Estimating Microalgal Density

Examples of Counting Devices Useful for Determining the Concentration of Algae in a Sample of Water

• Hemacytometer Counting Chambers
• Sedgwick-Rafter Counting Chambers
• Utermohl Devices

General Method for Determining the Density of Microalgae in Liquid Suspension

1. Concentrate or dilute the suspension of algae as necessary for accurate counts. Record the concentration or dilution factor.
2. Mix the algal suspension homogeneously, then place a sample of the suspension in a counting chamber designed to be observed under the microscope.
3. Count the number of algae within a unit volume of the counting chamber, like a hemocytometer. Repeat this procedure for other counting chambers of the same volume in order to obtain statistically accurate data.
4. Calculate the average number of algae per counting chamber. Since the volume of the counting chamber is known, the average number of algae per volume is calculated.
5. Correct for the concentration or dilution factor of the algae (Step 1 above) to determine the concentration of algae in the original sample.

Quick Method for Estimating Microalgal Density

1. Place 0.05 mL of the liquid algal suspension on a clean glass slide. Cover the liquid with a coverslip (22x22mm), avoiding any bubbles under the coverslip. The coverslip is then suspended almost exactly 0.1 mm above the surface of the glass slide.
2. Place the prepared slide under the microscope and count all algae that are visible in one field of view without moving the stage laterally, but focusing up and down as necessary. Record the magnification of the microscope.
3. Move the stage to a different position and count the number of algae in that field of view. Repeat this procedure several times to determine the average number of algae per field of view.
4. Keeping the microscope at the same magnification, use a stage micrometer to determine the diameter of the filed of view. From the value obtained, determine the surface area of the field of view.
5. Calculate the volume within a field of view as: surface area X height (0.1 mm). Then calculate the algal density as: 

            algal density = (average # of algae per field of view) ÷ (volume per field of view)