A sewage-polluted urban lake: choosing an intervention
A 60-hectare urban lake receives untreated sewage from a 5,000-person settlement. In summer, Secchi depth falls to 0.3 m, chlorophyll a is 120 µg/L, dissolved oxygen at 2 m depth is 1.5 mg/L at dawn, and Microcystis dominates the phytoplankton. BOD5 of the inflow is 60 mg/L, and total phosphorus is 0.4 mg/L. Fish kills occur at night. The lake has no stratification in winter but strong thermal stratification from April to June. The municipal engineer proposes three options: (1) dose the lake with copper sulphate to kill the algae; (2) install two 5-hp paddle aerators; (3) divert the sewage and create a 30-m vegetated buffer strip along the inflow. A citizens' group wants to add 2,000 filter-feeding silver carp and harvest them. The lake is also used for drinking-water abstraction and recreation.
1. Using the data, explain the night-time fish kills and predict the short-term effect of copper sulphate.
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The sewage supplies phosphorus (0.4 mg/L, well above the 0.02–0.05 mg/L range typical of eutrophic risk) and nitrogen, so phytoplankton biomass is high (chlorophyll a 120 µg/L). During the day, photosynthesis adds oxygen, but at night photosynthesis stops while all the algae, bacteria and fish continue respiring; decomposition of dead algae also consumes oxygen. The measured dawn oxygen at 2 m is already 1.5 mg/L, below the needs of most fish, so the night-time minimum is lethal. Microcystis can also produce toxins and surface scums that worsen oxygen loss.
Copper sulphate will kill the algae quickly. The dying cells sink and are decomposed by bacteria, which increases biochemical oxygen demand; the immediate result can be a worse oxygen crash and a larger fish kill. Lysed Microcystis releases microcystins, and copper itself is toxic to fish and zooplankton. Because the lake is used for drinking water, copper dosing and toxin release are unacceptable. The prediction is therefore short-lived clearing followed by more severe oxygen depletion; copper treats the symptom and adds a pollutant.
2. Compare the three municipal options and the carp proposal. Which should be the priority and why?
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Sewage diversion addresses the external nutrient load, which is the cause; the vegetated buffer strip further traps sediment and particulate phosphorus. This is the only option that reduces the input every day. Aerators add oxygen but do not remove nutrients; they are energy-dependent, can mix anoxic hypolimnetic water into the surface, and may help fish survive but not restore the lake. Copper is a short-term algicide with the harmful effects described.
Silver carp filter phytoplankton, including cyanobacteria, and harvesting them removes some nitrogen and phosphorus in fish biomass; however, the fish are invasive, can escape, disrupt the food web by removing zooplankton, and are a hazard to boaters because they jump. The harvest must be sustained and the biomass removed, not returned to the lake.
Priority should be sewage diversion plus the buffer strip, with monitoring of inflow and lake phosphorus. If internal phosphorus release from anoxic sediments keeps the lake eutrophic after diversion, consider sediment dredging or hypolimnetic oxygenation as a second phase. The carp should not be introduced into a drinking-water lake without a contained, evidence-based trial.
3. Why is total phosphorus, rather than BOD, the key management variable? What measurements would confirm internal loading?
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BOD measures the oxygen consumed by biodegradable organic matter over 5 days; it is useful for the oxygen sag and for sewage strength, but it does not measure the nutrient capital that sustains algal growth. In most freshwater lakes phosphorus is the limiting nutrient, so a small increase in phosphorus produces a large increase in algal biomass. The value of 0.4 mg/L total phosphorus is very high; even 0.02–0.05 mg/L can support mesotrophic conditions, so the lake is phosphorus-rich. Reducing BOD without reducing phosphorus would leave the bloom potential.
To test internal loading, measure soluble reactive phosphorus and total phosphorus in the hypolimnion through the stratified period (April–June), together with hypolimnetic oxygen; a rise in phosphorus as oxygen falls indicates release from iron–phosphorus complexes in the sediment. Measure sediment porewater phosphorus and sediment phosphorus fractions, and calculate the inflow–outflow phosphorus budget. A whole-lake phosphorus mass balance before and after diversion shows whether external loading has actually fallen. These measurements determine whether diversion alone will be enough or whether dredging or phosphorus-binding agents are needed.