A Lake Turns Green: Oxygen, Nutrients, and Fish Kill
A 40-hectare lake receives untreated sewage from a growing town. In June, Secchi depth falls from 1.8 m to 0.4 m; chlorophyll-a is 85 µg/L; total phosphorus is 120 µg/L, up from 20 µg/L. Night-time dissolved oxygen at 0.5 m is 2 mg/L; dawn dissolved oxygen at 3 m is 0 mg/L; and about 2,000 fish die. The lake stratifies thermally in summer, and the hypolimnion is anoxic. A citizen group proposes stocking tilapia and adding copper sulfate. The local government proposes a constructed wetland and sewage diversion. The lake is used for drinking water after treatment and for fishing. Local farmers also withdraw irrigation water. The town's population is 80,000 and growing 3% per year.
1. Diagnose the problem and explain the sequence that led to the fish kill, including why the kill may occur at dawn.
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The lake is eutrophic. Total phosphorus rose from 20 to 120 µg/L, chlorophyll-a is 85 µg/L, and Secchi depth fell to 0.4 m, indicating a dense algal bloom. Phosphorus is usually the limiting nutrient in freshwater, so sewage inputs relieved that limitation. During the day, algae photosynthesize and produce oxygen, but at night they respire and consume oxygen; bacteria decomposing dead algae also consume oxygen. Summer thermal stratification prevents mixing, so oxygen is not replenished in the hypolimnion, which becomes anoxic. At dawn, oxygen is lowest because respiration has continued all night without photosynthesis. Fish at 0.5 m face 2 mg/L, which is below the needs of many species, and the 3 m layer has 0 mg/L. The fish kill is therefore explained by hypoxia, possibly worsened by algal toxins.
2. Evaluate the two proposals. Which would you recommend and why? Include at least two ecological reasons.
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The constructed wetland and sewage diversion are the better choice because they address the cause: phosphorus loading. Diversion removes the nutrient source; a wetland can absorb and transform nutrients before they reach the lake. Copper sulfate is an algaecide, not a nutrient control; it kills algae, and their decomposition consumes oxygen, potentially worsening hypoxia and killing fish. Copper can also be toxic to non-target organisms and accumulate in sediment. Stocking tilapia is risky: tilapia can become invasive, compete with native fish, and its excretion adds nutrients. It may also disturb sediment. The citizen proposal treats the symptom; the government proposal treats the cause. The lake's use for drinking water makes copper and algal toxins especially unacceptable.
3. If external phosphorus loading is cut by 70%, why might the lake not recover immediately? Explain internal loading and monitoring.
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Cutting external phosphorus by 70% is necessary but may not produce immediate recovery because of internal loading. Phosphorus stored in sediments can be released back into the water when the hypolimnion is anoxic; this creates a positive feedback that keeps the lake eutrophic even after sewage diversion. The lake may therefore continue to bloom for years. Monitoring should include total phosphorus, dissolved oxygen, chlorophyll-a, and Secchi depth, with samples at different depths and seasons. If internal loading is severe, a carefully evaluated sediment treatment such as alum may be considered, but only after external loading is controlled. Recovery also depends on the lake's flushing rate and food-web changes. The public should be told that improvement will take time.