A pea-green lake, supersaturated surface and dying bottom-feeders
A 40-hectare shallow lake on the edge of a town receives secondary-treated sewage from a plant that removes solids and BOD but does no nutrient removal; a sugar mill also discharges upstream during the crushing season. In late June the water is pea-green and a Secchi disc disappears at 0.4 m. Measured values: chlorophyll-a 120 micrograms per litre; inflow nitrate 12 mg/L and phosphate 1.8 mg/L (N:P by mass 6.7:1); surface dissolved oxygen 9 mg/L at 05:00 and 14 mg/L at 16:00; bottom dissolved oxygen 1.2 mg/L; pH 9.2 at midday. Hundreds of bottom-feeding fish and snails are found dead, while carp are seen gulping air at the surface. The municipal committee proposes dosing the lake with a copper-based algicide before a festival and asks whether that will 'clean the water'.
1. Using the N:P ratio and the measured concentrations, identify the nutrient that should be controlled first and justify the choice. Which single intervention would you prioritise, and why not the other nutrient?
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Algal biomass has a carbon:nitrogen:phosphorus requirement of about 106:16:1 by atoms, roughly 7:1 by mass. The inflow's 6.7:1 is at or below that, so the lake is relatively nitrogen-poor compared with phosphorus, and nitrogen-fixing cyanobacteria gain an advantage - which fits the pea-green scum and the midday pH of 9.2 from intense photosynthetic CO2 removal.
Both nutrients are far above eutrophic thresholds (phosphate above about 0.05 mg/L, nitrate above about 1 mg/L), and chlorophyll-a of 120 micrograms per litre is hypereutrophic, so the problem is gross over-enrichment, not marginal shortage. Because phosphorus is the nutrient in relative excess, it is the one whose reduction will actually cap biomass; and because a large share arrives from one point source, it can be removed by ferric or alum dosing at the treatment plant or by diverting the outflow to a constructed wetland. Controlling nitrogen is a weaker lever: cyanobacteria can fix atmospheric N2 and will simply bypass the restriction.
Priority: phosphorus stripping at the plant. Expect a lag, because sediment releases stored phosphorus for years.
2. Explain how the surface water can be at 14 mg/L oxygen while fish die of suffocation. Account for the 05:00 reading and for why the deaths are mainly bottom-feeders.
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The surface reading is not an index of the whole lake. With a Secchi depth of 0.4 m, light is absorbed within the top fraction of the water column, so photosynthesis is confined there. At 16:00 that photosynthesis produces oxygen faster than it escapes, pushing surface water to 14 mg/L - above air saturation at summer temperature - which is why the pH also rises as CO2 is consumed.
Below the photic zone there is no photosynthesis, while settled algal cells, sewage solids and the sugar-mill load are decomposed by bacteria that consume oxygen. In a shallow lake in still, hot weather, thermal stratification prevents mixing, so the bottom layer is cut off from the surface and falls to 1.2 mg/L, below the roughly 4-5 mg/L that most fish need. Bottom-dwelling fish and snails cannot leave the zone and die; carp crowd the surface to use the oxygenated layer.
At 05:00 respiration has run all night without photosynthesis, so oxygen is at its daily minimum and may approach zero, which is often when the actual kills occur. Unionised ammonia, more toxic at pH 9.2, probably adds to the stress.
3. Evaluate the proposal to dose the lake with a copper algicide. Predict the sequence of events and propose two better interventions, ranked, with the expected timescale.
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The algicide treats the symptom and worsens the cause. Killing the bloom creates a large mass of dead algal cells that sinks and is decomposed by bacteria, consuming oxygen through the whole column and turning a bottom-layer problem into a lake-wide hypoxic event, with a second fish kill within days. Copper at algicidal doses is itself toxic to fish and invertebrates and accumulates in sediment. Lysis of cyanobacteria releases microcystins into water used for drinking and bathing. Because nitrogen and phosphorus remain, regrowth follows within weeks, so the exercise must be repeated and each round is worse.
Ranked interventions: first, nutrient removal at source - phosphorus stripping at the sewage plant or diversion into a constructed wetland - which addresses the load and is sustainable, taking one to three seasons to show in chlorophyll. Second, short-term hypolimnetic aeration or destratification to keep bottom oxygen above 4 mg/L while nutrient control takes effect. Sediment dredging is effective but costly and should wait until external loading is stopped, otherwise phosphorus simply returns.