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Reading passage Grade 8 English Language Arts

Why Thistledown Lake Runs Out of Oxygen on Summer Nights

An original grade 8 informational passage on how dissolved oxygen rises and falls in a fictional lake, with six evidence-citation questions.

Objectives

  1. Students will trace the linked causes of a summer oxygen drop across several paragraphs.
  2. Students will cite the strongest textual evidence to support an inference about the lake.

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.

Also: algae, bloom, decompose

Teacher overview

This passage follows a fictional lake through one hot week and explains why its fish crowd near the surface when oxygen drops. It gives eighth graders an informational text that rewards close reading, because the cause of the oxygen crash is built from several linked ideas rather than stated in a single sentence. Assign it near a unit on water quality or aquatic ecosystems, and use the questions to push students toward citing the strongest evidence rather than the first line they find.

Why Thistledown Lake Runs Out of Oxygen on Summer Nights — Grade 8 English Language Arts
Name: ____________________________ Date: ______________

Passage

Thistledown Lake sits at the edge of Ledford, a small fictional town in the hills. In winter the lake looks calm and cold. In summer it becomes a harder place to breathe, at least for the fish that live in it. To understand why, you have to think about a gas you cannot see: oxygen dissolved in the water.

Fish do not use the oxygen atoms locked inside water molecules. They pull in oxygen gas that is mixed into the water, the way sugar mixes into tea. Scientists measure this gas in milligrams per liter. Most fish stay healthy when the water holds more than 5 mg/L of dissolved oxygen. Below that level they grow stressed. They swim slowly, gulp at the surface, and can die if the shortage lasts.

Two things add oxygen to a lake. The first is the air above it. Wind stirs the surface and mixes oxygen down into the water. The second is plants. Tiny floating plants called algae, along with larger water plants, make oxygen during the day when sunlight reaches them. On a bright afternoon, the top of Thistledown Lake can hold plenty of oxygen.

Night changes the picture. In the dark, plants stop making oxygen. They keep using it instead, just as animals do. Bacteria in the mud use oxygen too, mostly when they break down dead leaves and dead algae. By early morning, before the sun returns, the oxygen in the water reaches its lowest point of the day. This is when fish are most likely to struggle.

Warm water makes the problem worse. Cold water can hold more dissolved oxygen than warm water can. When a July heat wave settles over Ledford, the top of the lake warms up. The warm layer floats on the cooler water below, and the two layers stop mixing. Oxygen from the air reaches the warm top layer but cannot travel down. The deep water slowly runs short, and fish are pushed out of the cool space they prefer.

The trouble often peaks after a large algae bloom. When farm runoff or lawn fertilizer washes into the lake, algae grow fast and thick. For a few days the water looks green and full of life. Then the algae die. Bacteria swarm the dead algae and use up oxygen as they break it down. A lake that looked healthy one week can lose much of its oxygen the next.

One August morning, a Ledford science club measured the lake before school. Near the surface the reading was about 8 mg/L, which is comfortable for fish. Six feet down, in the cooler water, the reading fell to about 3 mg/L. That is below the level most fish need. The students noticed sunfish and bass crowded into a thin band near the top, avoiding both the warm surface and the low-oxygen deep. The fish were caught between two problems the lake had created at the same time.

By late September the danger faded. Cooler air chilled the surface, the layers mixed again, and oxygen spread through the whole lake. Thistledown Lake had not been poisoned. It had simply run low on a gas that most people never think about, at the time of year when its fish needed that gas the most.

Questions

  1. According to the passage, what is the lowest level of dissolved oxygen most fish can handle before they become stressed?
  2. Name the two sources the passage says put oxygen into a lake.
  3. The passage states that oxygen is lowest in the early morning. Using details from the text, explain why the level drops during the night.
  4. Why does a summer heat wave make the oxygen problem worse? Quote the part of the passage that most strongly supports your answer.
  5. Explain how an algae bloom can lead to an oxygen crash, even though algae make oxygen. Use evidence from more than one part of the passage.
  6. When the science club measured the lake, the fish crowded into a thin band near the top. What does this detail suggest about the deep water and the warm surface water that morning?

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: Above 5 mg/L. The passage states that most fish stay healthy when the water holds more than 5 mg/L of dissolved oxygen, and that they grow stressed below that level.
  2. Answer: The air above the lake and plants. The passage says wind mixes oxygen from the air into the water, and that algae and larger plants make oxygen during the day.
  3. Answer: At night, plants stop making oxygen and keep using it, and bacteria in the mud also use oxygen to break down dead leaves and algae. With no new oxygen being made and several living things using it, the level falls to its lowest point by morning.
  4. Answer: A heat wave warms the top of the lake, and warm water holds less oxygen than cold water. The strongest supporting sentence is that the warm layer "floats on the cooler water below, and the two layers stop mixing," which traps low-oxygen conditions in the deep water.
  5. Answer: Algae make oxygen while they are alive, but after a bloom the algae die. Bacteria then swarm the dead algae and use up oxygen to break it down, so the same bloom that added oxygen later causes a shortage. Support comes from both the paragraph on algae blooms and the paragraph on bacteria using oxygen at night.
  6. Answer: It suggests the deep water was too low in oxygen and the surface water was too warm, so the only livable space was a thin band between them. The fish avoided both problems by crowding into that middle layer.

Differentiation

Support

Give students a blank cause-and-effect chart with the boxes labeled "heat wave," "layers stop mixing," "deep water loses oxygen," and "fish move up." Having students fill the arrows between boxes as they read helps them hold the multi-step cause together before they answer question 4.

Extension

Ask students to write a short paragraph predicting what a strong wind storm in the middle of the July heat wave would do to the lake's oxygen, using only evidence from the passage to justify the prediction.

ELL supports

Pre-teach "dissolved," "algae," "bloom," and "decompose" with a simple picture for each: sugar dissolving, green water, a spreading patch, and a leaf breaking apart. Allow students to answer questions 1 and 2 with short phrases or by pointing to the sentence in the text.