Your biology professor takes the class on a field trip to a local lake. You spend an hour studying the lake and writing down all observations. You hypothesize that the lake is oligotrophic. Which observations would support this hypothesis? (Check all that apply.)A) The shoreline is mostly rock and sand with very little vegetative matter.B) The lake has extensive aquatic plant beds.C) The lake has little planktonic growth.D) The bottom of the lake is easily visible.E) An oxygen probe reveals low oxygen content in the water.

Question

What will be an ideal response?

Answer 1

A, C, D

Clarify Question
• What is the key concept addressed by the question?
        o This question addresses eutrophic versus. oligotrophic lakes.
• What type of thinking is required?
        o This question is asking youto weigh and judge evidence, or evaluate, to choose the best of the possible answers.

Gather Content
• What do you already know about eutrophic vs. oligotrophic lakes?
        o Lakes and rivers that are low in algal nutrients (such as nitrate or phosphate) and low in the amount of algal material per unit of volume are termed oligotrophic. Such waters are often crystal clear. Oligotrophic lakes and ponds tend to be high in dissolved oxygen at all depths all year because they also have a low rate of oxygen use.
        o Eutrophic bodies of water are high in algal nutrients and often populated densely with algae. They are more likely to be low in dissolved oxygen, especially in summer. In a eutrophic body of water, decay microbes often place high demands on the oxygen available because when thick populations of algae die, large amounts of organic matter are made available for decomposition.
        o Human activities have often transformed oligotrophic lakes into eutrophic ones. For example, when people overfertilize their lawns or fields, nitrate and phosphate from the fertilizers wash off into local water systems. Lakes that receive these nutrients become more eutrophic.

Choose Answer
• Given what you now know, what information and/or problem solving approach is most likely to produce the correct answer?
        o An oligotrophic lake has low nutrient levels, and consequently little plant growth and clear water, with high oxygen content.
        o So these three observations fit an oligotrophic lake:
        o The shoreline is mostly rock and sand with very little vegetative matter.
        o The lake has little planktonic growth.
        o The bottom of the lake is easily visible.

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 weigh and judge evidence, or evaluate, to choose the best of the possible answers.
        o Did you recognize that clear water and little plant growth is the sign of an oligotrophic lake?