Measuring Water Clarity with a Secchi Disk


Maine Lakes Society and Lakes Alive!

Measuring Water Clarity with a Secchi Disk

Do you like to swim and boat in lakes that are clear and blue or murky and green? Most people like clear, clean water, and one way to gauge clarity is by measuring how deeply sunlight can penetrate down into the water column. The tool used to measure this is called a Secchi disk, which is a flat, round, black and white disk attached to a measuring tape.

Lake monitoring stewards slowly lower the disk down into the water until it is no longer visible through the Aqua Scope. An Aqua Scope is like a giant goggle that makes it easier to see through the uneven surface water; glare from the sun and waves often make it difficult to see very far. Once the Secchi disk disappears from sight, students record the depth shown on the measuring tape and compare their measurements to those of other lakes.


Essential Questions

  • What is the water clarity of the lake as measured by the Secchi disk?
  • How does this compare to readings at other times of the year or in other lakes?
  • What causes the clarity (Secchi depth) of a lake to change?

Learning Objectives

  • Secchi depth is a measurement of water clarity, which is a good indicator of water quality in most cases.
  • A lake is a dynamic system affected by the land that surrounds it (its watershed).
  • An overgrowth of microscopic plants, like plankton, floating in the water contributes to poor water clarity.
  • Phosphorus, a nutrient, is the limiting factor for plant growth and excessive amounts in a lake contribute to algal blooms, or an accelerated growth of phytoplankton. This causes the lake to become green, murky, and smelly.
  • Stormwater runoff often carries a lot of phosphorus. Vegetative buffers along the shore filter stormwater and remove much of the excess phosphorus before it enters the lake, helping to maintain high water clarity.
  • Poor water quality has a widespread effect across the watershed; affecting plant and animal life, people, homes, and local businesses.


Additional Info
Introduction to turbidity and how to use a Secchi Disk
Technical information about Secchi disc (Wikipedia)


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