A lake below a sewage outfall: nutrients, algae and oxygen
A river enters a lake and flows out at the far end. Four stations are sampled at 2 pm on a sunny July day: A upstream of the lake, B 50 m below a sewage treatment outfall, C 500 m downstream of B, and D 2 km further downstream. At each station, nitrate, phosphate, algal chlorophyll, dissolved oxygen at the surface and mayfly nymphs in a 0.25 m² kick sample are measured. Results: A: nitrate 2 mg/L, phosphate 0.02 mg/L, chlorophyll 5 µg/L, oxygen 9 mg/L, mayflies 18. B: nitrate 12 mg/L, phosphate 1.4 mg/L, chlorophyll 90 µg/L, oxygen 11 mg/L, mayflies 2. C: nitrate 10 mg/L, phosphate 1.2 mg/L, chlorophyll 60 µg/L, oxygen 4 mg/L, mayflies 0. D: nitrate 6 mg/L, phosphate 0.5 mg/L, chlorophyll 20 µg/L, oxygen 7 mg/L, mayflies 6. At C, oxygen at 5 am is 2 mg/L. The lake is shallow and mixes freely; the weather is calm.
1. Explain the pattern in algal chlorophyll and dissolved oxygen from A to C.
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Nitrate and phosphate rise sharply at B because the sewage outfall adds nutrients. Algal chlorophyll rises from 5 to 90 µg/L, so the algae bloom. At 2 pm at B, photosynthesis by the algae produces oxygen, so surface oxygen is high (11 mg/L) even though the water is polluted. At C the algae have begun to die and sink; bacteria decompose them and use oxygen in respiration, so oxygen falls to 4 mg/L at 2 pm and 2 mg/L at 5 am. The 5 am reading is lower because photosynthesis has stopped overnight but respiration continues. The high daytime oxygen at B is misleading; it does not mean the water is healthy. Oxygen demand from decomposition is the better indicator.
2. Why do mayfly nymphs decline from A to C, and what does this tell you about using them as indicators?
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Mayfly nymphs need high dissolved oxygen and are sensitive to organic pollution. At A oxygen is 9 mg/L and mayflies are 18; at C oxygen is 4 mg/L (2 mg/L at night) and mayflies are absent. The likely cause is oxygen depletion plus silt and sewage solids. At B mayflies are already down to 2 despite high daytime oxygen, which suggests that other factors such as ammonia or blanketing by sewage solids matter, or that the 2 pm oxygen reading does not represent the whole day. Mayflies are useful indicator species because their presence integrates conditions over time, not just one measurement. But the B result shows that a single oxygen reading can mislead; indicators should be used with chemical data.
3. Propose one management strategy and explain how it would reduce eutrophication.
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Upgrade the sewage treatment plant to tertiary treatment that removes phosphate (and nitrate), for example by chemical precipitation with iron or aluminium salts, before discharge. Phosphate is usually the limiting nutrient in freshwater, so reducing it limits algal growth. Fewer algae mean less dead organic matter, so bacterial decomposition and oxygen demand fall, and oxygen levels recover. Supporting measures include buffer strips along the river to intercept runoff, reducing fertilizer use on nearby land, and monitoring at D to check recovery. The strategy must target the limiting nutrient; if nitrogen is also high, both may need control. The lake will not recover immediately because nutrients stored in sediment can be released for years.