A Lake, a Bloom and a Fish Kill
A 40 ha lake receives treated effluent from a town of 6,000 people. Summer monitoring shows total phosphorus 0.18 mg L⁻¹ (target 0.02 mg L⁻¹), chlorophyll a 65 µg L⁻¹, Secchi depth 0.6 m, and surface dissolved oxygen 11 mg L⁻¹ at 14:00 but 2 mg L⁻¹ at 05:00. Surface pH is 9.2 at 16:00 and 7.4 at dawn. After a calm, cloudy week with a dense surface scum of cyanobacteria, about 400 carp are found dead. The council is considering two options: (A) a phosphorus-stripping plant at the sewage works, capital cost $4.2 million and $180,000 a year to run; or (B) spreading 20 t of barley straw in the lake each spring, costing $60,000 a year. A resident says, 'The fish were poisoned by the sewage. Anyway, algae make oxygen, so they cannot have killed the fish.' The lake also receives runoff from 300 ha of farmland and has a history of phosphorus release from sediments in late summer.
1. Explain the diel oxygen and pH changes and why the bloom can kill fish, despite producing oxygen by day.
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The lake is highly productive. By day, photosynthesis by the cyanobacteria removes CO₂, which raises pH from 7.4 to 9.2, and produces oxygen, so surface DO reaches 11 mg L⁻¹ at 14:00. At night, photosynthesis stops but the algae, the carp and the bacteria decomposing dead algae all respire, consuming oxygen and releasing CO₂; pH falls. The 05:00 DO of 2 mg L⁻¹ is the daily minimum. A calm, cloudy week reduces mixing and light, so photosynthesis is weak while respiration and decomposition continue. The surface scum also shades deeper algae, which then die and are decomposed by bacteria that use oxygen. The result is a night-time oxygen crash. High pH 9.2 is an additional stress and shifts ammonia towards the toxic un-ionised form. Fish can therefore die even though the algae produced oxygen earlier in the day.
2. Evaluate the resident's two claims.
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The claim that 'the sewage poisoned the fish' is not established by these data. Treated effluent can contain ammonia, pathogens and other contaminants, and ammonia is a plausible contributor, especially at pH 9.2. But the pattern, a bloom followed by a calm cloudy week and a dawn oxygen minimum of 2 mg L⁻¹, points to oxygen depletion as the proximate cause. The claim that 'algae make oxygen, so they cannot kill fish' confuses gross daytime production with the 24-hour oxygen budget. At night the bloom consumes oxygen, and when the bloom dies, bacterial decomposition consumes much more. A dense bloom can therefore be net oxygen-consuming over the day and night. To test the poisoning idea, the council would need ammonia, metals and pesticide measurements, plus fish gill histology; those data are not given.
3. Recommend between options A and B with reasoning, and state two monitoring measurements that would show whether A worked.
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Option A is the better primary investment, because phosphorus is the nutrient that limits this lake's algal biomass, and the sewage works is a continuous, identifiable point source. Reducing phosphorus at source addresses the cause, whereas barley straw is an algistat with variable and temporary effects and is unlikely to control a bloom at 65 µg L⁻¹ chlorophyll a. However, A alone may not be enough: 300 ha of farmland runoff and sediment phosphorus release in late summer will continue to load the lake, so the works should be coupled with buffer strips, fertiliser management and possibly sediment treatment. Monitoring should measure total phosphorus (and soluble reactive phosphorus), chlorophyll a, Secchi depth and dawn dissolved oxygen, plus the effluent load, for at least three summers. A decrease in phosphorus followed by lower chlorophyll and higher dawn oxygen would show that A worked; if phosphorus falls but blooms persist, internal loading is the explanation.