Algal Bloom, Dawn Fish Kills and a Lake Used by Villagers
A 40-hectare freshwater lake receives untreated domestic sewage from a town of about 20,000 people. In summer the surface carries a thick green scum and Secchi depth falls to 0.3 m. Over 24 hours, surface dissolved oxygen is 9 mg/L at 3 p.m. and 2 mg/L at 5 a.m.; the bottom 2 m is anoxic throughout the day. Biochemical oxygen demand is 18 mg/L, nitrate 6 mg/L and phosphate 1.2 mg/L. Hundreds of small fish are found dead along the shore at dawn, while the same fish are seen gulping at the surface in the afternoon. Villagers bathe and wash clothes in the lake. The municipality proposes spraying an algicide and dredging the bottom mud.
1. Explain why fish die at dawn but are seen gulping air at the surface in the afternoon, using the oxygen cycle of the lake.
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The lake has two opposing oxygen processes. Algal photosynthesis produces oxygen only in daylight, peaking in the afternoon, which is why surface water reaches 9 mg/L at 3 p.m. - often above atmospheric saturation. Respiration by algae, plants, animals and, above all, bacteria decomposing dead algae consumes oxygen around the clock.
At night photosynthesis stops but respiration continues, so oxygen falls steadily from dusk to a minimum at dawn. The dense bloom and the high BOD of 18 mg/L mean that the bacterial oxygen demand is very large, and the measured 2 mg/L at 5 a.m. is below the tolerance of many fish. The dawn kill is therefore the direct result of nocturnal oxygen debt.
During the afternoon, oxygen is plentiful in the top layer but the bottom 2 m remains anoxic because light does not penetrate the turbid water and decomposition consumes oxygen there. Fish avoid the anoxic depths, crowd near the surface and gulp air; surface scum and high ammonia also cause distress. The afternoon gulping and the dawn deaths are two expressions of the same imbalance, not separate events.
2. The municipality's algicide will clear the scum. Explain why the bloom returns and why spraying can make oxygen levels worse, and why phosphate rather than nitrate is the priority target.
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An algicide kills algae but leaves the nutrients in the lake. Once the algaecide is diluted or degraded, the same phosphate and nitrate load supports a new bloom, so the effect lasts weeks at best.
Worse, the killed algae die and sink. Bacterial decomposition of this sudden mass of organic matter creates a sharp rise in oxygen demand, so dissolved oxygen can fall even lower than before, particularly at depth and at night, and another fish kill may follow. Lysis of cyanobacterial cells also releases toxins into water used for bathing and washing.
Phosphate is the priority because freshwater productivity is usually limited by phosphorus. Cyanobacteria can fix atmospheric nitrogen, so lowering nitrate does not stop them, while lowering phosphate does. Phosphate also tends to be sediment-bound and recyclable, so each unit removed has a lasting effect. Nitrate control is still needed eventually, but phosphorus removal gives the earliest and most reliable improvement.
3. Evaluate the municipality's plan and propose a sequence of measures that would actually work.
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Spraying algicide and dredging are both in-lake, symptom-level measures, and they are expensive and temporary. Dredging removes stored nutrients once, but sewage continues to add new ones; the sediment soon returns to its previous state. Algicide alone is counterproductive for the reasons already given. The village's use of the lake for bathing also makes toxin release a direct health risk.
The first measure must be to stop the nutrient inflow: intercept and treat the town's sewage in a plant with phosphorus removal, or divert it, and control phosphate in detergents. Only after the inflow is cut do in-lake measures make sense. These can include aeration or artificial mixing to prevent bottom anoxia, constructed wetlands or buffer strips to strip nutrients from runoff, harvesting of algal biomass, and biomanipulation such as removing bottom-feeding fish that release sediment phosphorus.
Monitoring of dissolved oxygen, Secchi depth, phosphate and algal toxins should guide the sequence, and villagers should be advised against bathing while scums and toxins persist.