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A secchi disk contains alternating black and white quadrants and is attached to a line. This line is used to lower the disk into a body of water with the purpose of measuring the clarity of the water. The depth at which the disk can no longer be seen is called the secchi depth and is the measurement recorded.
Why do we measure it?
Secchi depth is important to measure because the clarity of water impacts the amount of light penetration and in turn can affect photosynthesis and the distribution of organisms. While people often focus on the negative aspect of losing clarity, completely clear water is usually not desirable either because that means the water is devoid of needed food like plankton.
Secchi disk readings are useful in comparing bodies of water or looking for changes to a specific body of water over time. Changes in clarity of water can be an indicator of a human threat to an ecosystem.
What affects it?
Clarity will decrease as color, abundance of algae or suspended sediments increase. The color of water is sometimes caused by staining, due to decay of plant material. Excess algae growth can occur where there is additional input of nutrients like phosphorus and nitrogen from agriculture and/or sewage treatment or septic system waste. An increase in suspended sediments can be the result of urban, agricultural or storm runoff.
Who uses the data?