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Your Fill of Chlorophyll
In the Lab, part 2
If you are a volunteer for the program, you’ve seen these instructions before: measure out 250 mL of lake water in the graduated cylinder, place filter rough side up...pour water into filter funnel, work hand-pump to create vacuum in receiving flask...place filter house with folded filter into desiccant and store in freezer. You turn in a can of filters in the fall and in the spring you receive numbers that represent the chlorophyll concentration from those filters. You may ask yourself - How do the little white filters become numbers? Hopefully this article will answer that question and more.
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| Chlorophyll analysis is a two step process. The first step involves extracting the chlorophyll from the f ilter before the actual analysis takes place. Extraction involves placing each dried chlorophyll filter into a test tube with 8 mL of ethanol (that’s 190 proof grain alcohol!). The tubes then go into a hot-water bath for 20 minutes, so the ethanol can heat up to around 160 ?F. The heated ethanol extracts the chlorophyll from the filters. Simply put, the algal cells that were caught on the filter burst open and the chlorophyll pigment that was inside of the algae is released into the ethanol. What we end up with is a tube of ethanol that has a green tint. |
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Chlorophyll analysis is completed using a fluorometer. Each sample is sipped into the machine where it enters a glass tube. The fluorometer sends light at a wavelength of 430 nanometers into the chlorophyll sample. This wavelength of light causes the chlorophyll molecules to fluoresce, or give off light. This fluorescence is then measured at a different wavelength (663 nanometers). The more chlorophyll in the tube, the more light that is given off at 663 nanometers. |
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| Prior to analyzing the samples, the fluorometer is calibrated with standards that contain a known amount of chlorophyll. The machine is able to use information about the fluorescence of the standards to calculate the amount of chlorophyll associated with the fluorescence of the unknown samples. |
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The chlorophyll value is then adjusted for the volume of water that was passed through the filter. This allows us to report all values in terms of the amount of chlorophyll that would be found in a liter of lake water. Each sample has two chlorophyll filters (hopefully!). We take the two values and average them to get the one number that we will use for that given sample from the given day. |
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Now some of you may be asking “Why chlorophyll?” Is this really the best method for determining the amount of algae that is in the lake? The simple fact is no, it is not the absolute best method. A better method would be to preserve the actual water (any where from 100 mL to 1000mL) and use a microscope to count, identify, and measure the algae that are actually there in the water. The problem with this method is that is extremely time-consuming and much more costly than doing chlorophyll filters (not to mention I develop a sort of motion sickness if I stare into a microscope for too long). The time required to analyze two sites worth of filters (32 filters) is about an hour for the set-up, and maybe another hour and a half for the analysis and clean-up. I couldn’t even begin to get through one sample in that time if I were using the microscope. |
Knowing the amount of algae in a lake is important because it is a way of monitoring the lake’s relative health. Algae are required for a healthy lake, as the algae are the base of the food chain and a source of dissolved oxygen. Just as a little algae are good, too much can be bad. Problems associated with excessive algae include: changes to the aquatic ecosystem that can affect the fishery, taste and odor problems, decreased recreational value, and loss of aesthetic value. Using filters to measure the chlorophyll concentration is a simple, relatively inexpensive and reliable way to gauge the amount of algae within the lake. With the help of volunteers, we are able to efficiently monitor this important component of lake ecosystem just by collecting a couple of little white filters.
Dan Obrecht |
Brought to you by the Lakes of
Missouri Volunteer Program |
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