A Lake That Keeps Blooming After Sewage Diversion
A 40-ha lake has a maximum depth of 12 m and a mean depth of 5 m. Sewage effluent was diverted 3 years ago, and the external phosphorus load fell from 2.8 to 0.6 g P/m²/yr. Summer chlorophyll a nevertheless remains 55–70 µg/L, Secchi depth is 0.7 m, and the hypolimnion has dissolved oxygen below 1 mg/L from July to September. Microcystis dominates the phytoplankton. Sediment cores contain 1,800 mg/kg total phosphorus in the top 10 cm but 600 mg/kg at 30 cm. Under anoxic conditions, internal phosphorus release is measured at 0.9 g P/m²/yr. Three proposals are on the table: (1) apply aluminum sulfate to bind phosphorus; (2) dredge the top 30 cm of sediment; (3) stock 5,000 grass carp to consume the algae. The lake is used for drinking water and recreation.
1. Explain why the lake has not recovered despite the reduction in external phosphorus load, using the data.
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External load reduction is necessary but not sufficient. The sediment has accumulated phosphorus over decades: 1,800 mg/kg in the top 10 cm versus 600 mg/kg at 30 cm. Under anoxia, iron-bound phosphorus is released, and the measured internal load of 0.9 g/m²/yr is now larger than the external load of 0.6 g/m²/yr. The lake therefore continues to fertilise itself. Anoxia also favours Microcystis, which can regulate buoyancy and access nutrients. The long water residence time and shallow mean depth of 5 m make the lake vulnerable. Recovery lag is expected because sediment phosphorus declines slowly.
2. Evaluate the three proposals, including risks and likely time scale.
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Aluminum sulfate can bind phosphorus and reduce internal loading, but it requires adequate alkalinity, careful dosing, and continued external control; it can be toxic to fish if pH falls. It is the most targeted option, with effects within months, but it is not permanent. Dredging removes the phosphorus-rich sediment but is expensive, destroys benthic habitat, and can release nutrients; it may be justified for a drinking-water lake but should be phased. Grass carp are mainly herbivores of macrophytes, not phytoplankton; removing macrophytes can increase algal blooms and reduce water clarity. Stocking 5,000 fish would likely worsen the problem. Prefer alum plus continued external control, with dredging as a later option.
3. Design a monitoring plan to judge success within 3 years, naming variables and thresholds.
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Monitor total phosphorus in surface and hypolimnetic water, chlorophyll a, Secchi depth, dissolved oxygen profiles at 1-m intervals, Microcystis biovolume, and microcystin. Sample fortnightly from May to October at three stations. Define success as summer chlorophyll a below 20 µg/L, Secchi depth above 1.5 m, hypolimnetic dissolved oxygen above 4 mg/L, and no microcystin above the drinking-water guideline for two consecutive years. Include a before-after-control-impact design and track rainfall and temperature, because climate variability can mask recovery. Judge alum by reduced sediment phosphorus release in sediment cores, not just by one season's water clarity.