The researchers studied zooplankton in two man-made lakes to see if they could tell how polluted the water was. They found that the lakes had too many nutrients, causing algae to grow too much. The most common zooplankton were tiny animals called rotifers, followed by copepods and water fleas. Some specific species of zooplankton were found in high numbers, indicating that the lakes were polluted. The amount of these animals in the lakes was affected by how clear the water was and how much chlorophyll was in it.