Can you identify independent and dependent variables?

To studi effects of common substraces on the heart rate of a tiny aquatic organism known as Daphnia , students placed a Daphnia in a drop of water on a gl slide .The student then added 1 or more drops of a test substance dissolved in water to the slide , waited 10 seconds, than counted heart beats for 10 seconds . The students used a clean slide and a new Daphia each time . Their data table is shown below . PLEASE PEOPLE HELPP!