Independent variable for bacterial growth

Exercise 1: Data Interpretation (2 pts each)

1. What patterns do you observe based on the information in Table 4?

No fish are present when the dissolved oxygen is zero. When there is more dissolved oxygen in the water, more fish are present. However, the number of fish tends to drop or level off when the dissolved oxygen is higher than 12 ppm.

2. Develop a hypothesis relating to the amount of dissolved oxygen measured in the water sample and the number of fish observed in the body of water.

Possible Hypotheses:

1. The amount of dissolved oxygen affects the number of fish that can live in a body of water.

2. As dissolved oxygen concentration increases, more fish can live in the body of water.

3. There is an ideal dissolved oxygen concentration for fish to live in.

3. What would your experimental approach be to test this hypothesis?

Possible Experimental Approach: Deposit an equal number of fish in several tanks. Maintain all other variables (temperature, light, food, etc.), but vary the dissolved oxygen concentration in each of the tanks. Observe the fish over time to determine how many fish can survive at different oxygen concentrations.

4. What are the independent and dependent variables?

Independent Variable: Dissolved oxygen concentration.

Dependent Variable: The number of fish.

5. What would be your control?

Possible Control: Aquarium with no fish. Measure the dissolved oxygen level in a fish tank at normal room conditions, and repeat this measurement every time you make an observation of the number of fish. [Use only one type of fish for your experiment, and control other variables such as light, food and temperature.]

6. What type of graph would be appropriate for this data set? Why?

A line graph is most appropriate because it can best display the relationship between the variables.

7. Graph the data from Table 4: Water Quality vs. Fish Population (found at the beginning of this exercise).

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8. Interpret the data from the graph made in Question 7.

The number of fish in the body of water increases along with dissolved oxygen up to about 12 ppm. When the concentration is higher than 12 ppm, the relationship is less clear. There may be an ideal dissolved oxygen concentration that supports the greatest number of fish, but that conclusion would require further testing.

Experiment 2: Experimental Variables (2 pts each part, 8 total/question)

Determine the variables tested in the each of the following experiments. If applicable, determine and identify any positive or negative controls.

1. A study is being done to test the affects of habitat space on the size of fish populations. Different sized aquariums are set up with six goldfish in each one. Over a period of six months, the fish are fed the same type and amount of food. The aquariums are equally maintained and cleaned throughout the experiment. The temperature of the water is kept constant. At the end of the experiment the number of surviving fish are surveyed.

A. Independent Variable: Habitat Space (Different sized aquariums are tested)

B. Dependent Variable: Size of Fish Populations (The number of surviving fish are surveyed)

C. Controlled Variables/Constants: Type of food, amount of food, equal maintenance and cleaning, water temperature

D. Experimental Controls/Control Groups: There are no control groups in this experiment.

2. To determine if the type of agar affects bacterial growth, a scientist cultures E. coli on four different types of agar. Five petri dishes are set up to collect results:

One with nutrient agar and E. coli

One with mannitol-salt agar and E. coli

One with MacConkey agar and E. coli

One with LB agar and E. coli

One with nutrient agar but NO E. coli

All of the petri dishes received the same volume of agar, and were the same shape and size. During the experiment, the temperature at which the petri dishes were stored, and at the air quality remained the same. After one week the amount of bacterial growth was measured.

A. Independent Variable: Type of agar (nutrient agar, mannitol-salt agar, MacConkey agar, LB agar)