Dissolved Oxygen in a Pond with an Algal Bloom
A shallow pond near a farm has dense green algae in summer. Students measure dissolved oxygen (DO) at the surface every few hours on a warm, sunny day: 6:00 3.2 mg/L; 10:00 6.8; 14:00 9.8; 18:00 7.1; 22:00 5.5. Water temperature stays between 18 and 24 °C. At dawn, several fish are seen gulping at the surface; by afternoon they are less active near the surface. The pond receives runoff containing nitrate and phosphate. A nearby unpolluted pond has DO between 6.5 and 8.0 mg/L all day. No measurements were taken below the surface.
1. Explain the daily pattern of dissolved oxygen and why the fish gulp at the surface near dawn.
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During daylight, algae and other producers photosynthesize and release O₂, so DO rises from morning to afternoon. At 14:00 the reading is highest because light has been strong for several hours.
After sunset, photosynthesis stops, but algae, animals, and decomposers continue respiring and consuming O₂. DO therefore falls through the night. The 6:00 reading is lowest because it follows the longest period without photosynthesis.
Fish gulp at the surface near dawn because low DO makes it hard to obtain enough oxygen. Surface water can also gain some oxygen by diffusion from the air, so fish crowd there when deeper water is especially low in oxygen.
2. Predict how a warm, cloudy day would change the 14:00 DO reading and the risk to fish by the next dawn. Justify your prediction.
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On a cloudy day, photosynthesis would be weaker, so the afternoon DO peak would be lower than 9.8 mg/L. Respiration would continue at a substantial rate, and warm water holds less dissolved oxygen than cool water.
With less oxygen produced during the day, the oxygen reserve entering the night would be smaller. Respiration through the night would still consume oxygen, so the next dawn DO could fall below 3.2 mg/L. The risk to fish would therefore increase, especially if the night is warm and the pond is poorly mixed.
3. The farmer claims the algae help the pond by producing oxygen. Use the data to evaluate this claim and propose one measurement that would help distinguish runoff-driven algal growth from untreated sewage.
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The claim is only partly true. Algae do produce oxygen in daylight, which explains the high afternoon DO. However, dense algae also respire at night, and dead algae are decomposed by oxygen-consuming microbes. The result can be a large daily swing with dangerously low DO before dawn, as the 3.2 mg/L reading shows. A dense bloom is therefore not simply beneficial.
To distinguish runoff from sewage, measure nitrate and phosphate concentrations in the pond and in incoming water. Farm runoff is likely to raise these plant nutrients strongly. Sewage may also add nutrients, but measurements such as ammonia, biochemical oxygen demand, or fecal bacteria would provide additional evidence. A below-surface DO profile would also show whether low-oxygen water extends beyond the surface.