Chemistry • Year 12 • Module 8 • Lesson 9

Nutrient Pollution & Eutrophication

Apply the eutrophication sequence to real NSW monitoring data, interpret a dissolved oxygen graph, and reason about cause-and-effect in a pollution event.

Apply · Band 4–5

1. Interpret NSW water-quality monitoring data

The table below shows monitoring data collected from three sites along a river in north-western NSW following heavy rain and fertiliser runoff from surrounding farms. ANZECC guideline trigger values are: nitrate ≤ 0.50 mg L⁻¹, phosphate ≤ 0.05 mg L⁻¹, dissolved oxygen ≥ 7.0 mg L⁻¹. 8 marks

Site	Nitrate / mg L⁻¹	Phosphate / mg L⁻¹	DO / mg L⁻¹	Visual observation
Site A (upstream)	0.30	0.03	8.6	Clear water, no algae visible
Site B (mid-river)	1.45	0.20	5.8	Greenish tinge, surface scum forming
Site C (downstream)	1.90	0.31	2.7	Dense green scum; dead fish near shore

1.1 Identify which sites exceed the ANZECC trigger values for all three parameters. Justify your answer by quoting specific values. 2 marks

1.2 Using the eutrophication sequence from the lesson, explain why the dissolved oxygen at Site C is so much lower than at Site A, even though the rain event was the same. 3 marks

1.3 Predict what would happen to Site B if no management action is taken and nutrient runoff continues over the next two weeks. Use data to justify your prediction. 3 marks

Stuck? Connect Cards 2, 4 and 5 from the lesson to the nutrient values at each site. Compare each measurement against the ANZECC limits.

2. Interpret the dissolved oxygen (DO) profile during an algal bloom event

The graph below shows dissolved oxygen (mg L⁻¹) measured at a single monitoring buoy in Lake Macquarie, NSW, over 28 days following a significant nutrient-loading event. 7 marks

Figure 2.1. Dissolved oxygen profile at a Lake Macquarie monitoring buoy following nutrient loading from catchment runoff. Adapted from illustrative NSW DPE water-quality monitoring data patterns (2022). ANZECC (2000) trigger level shown dashed.

2.1 Describe the trend in dissolved oxygen from Day 0 to Day 28, identifying the peak value, the minimum value, and the day on which each occurs. 2 marks

2.2 Explain why dissolved oxygen rises briefly after the initial nutrient loading event (Days 0–7), before eventually crashing. 2 marks

2.3 Between approximately which days would fish kills be most likely according to the ANZECC guideline? Justify using the graph and the ANZECC trigger value. 2 marks

2.4 The partial recovery in DO from Day 18 to Day 28 is still well below the ANZECC trigger. Suggest one process occurring in the water body that would explain the slow DO recovery. 1 mark

3. Cause-and-effect chain — Murray-Darling irrigation runoff

The diagram below shows five cause boxes (filled, left column) linked by arrows to five empty effect boxes (right column). Complete the effect boxes to trace what happens as nutrient-rich irrigation water from Murray-Darling farms enters a slow-moving river reach. An overall outcome box is provided below the chain. 6 marks

Cause 1: Heavy irrigation runoff carries dissolved NO₃⁻ and PO₄³⁻ into the river.	→	Effect 1 (nutrient and algal response): _______________

Cause 2: A dense algal bloom forms on the surface.	→	Effect 2 (impact on submerged plants): _______________

Cause 3: Submerged plants die from light deprivation.	→	Effect 3 (decomposition and BOD): _______________

Cause 4: Microbial decomposition consumes dissolved oxygen rapidly.	→	Effect 4 (DO and hypoxia): _______________

Cause 5: Dissolved oxygen falls below 2 mg L⁻¹.	→	Effect 5 (ecological outcome): _______________

Overall outcome (so…): Irrigation runoff from Murray-Darling farming

Stuck? Work through lesson Card 4’s 8-step chain, skipping the intermediate steps and just writing the key outcome of each cause.

4. Case study — Darling River cyanobacteria blooms, Menindee NSW (2018–2019)

5 marks Band 4–5

In late 2018 and January 2019, the Darling River near Menindee in far-western NSW experienced a series of severe cyanobacteria (blue-green algae) blooms during a prolonged drought. Nutrient levels in the river had been elevated by years of agricultural runoff from the upper catchment. When flows slowed and water temperatures rose, massive blooms of cyanobacteria developed. Over several weeks, hundreds of thousands of native fish — including large bream, perch and Murray cod — were found dead along a 40 km stretch of the river. Livestock drinking from the river also died. The NSW Government commissioned urgent monitoring of nitrate, phosphate, dissolved oxygen and cyanotoxin concentrations.

4.1 Using the eutrophication framework from the lesson, explain the sequence of events that led to the fish kills at Menindee. Refer specifically to: nutrient loading, algal bloom formation, decomposition, BOD and dissolved oxygen. 4 marks

4.2 Identify one way the cyanobacteria blooms created a hazard beyond the oxygen depletion mechanism. 1 mark

Stuck? Cyanobacteria (blue-green algae) are introduced in the lesson text and Key Terms. Connect the toxin production callout to the livestock deaths described in the stimulus.

Answers — Do not peek before attempting

Q1.1 — Sites exceeding ANZECC triggers for all three parameters

Both Site B and Site C exceed the ANZECC trigger values for all three parameters. Site B: nitrate 1.45 (limit 0.50), phosphate 0.20 (limit 0.05), DO 5.8 (below minimum 7.0). Site C: nitrate 1.90 (limit 0.50), phosphate 0.31 (limit 0.05), DO 2.7 (well below 7.0). Site A is below all three trigger thresholds.

Q1.2 — Why DO at Site C is much lower than at Site A (3 marks)

Site C has received the most nutrient loading (highest nitrate and phosphate), which has driven the most intense algal bloom [1]. As the bloom matures and begins to die downstream, bacterial decomposition of the dead algal biomass consumes large amounts of dissolved oxygen, significantly increasing BOD [1]. This oxygen demand exceeds the rate at which oxygen can be replenished from the atmosphere, driving DO down to 2.7 mg L⁻¹ at Site C compared to 8.6 at the unpolluted Site A [1].

Q1.3 — Prediction for Site B (3 marks)

Without management action, Site B will likely progress toward conditions similar to Site C [1]. The algal bloom visible at Site B will continue to grow and eventually die; the subsequent bacterial decomposition will increase BOD and further deplete dissolved oxygen [1]. Given that Site B already has elevated nutrients (1.45 nitrate, 0.20 phosphate) and reduced DO (5.8), continued runoff input will likely push DO below 4 mg L⁻¹, leading to hypoxic conditions and possible fish kills, as seen at Site C [1].

Q2.1 — Trend description

DO rises from 8.2 mg L⁻¹ on Day 0 to a peak of 11.4 mg L⁻¹ on Day 7 (algal bloom phase), then falls sharply to a minimum of 2.1 mg L⁻¹ on Day 18 (bloom crash/decomposition phase), before partially recovering to 5.8 mg L⁻¹ by Day 28. The overall trend across the full 28 days is a net decline from 8.2 to 5.8 mg L⁻¹.

Q2.2 — Why DO rises initially (2 marks)

During the active algal bloom phase (Days 0–7), the dense population of algae performs photosynthesis at a high rate, releasing oxygen as a byproduct [1]. This photosynthetic oxygen production temporarily exceeds the rate of oxygen consumption by respiration and decomposition, causing a net rise in dissolved oxygen — but this is transient, as it depends on the bloom remaining alive [1].

Q2.3 — Days when fish kills most likely

Fish kills would be most likely from approximately Day 13 to Day 24, when DO falls and stays below the ANZECC guideline of 7.0 mg L⁻¹. The risk is greatest around Days 16–21 when DO is at or near the minimum of 2.1 mg L⁻¹, well below the level most freshwater fish can tolerate (typically > 4 mg L⁻¹ for survival).

Q2.4 — Slow DO recovery

Continued microbial decomposition of partially decomposed algal biomass still on the lake bed or in suspension keeps BOD elevated, preventing rapid oxygen recovery. Sediments can also release stored nutrients (internal loading), sustaining low-level algal growth and further oxygen consumption.

Q3 — Cause-and-effect chain (sample answers)

Effect 1: Nutrient concentrations rise above limiting thresholds, stimulating rapid algal growth and eventual bloom formation.

Effect 2: The algal canopy blocks sunlight, preventing photosynthesis in submerged aquatic vegetation, which dies.

Effect 3: Dead plant and algal material accumulates; bacteria decompose it, dramatically increasing biochemical oxygen demand (BOD).

Effect 4: Dissolved oxygen is consumed faster than it can be replenished; the water body becomes hypoxic (dangerously low DO).

Effect 5: Fish, invertebrates and other aerobic organisms die or are forced from the reach — a fish kill event.

Overall outcome: Irrigation runoff from Murray-Darling farming ultimately causes hypoxic dead zones and fish kills in receiving river reaches through the eutrophication sequence.

Q4.1 — Eutrophication sequence at Menindee (4 marks)

Years of agricultural runoff from the upper Murray-Darling catchment elevated nitrate and phosphate concentrations in the Darling River (nutrient loading) [1]. When flows slowed and water warmed during drought, nutrient limitation was removed and cyanobacteria bloomed rapidly on the surface (algal bloom) [1]. As blooms died, bacterial decomposition of the massive biomass dramatically increased BOD, consuming dissolved oxygen faster than it could be replenished from the atmosphere [1]. Dissolved oxygen fell to critically low levels (hypoxia), creating conditions in which native fish could not survive — producing the mass fish kill [1].

Q4.2 — Hazard beyond oxygen depletion

Cyanobacteria can produce cyanotoxins (hepatotoxins and/or neurotoxins), which are toxic to mammals. The livestock deaths reported at Menindee were caused by animals drinking toxin-contaminated water, a hazard entirely separate from the dissolved oxygen depletion mechanism that killed the fish.