi disk to lake water quality).Ĭonclusions about the trophic state of a lake can be drawn by measuring the Secchi depth (Figure 3) since water transparency is directly related to Total Phosphorous and Chlorophyll-a.Īn example of changes in transparency due to human impact would be if there is construction going on near the lake shoreline. When the water transparency is high, the Secchi depth is high. The depth that the Secchi disk can no longer be seen through the water is the Secchi depth. A Secchi disk is a metal disk, 8 inches in diameter that is lowered into the water on a cord. Water transparency is measured with a Secchi disk (how to use a Secchi disk). In other words, when the water is murky or cloudy and contains a lot of particles, the light cannot penetrate as deeply into the water column. These particles can be algae or sediment from erosion, the more particles – the less water transparency. Water transparency depends on the amount of particles in the water. The area of the lake that is too dee p for plants to grow is called the limnetic zone (Figure 1). The area around the edge of the lake that is shallow enough for plants to receive light is called the littoral zone. The top section of the lake that sunlight penetrates is called the Euphotic zone. Plants and algae need sunlight to grow, so they are only able to grow in areas of lakes where the sun penetrates. In lakes this means how deep sunlight penetrates through the water. Transparency is how easily light can pass through a substance.

Secchi disk graph