For example, when students in one school found few invertebrates downstream of their football field they wondered whether a pesticide or other toxic chemical might have drained from the field during storms. Other explanations for the low counts might be that dissolved oxygen in the stream had intermittently dropped too low to support sensitive species, or the pH had become too high after liming of the fields. Chemical test kits can be used to measure dissolved oxygen and pH, but testing for pesticides or other toxic chemicals generally requires equipment and reagents that are too expensive or dangerous for high school use. Bioassays provide a feasible alternative. A bioassay is a test that uses biological organisms to determine toxicity, which is a measure of the degree to which a chemical or mixture of chemicals will harm living things.
All chemicals are potentially toxic, but some are deadly at much lower concentrations than others.
Rather than measuring the concentration of a specific chemical, bioassays provide an integrated measure of the toxicity of the mixture of chemicals in a solution. Types of bioassays Three popular test organisms for bioassays in the classroom are lettuce seeds, duckweed, and Daphnia . In our teacher programs at Cornell, we started with the lettuce seed bioassay-probably the simplest, since it requires no maintenance of cultures of test organisms. You simply place lettuce seeds in petri dishes along with the water, sediment, or chemical solution you wish to test. In duckweed bioassays, you float duckweed plants in your test solutions, then after several days of incubation you count how many new fronds, or leaves, have grown.