Definition: Secchi depth is a measurement of water transparency, which is affected by water color, algae and suspended sediments.
Why we monitor transparency: Secchi disks were originally designed to allow a simple measurement by any interested citizen. This measurement of lake transparency has been used to determine a lake’s health, often by relating transparency to a lake’s “trophic state”, or how productive the lake is at any point throughout the year. This is then used to determine if a lake is too productive (eutrophic) as a result of high nutrient concentrations. Increases in nutrients are often the result of human activity in the lake’s watershed. Turbid (cloudy) water also affects fish directly by limiting their ability to see predators, and affects our aesthetic enjoyment of a lake we use for recreation.
What influences transparency: Algae abundance the largest driver of transparency in most lakes and is influenced by the available nitrogen and phosphorus. Excessive nutrients are the results of fertilizer overuse, leaking septic systems, wastewater treatment plants, or manure runoff. Disturbances to the bottom sediments of shallow lakes from carp, wind, or loss of stabilizing rooted plants, may also result in increased turbidity.
Learn more about light and transparency here.
This is best to do when the sun is near its apex, generally between 10 AM and 2 PM. Also, you need to remove sunglasses before measuring.
On the shady side of a boat or dock, begin to lower the disk into the water.
Lower the Secchi Disk until it completely disappears. Then slowly retrieve the disk. As soon as you can make out the faint black and white markings stop.
Mark the tape at the surface of the water to record the depth to the nearest centimeter.
If you are on a dock and out of arm's reach of the water, use a clothespin to mark the height of the dock and then measure to the surface of the water. Subtract this from your final measurement.
Note: While Secchi depth is the preferred method to measure transparency in standing water, some volunteers may use turbidity tubes from the bank for small ponds or when shallow water is all that can be reached. In these cases, follow the Stream Instructions for turbidity tube, taking care to avoid disturbing the bottom sediments. Be sure to convert the units when recording on the data sheet and note the type of equipment used.