Detective 1

The graph below shows the relationship between IBI and the minimum dissolved oxygen (DO) level.  Study the graph and answer the questions.  Be prepared to explain the correct answers to your group.

Watershed Glossary

  1. Describe the relationship between DO and IBI - as DO goes down, the IBI ??
  2. Are there places/times where the relationship does not seem to be consistent?  Write those down.
  3. The reason that the IBI and DO are related in this way is because (choose the best answer and click on the letter):

    - The fish are bigger in some areas and use more oxygen.

    A is incorrect - The DO level is related more to the whether there are organic wastes decomposing and bacteria using the oxygen.

    - When the minimum DO is low, a lot of the fish cannot survive in that part of the stream.

    B is correct! - Many species of fish are sensitive to low oxygen levels and cannot survive.

    - The minimum DO is higher in places where there are a lot of algae producing oxygen through photosynthesis.

    C is incorrect - Algae do produce oxygen during the day, but a lot of algae actually leave a lot of organic material to decompose and use up oxygen.

    - More trees have been removed in areas with high DO, reducing the amount of leaves that decompose in the water and use up oxygen.

    D is incorrect - No this it backwards. Shading the stream leaves the water cooler. Cooler water can hold more DO.

  4. One major reason that the DO minimum level is lower is:

    - There are more trees shading the stream

    A is incorrect - No sorry. Remember shading will keep the water cooler. Cooler water holds more oxygen. What might use a lot of oxygen?

    - There are more fish using more oxygen

    B is incorrect - No the amount of oxygen consumed by fish is not the major determinant. What else might use a lot of oxygen?

    - There is less mixing of the water

    C is incorrect - Mixing of the water would actually increase the oxygen level.

    - There are larger amounts of organic wastes being released in these places

    D is correct! - Organic wastes are consumed by bacteria which use a lot of oxygen during the decomposition process.

  1. Oxygen levels can be lower in agricultural areas if:

    - There are more trees shading the stream

    A is incorrect - No sorry. Remember shading will keep the water cooler. Cooler water holds more oxygen.

    - Animal wastes get into the stream

    B is correct! - Decomposition of animal wastes will use a lot of oxygen.

    - Farmers use a lot of fertilizer

    C is incorrect - Fertilizer use will not directly effect oxygen levels. It may cause the growth of algae.

    - Farmers grow a lot of hay

    D is incorrect - The type of crops grown will not impact the oxygen level. What will consume a lot of oxygen?