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Secchi disc

An approximate evaluation of transparency of water can be made with a Secchi disc. Devised by an Italian oceanographer in the nineteenth century, this method is still widely used because of its simplicity. The Secchi disc is a weighted white or white and black disc, 20 cm in diameter. Transparency, estimated in this way, is the mean of the depths at which the Secchi disc disappears when viewed from the shady side of the boat and at which it reappears upon raising after it has been lowered beyond visibility.

Secchi disc transparency is basically a function of reflection of light from the surface of the disc and therefore is affected by the absorption characteristics of the water and of dissolved and particulate matter contained in the water. Although high concentrations of dissolved organic matter decrease transparency in a nonlinear way, as measured with a Secchi disc, reduction in light transmission as evaluated by the disc is influenced strongly by increasing scattering of light suspended by particulate matter. Depth of transparency is largely independent of surface light intensity but becomes erratic near dawn or dusk.

Observed Secchi disc transparencies range from a few centimeters in very turbid waters to over 40 m in a few clear lakes; most are in the range of 2 to 10m. Marked seasonal fluctuations occur in response to variations in concentrations of plankton or inorganic particles. Secchi disc transparencies correlate well with percentage transparency.

Procedure

1. Determine the Secchi disc transparency by lowering, on the shaded side of the boat, the disc just to the point where it is no longer visible. Record the depth using the black and red markings on the line.

2. Lower the disc below the point where it is no longer visible and slowly raise it until it just reappears. Record the depth. The Secchi depth is the average of these two measurements.