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Reading passage Grade 11 Science

Field Notes From Corrigan Lake: A Summer of Algae and Oxygen Loss

A grade 11 narrative reading passage of a limnology intern's field notes tracing nitrate runoff, an algae bloom, and a dissolved-oxygen crash at a fictional lake.

Objectives

  1. Students will trace how nitrate runoff, warm temperatures, and algae growth combined to cause a dissolved-oxygen crash.
  2. Students will interpret numeric water-quality data embedded within a narrative field-notes text.
  3. Students will apply the term eutrophication accurately to summarize the passage's events.

Vocabulary

Eutrophication
Eutrophication is what happens when a lake or bay gets overfed with nutrients: algae grows out of control, and when it dies and rots, the water loses the oxygen that fish need.
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: algae bloom, supersaturated

Teacher overview

Corrigan Lake (a fictional lake created for this passage) is monitored across one summer through the field notes of a limnology intern, giving students a narrative account of how nutrient runoff, warm weather, and algae growth combine to cause a dissolved-oxygen crash. The passage works well before or alongside a unit on aquatic chemistry, since the intern's notes include the kind of paired daytime-and-predawn readings real monitoring programs collect. This passage can also correct a common misconception directly: students often assume more algae automatically means less oxygen, when the truth is more specific: algae add more oxygen than they use during the day through photosynthesis, while at night respiration and decomposition continue with no photosynthesis to replace the oxygen, which is exactly why the crash in these notes happens before dawn rather than in the afternoon.

Field Notes From Corrigan Lake: A Summer of Algae and Oxygen Loss — Grade 11 Science
Name: ____________________________ Date: ______________

Passage

June 3. First sampling trip of the season at Corrigan Lake, a fictional lake used for this set of field notes. The lake looks the way it always does in early summer: clear near the shore, a light breeze keeping the surface moving. Dissolved oxygen at the dock station reads 8.5 mg/L, comfortably above the roughly 5 mg/L that most fish need to survive, and pH reads 7.4, close to the middle of the 6.5-to-8.5 range that water treated for drinking is generally expected to fall within. Nitrate-nitrogen reads a low 1.8 mg/L. Nothing here suggests anything unusual is coming.

July 10. A different lake today. Three days ago, a heavy overnight rain followed fertilizer application on the crop field that drains into the lake's northern inflow, and the water near that inflow has taken on a faint greenish tint. Nitrate-nitrogen at the dock station has climbed to 9.5 mg/L, more than five times the June reading. Worth flagging for the next visit: nitrogen this high can act like fertilizer for the lake itself, not just the field it washed off of.

August 2, 2:00 p.m. The greenish tint from July has become a thick mat of algae covering most of the cove near the inflow, and it has spread further than expected during this week's heat wave. Readings taken in mid-afternoon, while the algae is actively photosynthesizing in full sun, show dissolved oxygen at 11.2 mg/L, well above what the water could hold under normal conditions, a state worth noting as supersaturated. The pH has also climbed sharply, to 9.1, noticeably above the June reading and above the upper end of the 6.5-to-8.5 benchmark. Both changes trace back to the same cause: the algae's photosynthesis is pulling large amounts of dissolved carbon dioxide out of the water during daylight, which pushes the water's chemistry toward a higher pH at the same time it is releasing oxygen.

August 3, 6:15 a.m. Returned before sunrise after yesterday's numbers, and the lake tells a very different story in the dark. Dissolved oxygen at the same dock station has fallen to 2.1 mg/L, less than half of what most fish need to survive, and well below yesterday afternoon's reading at the same spot. Overnight, with no sunlight for photosynthesis, the algae and the bacteria feeding on dying algae cells kept consuming oxygen through respiration and decomposition without anything replacing it. A dozen dead sunfish were found along the cove's shoreline this morning, consistent with the overnight oxygen crash. This is eutrophication playing out from beginning to end: nutrient runoff triggering rapid algae growth, followed by an oxygen debt severe enough to kill fish once the algae's daytime oxygen production stopped.

August 20. Nitrate-nitrogen at the dock station has fallen back to near 2.0 mg/L, with no additional heavy rain events or fertilizer applications reported upstream since early August. The algae mat has thinned noticeably, and dissolved oxygen has stabilized around 7.5 mg/L with pH back near 7.6. The lake is recovering, but the dead sunfish along the shoreline earlier this month are a reminder of how quickly a single rain event that washes nutrients into a lake can set off consequences that take weeks to reverse.

Questions

  1. What was Corrigan Lake's dissolved oxygen reading on June 3, and how did it compare to the minimum level most fish need?
  2. What event directly preceded the nitrate spike recorded on July 10?
  3. Explain why the afternoon dissolved oxygen reading on August 2 (11.2 mg/L) was so much higher than the reading taken at the same spot the next morning before sunrise (2.1 mg/L).
  4. Based on the passage's explanation, why was the pH reading on the afternoon of August 2 (9.1) higher than the pH measured back in June (7.4)?
  5. What evidence in the passage shows that the dissolved-oxygen crash on the morning of August 3 harmed the lake's fish?
  6. Based on the full set of entries, explain what role the nitrate runoff recorded on July 10 most likely played in setting up the events of August 2 and August 3.
  7. Using the term "eutrophication" as the passage defines it, write one to two sentences summarizing what happened at Corrigan Lake this summer.

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: 8.5 mg/L, comfortably above the roughly 5 mg/L minimum most fish need. The passage states this reading directly in the June 3 entry and notes it is "comfortably above" the threshold fish require.
  2. Answer: A heavy overnight rain three days before the July 10 visit, which fell after fertilizer had been applied to the crop field draining into the lake's northern inflow. The July 10 entry states the rain followed the fertilizer application and washed nutrients toward the lake.
  3. Answer: During the afternoon, sunlight allowed the algae to photosynthesize, which adds oxygen to the water, producing the high 11.2 mg/L reading. Overnight, with no sunlight, photosynthesis stopped, but respiration and decomposition by the algae and bacteria kept consuming oxygen without anything replacing it, so the reading fell sharply by the next morning.
  4. Answer: The passage explains that the algae's photosynthesis pulls large amounts of dissolved carbon dioxide out of the water during daylight, and removing carbon dioxide pushes the water's chemistry toward a higher pH at the same time it raises dissolved oxygen.
  5. Answer: The August 3 entry reports that a dozen dead sunfish were found along the cove's shoreline that morning, consistent with the overnight oxygen crash to 2.1 mg/L, well below what most fish need to survive.
  6. Answer: The nitrate runoff acted as a fertilizer for the lake itself, most likely fueling the rapid algae growth described in the August 2 entry; that algae growth then set up the extreme day-to-night oxygen swing, since more algae means more oxygen produced by day but also more oxygen consumed by respiration and decomposition at night, which caused the fish-killing crash on August 3.
  7. Answer: Answers will vary but should describe the sequence correctly: nutrient (nitrate) runoff from the farm field triggered a rapid algae bloom, and once that bloom's oxygen production stopped overnight, the resulting oxygen debt from respiration and decomposition dropped dissolved oxygen low enough to kill fish, which is what eutrophication describes.

Differentiation

Support (below grade level)

Provide a simple timeline graphic (Rain + fertilizer → Nitrate spike → Algae bloom → Afternoon oxygen high → Overnight oxygen crash → Fish kill) for students to reference while answering questions, and allow question 6 to be answered as a short spoken response instead of writing.

Extension (above grade level)

Ask students to research (using only the passage and the dissolved-oxygen and pH benchmarks it cites, not outside sources) what a lake manager might measure the following summer to check whether the same conditions were building again before another fish kill occurred, and to justify their choice using evidence from this summer's entries.

ELL supports

Pre-teach "algae bloom," "dissolved oxygen," and "eutrophication" with a simple diagram showing algae producing oxygen bubbles in sunlight and consuming oxygen in the dark. Allow questions 1, 2, and 5 to be answered with short phrases pulled directly from the passage rather than full sentences.