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Worksheet Grade 7 Science

Kessler Creek Nutrients: When a Fertilizer Becomes a Pollutant

A grade 7 science worksheet using fictional Kessler Creek data: students compute percent change in nitrogen and phosphorus and trace how excess nutrients lower oxygen.

Objectives

  1. Students will calculate the percent increase in nitrogen and phosphorus between two creek sites.
  2. Students will explain how the same nutrient can act as a resource for plant growth and as a pollutant.

Vocabulary

Nitrate
Nitrate is a form of nitrogen that plants use as food. It gets into water from fertilizer, animal waste, and septic systems, and too much of it is unhealthy for both people and streams.
Dissolved oxygen
Dissolved oxygen is the oxygen gas mixed into water that fish and other underwater animals breathe. Cold, moving water holds more oxygen than warm, still water.
Algal bloom
An algal bloom is a fast, thick growth of algae that can turn a lake green and scummy. Blooms happen when the water gets lots of nutrients and warm sunshine, and some blooms make toxins.

Also: nutrient, phosphorus

Teacher overview

Nitrogen and phosphorus are nutrients that plants need to grow, which is why farmers spread them as fertilizer. When rain washes extra fertilizer off fields and into a stream, those same nutrients feed a heavy growth of algae, and the stream can lose the oxygen that fish depend on. This worksheet uses readings from a fictional stream called Kessler Creek to show how one substance can be both a resource and a pollutant, and it asks students to measure the change with percents.

Kessler Creek Nutrients: When a Fertilizer Becomes a Pollutant — Grade 7 Science
Name: ____________________________ Date: ______________

Student Worksheet

Kessler Creek is a fictional stream. Water quality was measured at two points: an upstream site above the farms and a downstream site below them.

Measurement Upstream site Downstream site
Nitrate 2 mg/L 8 mg/L
Phosphorus 0.05 mg/L 0.20 mg/L

Most fish need dissolved oxygen above 5 mg/L. During a summer algae bloom, the dawn dissolved oxygen at the downstream site dropped to 3 mg/L.

  1. What is the percent increase in nitrate from the upstream site to the downstream site? Use (downstream minus upstream) divided by upstream, times 100.
  2. What is the percent increase in phosphorus from the upstream site to the downstream site?
  3. Explain why nitrogen and phosphorus are called nutrients as well as pollutants. Give one example of each role.
  4. Fish need dissolved oxygen above 5 mg/L. During the bloom, the dawn reading was 3 mg/L. How many mg/L below the fish benchmark is that?
  5. Put these events in order to explain how extra nutrients lower the oxygen: bacteria use up oxygen; algae grow quickly; algae die and decompose; fertilizer washes into the creek.
  6. A farm changes its practices, and the downstream nitrate falls from 8 mg/L to 5 mg/L. What is the percent decrease?
  7. Classify each item as a resource use or a pollution effect: fertilizer helping corn grow; algae covering the creek; low dawn oxygen; nutrients feeding stream plants.
  8. A wastewater plant recovers phosphorus from its waste stream and sells it as fertilizer. Explain how this treats phosphorus as a resource rather than a pollutant.
  9. Based on the readings, is the creek healthier at the upstream site or the downstream site? Cite two measurements that support your answer.
  10. Suggest one action that people near Kessler Creek could take to reduce the nutrients washing into the water.

From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.

Answer Key
  1. Answer: 300%.
    (8 − 2) ÷ 2 × 100 = 6 ÷ 2 × 100 = 300 percent increase.
  2. Answer: 300%.
    (0.20 − 0.05) ÷ 0.05 × 100 = 0.15 ÷ 0.05 × 100 = 300 percent increase.
  3. Answer: They feed plants but also cause harmful algae growth.
    As a resource, nitrogen and phosphorus help crops and stream plants grow. As pollutants, too much of them feeds algae blooms that lower oxygen. Accept any answer that names both roles.
  4. Answer: 2 mg/L.
    5 − 3 = 2 mg/L below the fish benchmark of 5 mg/L.
  5. Answer: Fertilizer washes into the creek; algae grow quickly; algae die and decompose; bacteria use up oxygen.
    The decomposing algae feed bacteria, and those bacteria pull dissolved oxygen out of the water.
  6. Answer: 37.5%.
    (8 − 5) ÷ 8 × 100 = 3 ÷ 8 × 100 = 37.5 percent decrease.
  7. Answer: Resource use: fertilizer helping corn grow, nutrients feeding stream plants. Pollution effect: algae covering the creek, low dawn oxygen.
    The first two are helpful growth; the last two are harm caused by too much nutrient.
  8. Answer: It reuses the phosphorus instead of releasing it.
    By capturing phosphorus and selling it as fertilizer, the plant turns a would-be pollutant back into a useful nutrient, so it helps crops instead of feeding algae downstream.
  9. Answer: The upstream site is healthier.
    Upstream nitrate (2 mg/L) and phosphorus (0.05 mg/L) are far lower than downstream (8 mg/L and 0.20 mg/L), so the upstream water is less likely to grow the algae that starve fish of oxygen.
  10. Answer: Reduce fertilizer runoff.
    Accept any reasonable action, such as planting a buffer strip of grass along the creek, spreading less fertilizer, or keeping livestock away from the banks.

Differentiation

Support

Provide the percent-change formula on a reference card and complete item 1 together before students continue. Offer item 5 as cut-apart strips students can physically arrange in order.

Extension

Ask students to predict the downstream phosphorus level if a buffer strip removed 40 percent of the phosphorus load, and to show the calculation, then explain whether that would bring the creek closer to the upstream reading.

ELL supports

Pre-teach "nutrient," "fertilizer," "algae," and "dissolved oxygen" with a picture sequence showing runoff, algae growth, and fish in low-oxygen water. Allow items 3, 8, and 10 to be answered with a labeled drawing plus a short phrase.