Water Clarity is an important physical characteristic because it gives an indication of the overall water quality. High levels of nutrients cause dense algae blooms in a lake. The resulting brown or green color reduces water transparency significantly. Clarity, however, is not an indicator of pure water. Many toxins are invisible. Clarity is measured using a device called a Secchi disk. This is a disk-shaped piece of plastic divided into four quarters, painted alternating black and white. The disk is lowered into the water on the end of a calibrated line. When the disk disappears from view the depth is measured. As it is pulled up again, a second measurement is taken when the disk reappears. The average is recorded as the Secchi depth.A Secchi depth value of 4.5 meters is used to characterize very good water quality. Swimming is not recommended in water where the Secchi depth is less than 1.3 meters. Lake Singletary’s Secchi depth is generally between 3 and 4 meters, which is considered good, although we periodically experience clarity at or below 2 meters. Subsequent to an algae bloom in September of 2001 the Secchi dropped to 2.4 meters. In August and September of 1998, during a severe algae bloom, the Secchi depth dropped to below 2 meters - the worst ever recorded at Lake Singletary. The amount of Chlorophyll present is also an indicator of clarity. Chlorophyll is the photosynthetic pigment in all green plants, and is an inexpensive measure of the amount of algae present. In 1999 we added chlorophyll to our monthly sampling program. Lake water is filtered using special apparatus. The filter is then fan-dried and sent to the U Mass lab, which performs the analysis.