Drag the plankton net across the water to filter out a small sample of plankton.
Use the eyedropper to suck up the plankton sample. Place one drop of the sample on the slide.
Focus the microscope to see the entire drop. Count the number of organisms.
Multiply that number by 60 to get the average number of plankton in a teaspoon of water (one drop of water equals 1/60th of a teaspoon).
Multiply the number of plankton in one teaspoon by 3 to get an approximate number of plankton in one tablespoon (one tablespoon is 3 teaspoons).
Multiply the number of plankton in a tablespoon by 16 to get the number of plankton in a cup. Multiply the number of plankton in a cup by 4 to get the number of plankton in a quart. Multiply the number of plankton in a quart by 4 to get the number of plankton in a gallon of water. Multiply the number of plankton in a gallon by 3.785 to get the number of plankton populating one liter of water.