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Because phytoplankton respond quickly to changes in nutrient levels, their population density acts as an indicator of nutrient pollution and Bay health.
Scientists use a device called a Secchi disc to measure water clarity. A Secchi disc is a simple, black-and-white circle attached to a rope. The Secchi disc is lowered into the water until it disappears, then pulled up until it can just barely be seen. Scientists note the water line on the rope and measure the distance between the Secchi disc and the water line. The measurement is the water’s clarity: the depth that sunlight is able to penetrate through the water.