- Diatoms (single-celled algae) have frequently been used to evaluate the impact of domestic sewage or industrial waste on the aquatic environment. Use of these organisms in pollution impact evaluation is justified by reason of their sensitivity to pollutants. In a non-polluted environment there will usually be a very high diversity of kinds of different diatoms, while a polluted environment will show a very low diversity with only a few different kinds of diatoms.
- In this investigation, you will collect samples, digest the organic material leaving the silicone shells, and calculate the diatom diversity index (DDI).
MATERIALS:
some small containers with lids
- for holding the collected samples
100-mL beakers
*30 or 40 volume Hydrogen peroxide
Potassium dichromate
Microscope and slides*This is easily obtained from a beauty supply store. The hydrogen peroxide normally used around the home is only 3%, and not strong enough for this purpose.
PROCEDURE:
COLLECTION:
1. Diatom scrapings are taken from rocks or other submerged objects. They are the slick brown coating on the rocks.
PREPARATION:
2. Place approximately a 5 mL sample in a 100-mL beaker.
3. Add a small "pinch" of Potassium dichromate to the sample. You will not need much, and will not need to add any additional, as this is acting as the catalyst and will not be consumed in the process.
4. Add 10 mL of hydrogen peroxide to the sample. The reaction will bubble and produce heat. The mixture will turn dark and return to pale yellow when complete.
5. Add 10 mL of hydrogen peroxide 2 more times. Waiting for the reaction to complete between additions.
6. When the final reaction has completed, add approximately 30 mL of distilled water and let settle for 48 hours.
DATA COLLECTION:
7. The residue from digesting the sample is mounted on microscope slides and examined under the microscope.
- Diatom diversity indices (DDI) are computed using the Sequential Comparison Index of Cairns et al.1 By this method of analysis, 2,000 individual diatoms are sequentially examined along transects of the microscope slides. Each diatom is compared with that examined immediately before, and if it differs in cell structure, a difference in species is assumed, whereupon a "run" is scored.
- Use the data table to score your diatom count. The table has 200 squares, so you will need 10 tables. If a diatom is the same as the one immediately before it, place an "S" in the box. If a diatom differs in cell structure from the one immediately before it, place a "D" in the box. Each "D" will count as a "run".
Below is an example of this.
8. The DDI is calculated from the ratio:
where increasing values of DDI are indicative of increasing species diversity in the range, 0 - 1. Natural surface waters of high quality will usually approach the upper limit, which generally indicates unstressed diatom communities in streams. Where the DDI approaches medial or low values, indicating continuing degrading stress conditions, stream waters are of lower quality.
1Cairns, J., D.W. Albaugh, F. Busey, and M.D. Chaney, "The Sequential Comparison Index - A Simplified Method for Non-Biologists to Estimate Relative Differences in Biological Diversity in Stream Pollution Studies," JWPCF, 40 (9) : 1607 - 1613 (1968)
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- In this investigation, you will collect samples, digest the organic material leaving the silicone shells, and calculate the diatom diversity index (DDI).
DIATOM DIVERSITY
INTRODUCTION: