Free response · Unit 8 Beta

Phosphate, algae and dissolved oxygen in pond-water tanks

Q1: Design an investigation · scientific calculator allowed · 10 points

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The question

Name: ______________________   Date: ____________   Phosphate, algae and dissolved oxygen in pond-water tanks (10 points)

Model or diagram

Algae and dissolved oxygen in tanks with different phosphate additions

00.250.50.7510102030405060Phosphate added (mg per L)Algae (µg per L)0246810Dawn DO (mg per L)Algae (left axis)Dawn DO (right axis)

  1. (a) A student set up four identical 40-liter tanks of pond water in a sunny greenhouse and added 0, 0.1, 0.5 or 1.0 mg/L of phosphate. Algae were measured after 14 days, and dissolved oxygen was measured at dawn on day 21, after many algae had died. Write a testable hypothesis for the effect of phosphate on algae. [1 point]

  2. (b) Identify the independent variable and the dependent variable for the algae measurement. [2 points]

  3. (c) Identify the control in this experiment, and describe its purpose. [2 points]

  4. (d) Identify one variable that should be held constant across the tanks. [1 point]

  5. (e) Using the graph, explain why dissolved oxygen on day 21 is lowest in the tank with the most phosphate. Include every step. [2 points]

  6. (f) The student wants to test thermal pollution instead. Describe one change to the design, and predict its effect on dissolved oxygen. [1 point]

  7. (g) The pond the water came from drains a watershed of corn fields. Propose one practice the farmers could use to reduce phosphate reaching the pond, and explain how it works. [1 point]