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Worksheet Grade 8 Science

Kestrel Lake's Oxygen Crash: Reading a Summer Depth Profile

A grade 8 science worksheet where students read a fictional lake's dissolved-oxygen depth profile, find the low-oxygen zone, and trace a nutrient-driven fish kill.

Objectives

  1. Students will interpret a dissolved-oxygen depth profile to locate the zone where fish cannot survive.
  2. Students will explain how a nutrient acts as both a crop resource and a pollutant that drives an oxygen crash.

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.
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, decomposition

Teacher overview

Fictional Kestrel Lake sits below a farm whose fertilizer runoff feeds heavy algae growth each summer. When the algae die, bacteria decompose them and use up oxygen deep in the lake, which can leave fish with nowhere to breathe. This worksheet has students read a dissolved-oxygen depth profile, find the depths that fall below the 5 mg/L most fish need, and trace the chain that connects a helpful crop nutrient to a harmful oxygen crash.

Kestrel Lake's Oxygen Crash: Reading a Summer Depth Profile — Grade 8 Science
Name: ____________________________ Date: ______________

Student Worksheet

Suppose a class measures dissolved oxygen at five depths in the fictional Kestrel Lake on a July afternoon. Fish generally need dissolved oxygen above 5 mg/L to survive.

Depth (meters) Dissolved oxygen (mg/L)
0 (surface) 9
2 8
4 6
6 3
8 1
  1. At which depths is the dissolved oxygen below the 5 mg/L level that most fish need?
  2. What is the difference in dissolved oxygen between the surface and the 8-meter reading?
  3. Which depths make up the zone where fish can still survive?
  4. The same fertilizer nitrogen that helps crops grow also feeds the lake's algae. Explain how one nutrient can be both a resource and a pollutant.
  5. Put these events in the correct cause-and-effect order: algae die; fertilizer runs off into the lake; bacteria use oxygen to decompose the algae; algae grow quickly; fish in deep water suffocate.
  6. Calculate the average dissolved oxygen across the five depths.
  7. Is that average above or below the 5 mg/L threshold, and by how much?
  8. Which single depth reading is closest to the 5 mg/L threshold?
  9. If the bloom gets worse next summer, predict what will happen to the size of the zone where fish can survive, and support your prediction with the data.

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: 6 meters and 8 meters. Their readings of 3 mg/L and 1 mg/L are both below the 5 mg/L threshold.
  2. Answer: 8 mg/L. The surface reads 9 mg/L and the 8-meter depth reads 1 mg/L, so 9 − 1 = 8 mg/L.
  3. Answer: 0, 2, and 4 meters. Their readings (9, 8, and 6 mg/L) are all at or above the 5 mg/L threshold.
  4. Answer: Nitrogen is a resource when it helps crops grow, but the same nitrogen washing into the lake feeds algae that later die and rob the water of oxygen, which makes it a pollutant in that setting.
  5. Answer: Fertilizer runs off into the lake; algae grow quickly; algae die; bacteria use oxygen to decompose the algae; fish in deep water suffocate.
  6. Answer: 5.4 mg/L. (9 + 8 + 6 + 3 + 1) ÷ 5 = 27 ÷ 5 = 5.4 mg/L.
  7. Answer: Above, by 0.4 mg/L. The average 5.4 mg/L minus the 5 mg/L threshold is 0.4 mg/L, even though two depths are far below it.
  8. Answer: 4 meters. Its 6 mg/L reading is only 1 mg/L from the threshold, closer than the 3 mg/L reading at 6 meters, which is 2 mg/L away.
  9. Answer: The survivable zone will shrink. A worse bloom means more algae decompose and use more oxygen, so the low-oxygen layer would rise toward the surface and the 4-meter reading would likely drop below 5 mg/L.

Differentiation

Support

Give students a blank number line of depths with the 5 mg/L threshold already marked, so they can plot each reading and see the crossing point before answering. Work item 6 as a guided example of averaging five values.

Extension

Ask students to sketch what the depth profile might look like in winter, when the lake is cold and mixed, and explain in a short paragraph why cold, well-mixed water usually holds more oxygen at every depth.

ELL supports

Pre-teach "depth," "dissolved oxygen," "runoff," and "decompose" with a labeled cross-section of the lake showing algae at the top and low oxygen at the bottom. Allow students to answer items 4 and 9 orally or with a labeled drawing.