Bioassay – Assessing the Effects of Aquatic Pollutants on Invertebrates
Background
Organisms, including humans, are exposed to thousands of human-created chemicals. The potential health and environmental effects of different chemicals vary greatly. In a sense, the “dose makes the poison”.
The effect of a chemical on an organism is also determined by the way in which the organisms is to a chemical exposed (example: respiratory system vs. digestive system), and the length of time over which an organism is exposed. Another issue is that some individuals in a population may be affected differently than other individuals. Chronic exposure is exposure to low doses over long periods of time. Acute exposure is higher levels of exposure over a short time frame.
Given all of these variables, it is difficult to determine the exact effects of a chemical. However, it is very important that toxicologists screen the many chemicals that enter our air and water to determine those that have the most negative effects. These “most toxic” chemicals are then further assessed.
One way of comparing different chemicals is the LD-50 analysis. LD-50 is the concentration (dosage) of a chemical that results in 50% mortality of a species in a specified period of time. When studying aquatic invertebrates, the LD-50 is often determined over a 48 hour period.
Pre Lab Questions: (answer in your notebook)
1. What chemical are you testing? What are the other chemicals being tested in your class?
2. What might be the sources of this chemical in an aquatic environment?
3. In your words, what does the “dose makes the poison” mean?
4. What are at least three reasons that the exact effects of a chemical are difficult to determine?
5. Hypothesis: of the chemicals being tested, list in order from most toxic to least toxic.
6. Is this test an acute or chronic assessment of the chemical? Explain.
Toxicity Screening Test:
1. Make several dilutions of your chemical. This will be a wide range of concentrations since this is a screening.
A. Antifreeze: Make a 10% solution by mass of the antifreeze and volume of water.
i) Weigh 10 grams antifreeze; add to 90 ml pond or dechlorinated water
ii) Make 100 ml of three additional serial dilutions: 1%; 0.1%; and 0.01%
B. Sodium Hypochlorite (bleach)
i) The solution you obtain is a 6% solution.
ii) Dilute 4:1. This is 75 ml water to 25 ml bleach. This is a 1.5% solution
iii) Make three serial dilutions of the 1.5%: these will be 0.15%; 0.015% and 0.0015%
C. Solid chemicals
i) Make 100 ml of a 10 % dilution. This is 10 grams to 100 ml water
ii) Make 100 ml of three additional serial dilutions : 1%; 0.1%; and 0.01%
2. Label the small beakers with your name, your science class, and the % solution.
3. Add the specified number of scuds to each beaker
4. Make a neat data table in your notebook. This will be handed in with the lab
5. Count the living scuds in each dilution for 2 to 3 days.
Data:
1. Enter your data into the class spreadsheet.
2. Print the class spreadsheet
3. Graph the averages for each of the tested chemicals. Put all three graphs on the same page. Each person must make their own graph.
Analysis:
1. Do you see any significant differences between individual data and the class averages? Be specific in stating the differences.
2. Why is it “good science” to graph the averages rather than your individual data?
3. Based on this data, are any of the concentrations close to a 48-hour LD-50? EXPLAIN your answer.
4. For your chemical, how would you determine the LD-50. Briefly state a procedure to determine this.
5. Do you see significant differences in the different chemicals? Look for 10% or greater difference to assume a statistical significant difference.
6. Based on the results, what generalizations can you make about EACH of the chemicals? Do you think an effort should be made to reduce levels of the chemical in aquatic ecosystems? When you answer this, INCLUDE some of the data in your discussion.