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Worksheet Grade 7 Science

Tanner Creek Water Quality: Oxygen, Temperature, and pH

A grade 7 science worksheet comparing dissolved oxygen, temperature, and pH at two sites on fictional Tanner Creek, with percent-change work and a fish habitat benchmark.

Objectives

  1. Students will compare dissolved oxygen, temperature, and pH data from two stream sites.
  2. Students will calculate the percent decrease in dissolved oxygen and judge each site against the 5 mg/L fish benchmark.

Vocabulary

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.
pH
pH is a number that tells whether water is acidic, like lemon juice, or basic, like baking soda mixed in water. Pure water sits in the middle of the scale at a pH of 7.

Also: aquatic habitat, benchmark

Teacher overview

Three simple measurements tell a lot about whether a stream can support fish: dissolved oxygen, temperature, and pH. Warm water holds less dissolved oxygen than cold water, so a sunny, unshaded stretch of a creek often has less oxygen than a cool, shaded one. This worksheet gives students paired readings from a fictional stream called Tanner Creek and asks them to compare the two sites, calculate the change in oxygen, and decide which site is better habitat.

Tanner Creek Water Quality: Oxygen, Temperature, and pH — Grade 7 Science
Name: ____________________________ Date: ______________

Student Worksheet

Tanner Creek is a fictional stream. A class measured two sites on the same afternoon: a cool, shaded site upstream and a warm, sunny site downstream.

Measurement Upstream (cool, shaded) Downstream (warm, sunny)
Temperature 10 degrees C 22 degrees C
Dissolved oxygen 10 mg/L 6 mg/L
pH 7.0 7.8

Most fish need dissolved oxygen above 5 mg/L. For reference, the pH range used for drinking water is 6.5 to 8.5.

  1. What is the temperature difference between the two sites?
  2. What is the dissolved oxygen difference between the two sites?
  3. What is the percent decrease in dissolved oxygen from the upstream site to the downstream site? Use (upstream minus downstream) divided by upstream, times 100.
  4. Fish need dissolved oxygen above 5 mg/L. Are both sites above that benchmark? For the downstream site, state how many mg/L it clears the benchmark by.
  5. Explain why the warmer downstream site has less dissolved oxygen than the cooler upstream site.
  6. Are both pH readings inside the 6.5 to 8.5 range used for drinking water? Show how you decided.
  7. Suppose the downstream site warmed further and its dissolved oxygen fell to 4 mg/L. Would it still meet the 5 mg/L benchmark? What percent below the benchmark would 4 mg/L be?
  8. Name one thing people could do near a creek that would warm the water and lower its dissolved oxygen.
  9. Which site is better habitat for cold-water fish? Cite two data points that support your answer.
  10. Explain why measuring temperature, dissolved oxygen, and pH together tells you more about stream health than measuring only one of them.

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: 12 degrees C.
    22 − 10 = 12 degrees C difference.
  2. Answer: 4 mg/L.
    10 − 6 = 4 mg/L difference.
  3. Answer: 40%.
    (10 − 6) ÷ 10 × 100 = 4 ÷ 10 × 100 = 40 percent decrease.
  4. Answer: Yes, both are above the benchmark.
    Upstream is 10 mg/L and downstream is 6 mg/L, both above 5 mg/L. The downstream site clears the benchmark by 6 − 5 = 1 mg/L.
  5. Answer: Warm water holds less dissolved oxygen.
    The downstream water is 12 degrees warmer, and warmer water cannot hold as much dissolved oxygen, so its reading is lower.
  6. Answer: Yes, both are inside the range.
    7.0 and 7.8 both fall between 6.5 and 8.5, so both readings are inside the reference range.
  7. Answer: No, it would not meet the benchmark.
    4 mg/L is below 5 mg/L. The percent below the benchmark is (5 − 4) ÷ 5 × 100 = 20 percent.
  8. Answer: Remove shade trees along the bank.
    Accept any answer that would add heat, such as cutting down streamside trees or releasing warm water, since both would raise the temperature and lower the oxygen.
  9. Answer: The upstream site is better habitat.
    It has higher dissolved oxygen (10 mg/L versus 6 mg/L) and a cooler temperature (10 degrees versus 22 degrees), both of which cold-water fish need.
  10. Answer: Each measurement shows a different part of stream health.
    Temperature affects how much oxygen the water can hold, dissolved oxygen tells whether fish can breathe, and pH shows whether the water is too acidic or basic. Together they give a fuller picture than any single reading.

Differentiation

Support

Give students the percent-change formula on a reference card and complete item 3 together. Provide a number line marked at 5 mg/L so students can see at a glance whether each oxygen reading clears the benchmark.

Extension

Ask students to predict the dissolved oxygen at a third site that is 16 degrees C, halfway between the two temperatures, and to explain what other information they would need before trusting that prediction.

ELL supports

Pre-teach "dissolved oxygen," "temperature," "pH," and "benchmark" with a labeled thermometer and a simple pH strip image. Allow items 5, 8, and 10 to be answered with a short phrase or a labeled sketch rather than full sentences.