Writing prompt Grade 11 Science
Fallowmere Reservoir's Late-Summer Oxygen Loss
Grade 11 students read four months of depth profiles from a fictional reservoir and explain stratification, deep-water anoxia, and the phosphorus release that follows.
Objectives
- Students will explain how temperature-driven density differences keep a reservoir from mixing in summer.
- Students will use monitoring data to trace oxygen loss, phosphorus release, and manganese release to a shared cause.
- Students will describe a feedback in which one summer's oxygen loss makes the following summer's bloom larger.
Vocabulary
Key words: stratification, thermocline, hypolimnion, anoxia, internal loading, turnover
Teacher overview
Deep water in a reservoir can run out of oxygen by late summer while the surface layer stays well supplied, and that change alters the chemistry of the sediment underneath. This prompt supplies four monitoring profiles from a fictional reservoir and asks students to write a scientific explanation of the sequence behind them. The required explanation turns on a physical barrier rather than a biological one, because density differences stop the layers from trading water while bacteria below keep consuming oxygen. The prompt also asks students to trace the feedback that can make the next summer's bloom larger than this one.
Prompt
Background. Fallowmere Reservoir is a fictional water supply used for this assignment. It is 62 feet deep at the dam and holds the raw water for the fictional Fallowmere Water District. The district samples four depths on the same buoy line about every six weeks. Staff record temperature in degrees Celsius, dissolved oxygen, total phosphorus, and dissolved manganese. Every figure in the table below is invented for this scenario.
| Date | Depth (ft) | Temp (C) | Dissolved oxygen (mg/L) | Total phosphorus (mg/L) | Dissolved manganese (mg/L) |
|---|---|---|---|---|---|
| May 12 | 5 | 14.5 | 9.6 | 0.02 | 0.02 |
| May 12 | 20 | 13.8 | 9.4 | 0.02 | 0.02 |
| May 12 | 35 | 12.9 | 9.1 | 0.03 | 0.03 |
| May 12 | 55 | 11.8 | 8.8 | 0.03 | 0.03 |
| July 8 | 5 | 25.6 | 8.2 | 0.02 | 0.02 |
| July 8 | 20 | 22.4 | 7.4 | 0.02 | 0.02 |
| July 8 | 35 | 13.1 | 3.1 | 0.06 | 0.30 |
| July 8 | 55 | 11.4 | 0.4 | 0.14 | 1.20 |
| Aug 26 | 5 | 26.1 | 8.0 | 0.03 | 0.02 |
| Aug 26 | 20 | 23.9 | 6.9 | 0.03 | 0.03 |
| Aug 26 | 35 | 13.4 | 0.6 | 0.19 | 1.90 |
| Aug 26 | 55 | 11.6 | 0.0 | 0.41 | 3.40 |
| Oct 14 | 5 | 14.2 | 8.9 | 0.09 | 0.35 |
| Oct 14 | 20 | 14.1 | 8.8 | 0.09 | 0.34 |
| Oct 14 | 35 | 14.0 | 8.6 | 0.09 | 0.33 |
| Oct 14 | 55 | 13.9 | 8.4 | 0.09 | 0.32 |
Three points of context will help you read the table. First, water is densest a little above its freezing point, so warm water floats on cold water and needs wind energy to mix downward. Second, bacteria in the sediment consume oxygen as they break down settled algae and leaves. Third, phosphorus is held in the sediment by iron minerals, and manganese by manganese minerals, and both kinds stay solid only while oxygen is present; when the oxygen is gone, those minerals dissolve and release what they held. Most fish need dissolved oxygen above 5 mg/L, a standard water-quality benchmark. In mid-October the district's customers reported gray and brown water, and the plant reported a sudden manganese load at its intake.
Writing task. Write an explanation for the board of the fictional Fallowmere Water District that accounts for the pattern in the table. Your explanation must do five things. Explain why the reservoir separated into layers between May and July. Explain why oxygen fell sharply only below 20 feet. Explain why phosphorus and manganese rose in the same water where oxygen fell. Explain what the October profile shows and why customers saw discolored water that month. Finally, explain the feedback that could make next summer's algae bloom larger than this one. Cite specific values from the table for each claim, and use the terms stratification, thermocline, hypolimnion, anoxia, internal loading, and turnover correctly.
Planning Organizer
Fill in every row before you draft, and use the rows to plan the five parts of your explanation.
| Part of the explanation | Data you will cite | The mechanism in your own words |
|---|---|---|
| Why the layers formed between May and July | ||
| Why oxygen fell sharply only below the thermocline | ||
| Why phosphorus rose in the deep water | ||
| Why manganese rose in the deep water | ||
| What the October profile shows | ||
| Why customers saw discolored water in October | ||
| The feedback that affects next summer | ||
| Terms I must define for the board |
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
In May the Fallowmere profile was nearly uniform. Temperature fell only from 14.5 to 11.8 degrees Celsius between 5 and 55 feet, and dissolved oxygen stayed between 9.6 and 8.8 mg/L at every depth. Wind could still push the whole column into motion, so the surface kept resupplying the bottom with oxygen. By July 8 the surface had warmed to 25.6 degrees Celsius while the water at 55 feet was still 11.4 degrees. Warm water is less dense than cold water, so the heated surface layer floated on top and the wind no longer had enough energy to mix the two. That separation is stratification, and the narrow zone of rapid temperature change between 20 and 35 feet is the thermocline. Below it lies the hypolimnion, which was cut off from the atmosphere for the rest of the summer.
Once the hypolimnion was isolated, its oxygen supply stopped but its oxygen demand did not. Bacteria in the sediment continued to break down settled algae and leaf material, and that decomposition consumes oxygen. Oxygen at 55 feet fell from 8.8 mg/L in May to 0.4 mg/L in July and to 0.0 mg/L on August 26. The surface held near 8.0 mg/L in July and August because it remained in contact with the air and with photosynthesizing algae. Oxygen at 5 and 20 feet fell only modestly, to 8.0 and 6.9 mg/L, partly because warmer water holds less oxygen, and it stayed above the 5 mg/L most fish need because those depths sit above the thermocline, so the severe loss tracks the physical barrier rather than the whole reservoir.
Phosphorus and manganese rose in exactly the water that lost its oxygen. Total phosphorus at 55 feet climbed from 0.03 mg/L in May to 0.14 mg/L in July and 0.41 mg/L in August, and dissolved manganese climbed from 0.03 to 3.40 mg/L over the same months. Iron minerals in the sediment hold phosphate, and manganese minerals hold manganese, as long as oxygen is present. When the hypolimnion went anoxic, those minerals dissolved and released both into the water above the mud. This release from the reservoir's own sediment is called internal loading, and it explains why the deep concentrations climbed with no new pollution entering the reservoir.
The October profile shows turnover. Temperature was nearly identical from top to bottom, between 14.2 and 13.9 degrees Celsius, because autumn cooling erased the density difference and wind mixed the full column. Oxygen returned to 8.4 mg/L even at 55 feet. Phosphorus and manganese were also spread evenly, at about 0.09 and 0.33 mg/L at every depth, which is higher than the surface values of the summer. That mixing carried dissolved manganese to the intake, and once the plant's chlorine oxidized it, it formed dark solid particles, so customers saw gray and brown water in mid-October.
The feedback matters for next year. The phosphorus released from the sediment does not leave the reservoir at turnover; it is redistributed into the surface water, where algae can use it the following spring. A larger bloom produces more organic material that settles into the hypolimnion, which raises oxygen demand there, which deepens next summer's anoxia, which releases still more phosphorus. The board should understand that one warm, stratified summer can therefore make the following summer worse without any change in what the watershed sends to the reservoir.
Rubric
| Criterion | Emerging | Developing | Proficient | Advanced |
|---|---|---|---|---|
| Physical mechanism | Does not connect temperature to mixing | States that layers formed but does not tie the layering to density | Explains that warming lowers surface density and blocks mixing, naming the thermocline | Explains the density barrier and uses the depth of the temperature break to locate the thermocline in the data |
| Use of the data | Cites no values from the table | Cites one or two values without linking them to a claim | Cites values from at least three dates and links each to a stated claim | Cites values across all four dates and contrasts depths above and below the thermocline to rule out alternatives |
| Chemical release | Does not mention phosphorus or manganese | Notes that both rose without a cause | Ties the rise in both to the loss of oxygen and the dissolving of iron and manganese minerals | Ties the release to anoxia, names it internal loading, and explains why concentrations differ by depth |
| Feedback reasoning | No mention of the following year | States next year could be worse without a mechanism | Traces released phosphorus to a larger bloom and heavier oxygen demand | Traces the full loop and explains why it can worsen without new watershed inputs |
| Organization and conventions | Ideas are out of causal order and errors block meaning | Some causal order; errors are noticeable | Clear causal order with correct use of the required terms | Causal order, precise terminology, and a closing statement written for a board audience |
Differentiation
Support
Give these students a version of the table with the two thermocline depths shaded, and provide the first sentence of each of the five required paragraphs. Allow them to write the explanation as five labeled sections rather than as connected prose, and let them draw a labeled cross-section of the reservoir before drafting to place the epilimnion, thermocline, and hypolimnion.
Extension
Ask these students to add a sixth paragraph that recommends one monitoring change and one operating change, such as drawing from a shallower intake port while the reservoir is stratified in late summer, and to state what evidence in the table supports each recommendation.
ELL supports
Pre-teach stratification, thermocline, hypolimnion, anoxia, and turnover with a labeled diagram and a cognate check for students whose first language shares Latin or Greek roots for these terms. Provide the sentence frames "The data show ___ because ___" and "Once the oxygen fell to ___, the sediment released ___." Allow the Planning Organizer to be completed in phrases before drafting.
Extensions
Have students compare their finished explanation with a second fictional reservoir that is only 18 feet deep and never separates into layers, and write a short paragraph predicting how that reservoir's deep-water chemistry would differ through the summer. Students can also research how water suppliers manage manganese at the treatment plant and add a note on which of those methods would address the cause rather than the symptom.