The Secchi disc is a simple and inexpensive device used to measure water turbidity. It consists of an 8-inch disc attached to a depth-marked line, and readings are taken by lowering it into the water until it is no longer visible. The same observer should take the readings for consistency, and readings are usually taken in the middle of the day from the shaded side of a boat. Secchi disc readings are used in water quality monitoring programs worldwide and are archived by water quality monitoring organizations. Citizen scientists often take Secchi disc readings throughout the year to monitor changes in water quality.
The Secchi disc is a device that can be used to take quick measurements of the turbidity of water. These devices can be used in any body of water and are used in water quality monitoring programs around the world. While other devices can measure turbidity more accurately, the Secchi disk is very easy to use, cheap to implement, and easy to train. Someone can learn to use the disc in minutes to make quick turbidity measurements.
Turbidity or clarity refers to the visibility in the water column. When turbidity is high, many sediments and suspended organisms obscure the clarity of the water, greatly reducing visibility. When turbidity is low, there is less material in the water and it can be clear to a significant depth. This impacts water-dwelling organisms, as many are adapted to survive in specific types of conditions, and changes in clarity can cause die-offs or other problems.
Secchi discs consist of discs eight inches (20 centimeters) in diameter, classically painted in black and white dials, although some are only white. The disc is attached to a line marked with depth measurements. To use the disc, someone lowers it into the water until it is no longer visible, and notes the reading on the line. Some people lower the disc to the invisible point, raise it to the visible point, and average the two measurements for the most accurate reading.
Readings with a Secchi disk are usually taken in the middle of the day, from the shaded side of a boat so that glare does not interfere with the reading. For consistency, the same observer should take the reading each time, because different people have different levels of visual acuity, and some people may see the disk more clearly than others. Therefore, if two separate individuals take the readings, the readings may not match; using the same observer keeps the records constant.
Secchi disk readings are archived by water quality monitoring organizations, and readings from various bodies of water are sometimes published for convenience. In some countries, volunteers take Secchi disk readings throughout the year to monitor changes in the water as the year progresses. These citizen scientists provide a valuable service to water quality monitoring organizations, which typically lack the staff to monitor many bodies of water themselves.
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