Writing prompt Grade 6 Science
Why Stagbrook's Oxygen Runs Lowest Just Before Sunrise
A grade 6 science argument prompt: students use a 24-hour oxygen and temperature record from a fictional creek to explain the dawn low that killed minnows.
Objectives
- Students will make a claim about what caused a dawn low in dissolved oxygen.
- Students will support that claim with at least three readings from a 24-hour data record.
- Students will explain how plants and algae can both raise and lower the oxygen in a creek.
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.
- 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.
Also: photosynthesis, respiration, algae, nutrient
Teacher overview
Students write a short scientific argument explaining why dissolved oxygen in a fictional creek dropped to its lowest point just before sunrise, and why the drop was far larger below a pasture ditch than above it. The data set gives them oxygen and temperature together, which lets them test an explanation rather than guess at one. One misconception to address before students write: many students learn that warm water holds less oxygen and then apply that rule to every oxygen problem they meet. The rule is true, yet in this record the warmest readings carry the most oxygen and the coolest reading carries the least, because the plants in the creek are adding oxygen all afternoon and using it all night. Students who reach for temperature alone will write a confident argument that the data contradict.
Prompt
Background
Stagbrook is a fictional creek in the fictional town of Yarnwick. Everything here about Stagbrook and Yarnwick was written for this assignment.
A ditch drains a pasture into Stagbrook. Below that ditch the creek bed is covered with thick mats of green algae. Above the ditch the bed is mostly clean gravel.
Green plants and algae do two things with oxygen. In daylight they make sugar and give off oxygen. That is photosynthesis. Day and night, they also use oxygen to stay alive. That is respiration. Animals in the creek respire too, and so do the bacteria in the mud.
Last August a class measured Stagbrook for 24 hours. They set up at two sites. Site A sits just above the pasture ditch. Site B sits about 200 meters below it. The class measured dissolved oxygen and water temperature every three hours. Dissolved oxygen is the oxygen mixed into the water itself, which is what fish breathe through their gills.
On the morning of the second day, the class found 40 dead minnows in a pool at Site B. They found none at Site A.
Table 1. Stagbrook 24-hour record (fictional class data)
| Time | Site A temperature | Site A oxygen | Site B temperature | Site B oxygen |
|---|---|---|---|---|
| 3 p.m. | 21 degrees Celsius | 9.1 mg/L | 22 degrees Celsius | 13.2 mg/L |
| 6 p.m. | 21 degrees Celsius | 9.4 mg/L | 22 degrees Celsius | 13.8 mg/L |
| 9 p.m. | 19 degrees Celsius | 8.6 mg/L | 20 degrees Celsius | 9.9 mg/L |
| 12 a.m. | 18 degrees Celsius | 8.1 mg/L | 19 degrees Celsius | 7.0 mg/L |
| 3 a.m. | 17 degrees Celsius | 7.6 mg/L | 18 degrees Celsius | 4.6 mg/L |
| 6 a.m. | 16 degrees Celsius | 7.2 mg/L | 17 degrees Celsius | 3.1 mg/L |
| 9 a.m. | 18 degrees Celsius | 8.3 mg/L | 19 degrees Celsius | 6.4 mg/L |
| 12 p.m. | 20 degrees Celsius | 9.0 mg/L | 21 degrees Celsius | 11.0 mg/L |
Two more measurements from the same fictional trip
The class also tested nitrate, a nutrient that helps algae grow. Site A read 0.4 mg/L. Site B read 3.8 mg/L. The pasture ditch was running on the day of the test.
One benchmark from outside the scenario
Most fish need dissolved oxygen above 5 mg/L. (Source: standard water-quality guidance from EPA and USGS.)
Your task
Write a scientific argument of four or five paragraphs. Answer this question: what caused the low oxygen that killed the minnows at Site B?
State a clear claim. Support it with at least three specific readings from Table 1 or from the nitrate results. For each piece of evidence, explain what it shows. Use the words photosynthesis and respiration correctly at least once each. Finish with a sentence about what the class should measure next to check your explanation.
Planning Organizer
Fill in every row before you draft. Each row matches one part of the rubric.
| Part of the argument | What to write | My notes |
|---|---|---|
| Claim | One sentence naming the cause of the low oxygen at Site B. | |
| Evidence 1 | A reading from Table 1 that shows when the oxygen was lowest at Site B. Give the time and the number. | |
| Reasoning 1 | Why does that time of day point to plants and algae rather than to something else? | |
| Evidence 2 | A pair of readings that shows what temperature was doing at the same time. Give both numbers. | |
| Reasoning 2 | If warm water were the cause, when would the lowest oxygen appear? What does the record show instead? | |
| Evidence 3 | A comparison of Site A and Site B, either the oxygen swing or the nitrate result. | |
| Reasoning 3 | What does the difference between the two sites tell you about the ditch? | |
| Conclusion | Why the minnows died at Site B and not at Site A, and one measurement the class should take next. |
From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.
Exemplar & Rubric
Exemplar response (written at grade 6 level)
The minnows at Site B died because the algae and plants in the creek used up the oxygen during the night. All day the algae made oxygen faster than the creek used it. After dark the algae kept using oxygen but stopped making it, so the oxygen fell hour by hour until the sun came up.
The clearest evidence is the timing. At Site B the oxygen was highest at 6 p.m., at 13.8 mg/L, and lowest at 6 a.m., at 3.1 mg/L. That low point came at the end of the night, right before sunrise. Then the oxygen climbed again during the morning, reaching 6.4 mg/L by 9 a.m. A cause that turns off in the dark and back on in the light fits photosynthesis.
Temperature does not explain the pattern. Warm water does hold less oxygen than cool water, so if heat were the cause, the lowest reading should come in the afternoon when the creek is warmest. The record shows the opposite. Site B was 22 degrees Celsius at 6 p.m. with 13.8 mg/L of oxygen, and 17 degrees Celsius at 6 a.m. with only 3.1 mg/L. The coolest water of the day held the least oxygen.
The two sites also differed in a way that points to the ditch. Site A swung between 9.4 mg/L and 7.2 mg/L, a change of about 2 mg/L. Site B swung from 13.8 mg/L down to 3.1 mg/L. Site B also read 3.8 mg/L of nitrate while Site A read 0.4 mg/L. Nitrate is a nutrient that feeds algae, and thick algae mats grow only below the ditch. More algae means a bigger rise in the day and a bigger fall at night.
The minnows died at Site B because the oxygen there fell to 3.1 mg/L, well under the 5 mg/L that most fish need, while Site A never dropped below 7.2 mg/L. To check this explanation, the class should measure oxygen every three hours again on a cloudy day. If the algae are the cause, a cloudy day should give a smaller daytime rise and the whole cycle should flatten out.
Rubric
| Criterion | Emerging | Developing | Proficient | Advanced |
|---|---|---|---|---|
| Claim | No claim, or the claim restates the question | Names a cause but leaves it vague | States one clear cause of the low oxygen at Site B | States a clear cause and names the time pattern it must explain |
| Evidence | No readings from the record | Uses one reading, or quotes numbers without times or sites | Uses at least three specific readings, each with its time and site | Uses three or more readings chosen to test the explanation, including the temperature pair |
| Reasoning | Restates the number without explaining it | Explains one piece of evidence | Explains what each piece of evidence shows about oxygen in the creek | Explains each piece and connects photosynthesis and respiration to the daily rise and fall |
| Organization and conventions | Hard to follow; frequent errors interrupt meaning | Some order; errors distract in places | Clear paragraphs, correct use of photosynthesis and respiration, few errors | Clear paragraphs, precise science vocabulary, a closing next measurement, and clean conventions |
Differentiation
Support
Give these students a copy of the table with the Site B oxygen column already graphed as a line, and mark sunrise and sunset on the time axis. Provide sentence frames: "The oxygen at Site B was lowest at ___, when it read ___." and "This points to plants because ___." Allow a three-paragraph response, and accept a labeled sketch of the day and night oxygen cycle in place of the final paragraph.
Extension
Ask students to predict, in writing, what the same 24-hour record would look like at Site B after a week of heavy rain that scoured the algae mats away, and to state which two readings would change the most. A second option is to write the short note the class would send to the fictional pasture owner, explaining what nitrate has to do with the dead minnows.
ELL supports
Pre-teach "dissolved," "oxygen," "algae," "nutrient," "photosynthesis," and "respiration" with a labeled two-panel drawing: a creek in daylight with arrows showing oxygen going into the water, and the same creek at night with arrows showing oxygen leaving. Provide a bilingual glossary card. Allow students to talk through the organizer with a partner before drafting, and accept a first draft that uses the sentence frames word for word.
Extensions
Have students look up whether their state posts dissolved oxygen data for a nearby stream, and report the time of day the sample was taken. Ask them why a single midday sample could make a struggling stream look healthy. A second option is to plan the fictional class's next trip: list the sites, the times, and the reason for each choice in one short paragraph.