The Secchi disk is a device which can be used to take quick measurements of water turbidity. These devices can be used in any body of water, and they are used in water quality monitoring programs all over 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 on. Someone can learn to use the disk in just a few minutes to take quick turbidity measurements.
Turbidity or clarity refers to the visibility in the water column. When turbidity is high, lots of suspended sediments and organisms obscure the water clarity, cutting visibility down significantly. When turbidity is low, less material is present in the water, and it can be clear to a significant depth. This has an impact on organisms which live in the water, as many are adapted to survive in specific types of conditions, and changes in clarity can cause dieoffs or other problems.
Secchi disks consist of disks which are eight inches (20 centimeters) in diameter, classically painted in black and white quadrants, although some are just white. The disk is attached to a line marked with depth measurements. To use the disk, someone lowers it into the water to the point where it is no longer visible, and takes note of the reading on the line. Some people lower the disk to the invisible point, raise it to the visible point, and average the two measurements for a more accurate reading.
Readings with a Secchi disk are generally taken during the middle of the day, from the shady side of a boat so that glare does not interfere with the reading. For consistency, the same observer needs to take the reading every time, because different people have differing levels of visual acuity, and some people may see the disk more clearly than others. Thus, if two separate individuals take readings, the readings may not match; using the same observer keeps records constant.
Secchi disk readings are kept on file by organizations which monitor water quality, and sometimes readings from various bodies of water are published for convenience. In some nations, volunteers take Secchi disk readings throughout the year to monitor changes in the water as the year goes by. These citizen scientists offer a valuable service to water quality monitoring organizations, which usually lack the personnel to monitor numerous bodies of water themselves.