Algae, phosphate and fish kills in Lake M

Question 1: Design an investigationAbout 23 minutes · 10 points

Lake M is a small lake surrounded by farm fields that are fertilized each spring. In recent summers the lake has had thick blooms of algae, followed by fish kills. The diagram shows a model of the process that scientists think is taking place.

A class tested how phosphate affects the growth of algae. The students set up 12 identical 20-liter aquariums, each filled with lake water and the same small amount of algae from Lake M. Three aquariums received no added phosphate, three received phosphate to a concentration of 0.1 milligram per liter (mg/L), three received 0.5 mg/L and three received 1.0 mg/L. All aquariums were kept at 22 degrees Celsius under the same lamps for 14 days. On day 14, the students measured the concentration of algae in each aquarium.

(A) Using the diagram, explain why the decomposition of dead algae causes the dissolved oxygen in Lake M to fall. 1 point

(B) Identify the independent variable in the class's experiment. 1 point

(D) Identify the control group in the class's experiment. 1 point

(E) Identify one variable that the class held constant in the experiment. 1 point

(F) Identify a testable hypothesis for the class's experiment. 1 point

(G) If the students had placed the three 1.0 mg/L aquariums by a sunny window while the other nine stayed under the lamps, explain how this change would affect the conclusion the class could draw about phosphate. 1 point

(H) Describe one environmental problem, other than fish kills, that algal blooms can cause in a lake. 1 point

(I(i)) Propose a realistic solution that would reduce the amount of phosphate reaching Lake M from the surrounding farm fields. 1 point

(I(ii)) Justify the solution proposed in part I(i) by explaining one advantage of it other than reducing phosphate in Lake M. 1 point