Student Experience 3: What happens when nutrients are added to pond water?

Purposes

To have students observe that adding nutrients to pond water promotes algae growth.
To help students see that increased algae growth causes cloudy water.

Description

Recall that students have monitored samples of pond water that were exposed to sunlight and different nutrient conditions. [Click here to view the instructions for the initial set up of this experiment.] A short video showing algae growth in samples exposed to the same conditions as used in this student experience is available here. Students watch this video as a quick refresher of what they observed over the three weeks. Students then review and discuss their recorded observations of this experiment. In particular, they make comparisons across the different samples for different time points.

Samples with added nutrients should become cloudy and develop color (likely some shade of green), but samples with no added nutrients should remain more clear and less colored. Students may also notice that the cloudiness and color changes depend on different levels or different types of nutrients. Encourage students to consider the water source (pond or tap) when interpreting their observations and lead a discussion aimed at identifying algae as a possible source of the colored, cloudy water. Students should come away from the discussion with the idea that excess nutrients can promote algae growth and cause pond water to become cloudy.

Science Practices

Analyzing and Interpreting Data By comparing observation notes with peers and engaging in discussion, students make sense of the observations that they have recorded over the past several days in order to reach consensus on patterns observed in the data. In addition, this experience provides an opportunity to engage students in discussion about what data they are finding most useful and how the organization of data can facilitate sense making.

Pond water jars after 3 weeks (from left to right: (1) tap water, (2) tap water with medium amount of fertilizer, (3) pond water, (4) pond water with low amount of fertilizer, (5) pond water with medium amount of fertilizer, and (6) pond water with high amount of fertilizer; see example conditions for more detail)

Questions to Ask Students When Discussing Results

Why do you think we have one pond water jar without any added nutrients?
What difference do you notice between the sample with no nutrients and those that had nutrients added?
What could cause the water to have a more greenish color?
How do you think the cloudy water affects sunlight going through it to the bottom of the pond?

Crosscutting Concepts

Patterns As students work to analyze their observations, emphasize the importance of attending to patterns in the data. For example, prompt students to look at how one observed characteristic (e.g., color) varied based on the amount of nutrients added.

Cause and Effect By having a control sample (i.e., jar with no added nutrients) and jars of varying concentrations of fertilizer, this investigation helps students to understand the effects of an abiotic factor (i.e., nutrients, in the form of fertilizer) and observe how nutrients promote algae growth, resulting in cloudiness in the water.

Student Thinking

Again, some students believe that plants get their food from soil nutrients or equate fertilizer with plant food. Some students may continue to have trouble accepting that plants or other producers, like algae, live within the pond water. Because most pond algae are microscopic and therefore not visible when present in low concentrations, students may find it difficult to accept that they are in the relatively clear pond water at the start of the experiment.

Implementation Tips

(Note: the example conditions description includes other practical guidance)

Encourage students to be careful with vocabulary in this experience. Students are likely to equate or interchange “food” with “nutrient.” Our use of the term “nutrient” in this experience is deliberate and should be distinct from “food.” Nutrients are what the fertilizers supply: non-food chemicals that plants use to grow. Producers, including algae, make their own food (sugar) using photosynthesis, so their food does not come from fertilizers.
One sample with no added nutrients is important so that students can compare results to see the specific changes that depend on the presence of nutrients. You might introduce the term “control condition” (no nutrients) here and contrast it with the “experimental conditions” (conditions with nutrients).
When discussing the meaning of the results, a student may or may not come up with algae as the source of the greenish, cloudy water based on their own knowledge/experience. If not, emphasize the conditions that produced the growth (sunlight and fertilizer) and lead students in describing what kind of organism could produce these results (i.e., grows more with sunlight and nutrients, floats in water). If no student suggests “algae” as the source after discussing the characteristics of the growth, tell students that “algae” fits the conditions and observations the students made.
The growth of algae provides an opportunity for students to recognize that microscopic organisms can produce changes that they can readily observe. Call attention to the microscopic size of the algae to help counter the idea that an organism’s importance in an ecosystem correlates with its size.
Because components of the jars may be harmful to plumbing, DO NOT empty them into sinks. When the pond water investigation is complete, empty the jars in an outdoor natural area (e.g., grassy area, field, woodland) and rinse them with tap water. The volumes of the samples are very small relative to an outdoor environment; nonetheless, use plenty of water to minimize any impact. Soil-based decomposers will break down the accumulated algae.