The Secchi Disk provides a convenient method for measuring light penetration below the water surface. By using the Secchi Disk, one can determine the transparency or limit of visibility of the water and estimates of light transmission can be derived. The limit of visibility is approximately the region of transmission of 5% sunlight (Reid, 1961). Once the limit of visibility is established, calculations can be made to determine the lower limits of the euphotic zone (light) which is usually three times the Secchi Disk depth (Welch, 1948).

      Welch (1948) lists the following conditions for Secchi Disk measurements:

and any departure from the conditions should be specifically stated in the records."

Notes: High concentrations of dissolved substances such as fats, proteins, and carbohydrates and particulate organic matter such as phytoplankton, zooplankton, detritus, and colloidal substances which are found in the highly productive coastal waters, cause most of the light to be absorbed within the first few meters of water. In the clearest ocean water with low concentrations of dissolved substances and particulate organic matter may have Secchi Disk readings of 40 meters or more.

Secchi Disk (LaMotte (1062)). Lowering into the water

The mark is at the bend, the red lines are 10cm each..(there are 12 lines to the bend (120cm))..so

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