Lake Algal Bloom: Nutrient Limitation and Oxygen Swing
A lakeside town is concerned about a summer algal bloom. Monitoring on a calm, sunny day gives surface water temperature 26 °C, total phosphorus 0.09 mg L⁻¹, total nitrogen 1.2 mg L⁻¹, and Secchi depth 0.8 m. Dissolved oxygen at the surface is 12 mg L⁻¹ at 15:00 but 4 mg L⁻¹ at 05:00. The lake stratifies in summer, and the town's wastewater plant discharges treated effluent rich in phosphate. Nearby farms use nitrogen-rich fertiliser. A consulting firm proposes adding aluminum sulfate (alum) to bind phosphorus in the lake sediment, while a citizens' group proposes planting vegetated buffer strips along streams and upgrading the wastewater plant. The town council wants to know which nutrient is most likely limiting, why oxygen swings so widely, and how to compare the two proposals.
1. Calculate the mass N:P ratio and infer the most likely limiting nutrient. State a caveat about this inference.
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Mass N:P = 1.2/0.09 = 13.3:1. The Redfield mass ratio for algae is about 7.2:1 N:P (molar 16:1). A water N:P of 13.3 is above 7.2, meaning nitrogen is relatively abundant compared with phosphorus; phosphorus is the more likely limiting nutrient. This is consistent with the wastewater effluent being phosphate-rich and with the proposed alum treatment.
Caveat: total N and total P include forms not available to algae, and the ratio alone does not prove limitation. A bioassay, adding N or P to lake water and measuring algal growth, or measurement of soluble reactive phosphorus would strengthen the conclusion. If the bioassay showed growth with N addition, the conclusion would change.
2. Explain why dissolved oxygen is 12 mg L⁻¹ at 15:00 but 4 mg L⁻¹ at 05:00, and why the 05:00 value matters for fish.
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Dissolved oxygen rises during the day because photosynthesis by algae produces O2 and consumes CO2. At night, photosynthesis stops but algae, bacteria and other organisms continue to respire, consuming O2. Just before dawn, respiration has been removing O2 for many hours without replacement, so DO reaches its minimum. The 12 mg/L afternoon value may include supersaturation from intense photosynthesis; the 4 mg/L pre-dawn value is the more critical for fish.
Warm water at 26 °C holds less dissolved oxygen than cold water, and the bloom's respiration, plus bacterial decomposition of dead algae, increases oxygen demand. A pre-dawn DO of 4 mg/L is stressful for many fish and invertebrates; if it falls below about 2 mg/L, fish kills become likely. The wide swing itself is evidence of high biological activity, not necessarily clean water.
3. Compare alum treatment with buffer strips and wastewater upgrading. Which proposal is more defensible, and what evidence would you require before choosing?
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Alum binds phosphate and can lower algal growth quickly, but it treats the symptom. It is a recurring cost, the floc can smother benthic habitat, and if the lake becomes anoxic, phosphorus may be released again from sediment. It also does not reduce nitrogen or the cause of the bloom. Buffer strips and wastewater upgrading reduce phosphorus inputs at source and are more sustainable, but they take time, require maintenance and enforcement, and may be expensive.
A good comparison would use: capital and operating costs; expected kg P removed per year; time to effect; ecological risks; and monitoring. The best plan is likely source control first, with alum as a temporary measure only if the lake is already severely impaired. The council should also monitor soluble reactive phosphorus and DO before and after any intervention.