Oxygen levels in the photic zone fluctuate over the course of a 24-hr period. If you were measuring dissolved oxygen over the course of the day, when would you expect to see the lowest levels of oxygen and why?

Question

A.  Mid-morning -- because fish are most active and their respiration rates are highest.
B.  Early afternoon -- when the surface water warms up and sinks to the bottom.
C.  Mid-afternoon -- because the activity of the Calvin cycle (the dark reactions) of photosynthesis are at their peak.
D.  Early evening -- when animals come to feed and disturb the surface waters.
E.  During the night -- because the light reactions of photosynthesis are inactive.

Clarify Question
· What is the key concept addressed by the question?
· What type of thinking is required?
 
Gather Content
· What do you already know about oxygen levels?
 
Choose Answer
· Given what you now know, what information and/or problem solving approach is most likely to produce the correct answer?
 
Reflect on Process
· Did your problem-solving process lead you to the correct answer? If not, where did the process break down or lead you astray? How can you revise your approach to produce a more desirable result?

Answer 1

E.  During the night -- because the light reactions of photosynthesis are inactive.

Clarify Question
· What is the key concept addressed by the question?
        o This question addresses oxygen levels.
· What type of thinking is required?
        o This question is asking you to analyze the information given, using logic, to dissect the problem and determine the answer.
 
Gather Content
· What do you already know about oxygen levels?
        o The light reactions of photosynthesis create oxygen, but only during the daytime when light is available, and only in the photic zone.
        o Water absorbs light passing through it, and the intensity of sunlight available for photosynthesis decreases sharply with increasing depth. In deep lakes, only water relatively near the surface receives enough light for phytoplankton to exhibit a positive net primary productivity. Those waters are described as the photic zone.
        o The supply of dissolved oxygen to the deep waters of a lake can be a problem because oxygen enters aquatic systems near their surface, where gas exchange occurs with the air and photosynthesis occurs in aquatic organisms. In the still waters of a lake, mixing between the surface and deeper layers may not occur except occasionally as a result of thermal stratification; consequently, oxygen only rarely enters the deep waters from the surface waters.
 
Choose Answer
· Given what you now know, what information and/or problem solving approach is most likely to produce the correct answer?
        o Oxygen levels would be lowest at night when the light reactions of photosynthesis are quiet. The light reactions produce oxygen not the Calvin cycle.
 
Reflect on Process
· Did your problem-solving process lead you to the correct answer? If not, where did the process break down or lead you astray? How can you revise your approach to produce a more desirable result?
        o The question required you to analyze the information given, using logic, to dissect the problem and determine the answer.
        o Did you recognize that photosynthesis produces oxygen only while the sun is up?