How to Count Phytoplankton With an Inverted Microscope
Trillions of phytoplankton exist in Earth's lakes, oceans, rivers and seas. Phytoplankton are small, photosynthetic, generally single-celled organisms that resemble microscopic plants. The most numerous type of phytoplankton is the diatoms. Diatoms range from 50 to 150 microns in size, so you need a microscope to see them in any detail. You can count the phytoplankton that exist in a water sample using a microscope and a gridded microscope slide. Using an inverted microscope for this process allows you to view the sample from the bottom, where organisms tend to settle in a suspension of water.
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Things You'll Need
- Gridded microscope slide (2 millimeter x 2 millimeter grid squares)
- Dropper
Instructions
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1
Place a drop of water containing phytoplankton onto your gridded microscope slide using your eyedropper. Tilt the slide back and forth a little to flatten the drop out so that it isn't bulging from the surface of the slide.
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2
Count the number of grid squares that the water sample covers. Suppose, as an example, that the water covers 20 grid squares.
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3
Place the microscope slide on the tray of your inverted microscope. Turn the microscope on and focus on the slide using a low to medium power lens. A high power lens will likely zoom in too close to the phytoplankton and grid to be useful.
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4
Count and record the number of phytoplankton organisms in one 2 mm x 2 mm grid square. Record the data. Repeat the count for four more grid squares, then add up the total number of phytoplankton that you counted. Suppose there were 11 organisms total in the 5 grid squares.
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5
Divide the total number of grid squares covered by the water by 5. In the example, 20 grid squares were covered by the water, so 20 / 5 = 4.
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6
Multiply your answer from the previous step by the total number of organisms that you counted to estimate the total number of organisms on the slide. In the example, you counted 11 organisms, so the whole slide contains 11 x 4 = 44 organisms.
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7
Multiply your answer from the preceding step by 20 to estimate the total number of organisms in 1 milliliter (mL) of water from your sample. You multiply by 20 because 1 mL of water is equal to 20 drops from a standard dropper. The calculation would look like this:
Number of organisms in 1 mL of water = 44 per drop x 20 drops / mL = 880 organisms per mL of water.
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References
- Photo Credit moon jellyfish and planktons in blue water image by Lijuan Guo from Fotolia.com
