Secchi Depth
Secchi depth is a measurement of water clarity or transparency, which is affected by water color, algae and suspended sediments. A Secchi disk is a 20 cm diameter circular plate divided into quarters painted alternately black and white that is lowered by hand into the water to the depth at which it vanishes from sight. 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. The distance to vanishing is then recorded as Secchi Depth. The clearer the water, the greater the distance. Secchi disks are simple to use and inexpensive. For river monitoring they have limited use, however, because in most cases the river bottom will be visible and the disk will not reach a vanishing point. Lakes and deeper, slower moving rivers are the most appropriate places for Secchi depth measurement although the current might require that the disk be extra-weighted so it does not sway and make measurement difficult. Secchi disks cost about $40 and can be homemade. The line attached to the Secchi disk may be marked in meters in waterproof ink and using a measuring tape in meters helps having precision to the centimeter.
Transparency can be affected by the color of the water, algae, and suspended sediments. Transparency decreases as color, suspended sediments, or algal abundance increases. Water is often stained yellow or brown by decaying plant matter. In bogs and some lakes the brown stain can make the water the color of strong tea. Algae are small, green aquatic plants whose abundance is related to the amount of plant nutrients, especially phosphorus and nitrogen. Transparency can therefore be affected by the amount of plant nutrients coming into the lake from sources such as sewage treatment plants, septic tanks, and lawn and agricultural fertilizer. Suspended sediments often come from sources such as resuspension from the lake bottom, construction sites, agricultural fields, and urban storm runoff.
Transparency is an indicator of the impact of human activity on the land surrounding the lake. If transparency is measured through the season and from year to year, trends in transparency may be observed. Transparency can serve as an early warning that activities on the land are having an effect on a lake.