The Lake, the Golf Course, and the Fish Kill
Students in an environmental science class are monitoring a 40-hectare lake that receives runoff from a golf course and from aging septic systems. From May to September they record: total phosphorus rising from 12 to 48 µg/L; chlorophyll a rising from 4 to 28 µg/L; Secchi depth falling from 2.4 m to 0.6 m; dissolved oxygen at 8 m falling from 8 mg/L to 2 mg/L; nitrate staying between 0.05 and 0.08 mg/L all summer. Surface temperature rises from 14 °C to 25 °C and the lake stratifies in July. A student writes, 'The fertilizer nitrogen is feeding the algae, and that is why the fish are dying.' The class also runs a two-week bottle bioassay with lake water: bottles spiked with phosphate reach chlorophyll a of 30 µg/L, bottles spiked with nitrate reach 8 µg/L, and controls reach 7 µg/L. The teacher wants them to evaluate the nitrogen claim and use the evidence to recommend a management action.
1. Evaluate the student's nitrogen claim using the monitoring data and the bioassay. Which nutrient is most likely controlling the algal bloom, and what is the evidence?
Read the worked answer
The nitrogen claim is not supported by the class's own evidence.
- Nitrate is low and flat. It stays between 0.05 and 0.08 mg/L all summer while total phosphorus rises fourfold (12 → 48 µg/L). If nitrogen were driving the bloom, it would be drawn down and would vary with algal biomass.
- Chlorophyll tracks phosphorus, not nitrate, across the season.
- The bioassay is a direct test. Phosphate addition raised chlorophyll a to 30 µg/L, while nitrate addition (8 µg/L) was barely above the control (7 µg/L). The bottle experiment points to phosphorus as the limiting nutrient.
In freshwater lakes, phosphorus is usually limiting because nitrogen can be supplied by biological fixation and atmospheric deposition, whereas phosphorus has no atmospheric reservoir and comes mainly from runoff, sewage, and sediment. The student also skips a step: even if nitrogen were involved, the fish kill needs a mechanism (oxygen depletion), not a direct toxic effect of fertilizer.
Caveat worth stating: a bottle bioassay is short-term and enclosed. The conclusion should be 'phosphorus is the most likely limiting nutrient,' not 'phosphorus is proven to be the only cause.'
2. Explain the causal chain from nutrient input to low dissolved oxygen at 8 m, including why the deep water is affected more than the surface.
Read the worked answer
The chain has four links.
- Loading. Runoff from the golf course and septic systems delivers phosphorus. Some is also released from anoxic sediments, a positive feedback that keeps the lake enriched even after external loads fall.
- Growth. Added phosphorus relieves the limiting nutrient, so algae (including cyanobacteria) multiply; chlorophyll rises from 4 to 28 µg/L.
- Light and sinking. Dense algae reduce Secchi depth from 2.4 m to 0.6 m, shading deeper water and submerged plants. Algae die and sink as organic matter.
- Decomposition and oxygen. Bacteria oxidize the sinking organic matter, consuming dissolved oxygen. In July the lake stratifies: warm, less dense surface water sits on cold, denser deep water and the layers do not mix. The hypolimnion at 8 m is cut off from the atmosphere and from photosynthesis, so its oxygen falls from 8 mg/L to 2 mg/L and keeps dropping. Fish that need more than about 2–3 mg/L suffocate or are squeezed into a shrinking layer.
The surface can still be oxygen-rich, even supersaturated in daytime, which is why the deep measurements matter most.
3. The bioassay is a bottle experiment. Identify two limitations of that approach and describe one additional field test that would strengthen the case for a management action.
Read the worked answer
Limitations of the bottle bioassay:
- Enclosure effects. Bottles exclude grazers, fish, and sediment, alter light and mixing, and allow algae to settle or stick to walls; a two-week bottle is not the lake.
- No true replication at the lake scale. There is one lake and one season, and bottles are subsamples of one water body, so the class cannot generalize from this design.
- Nutrient recycling. Bottles can recycle nutrients between water and algae, and the sediment - an important phosphorus source - is missing.
A stronger field test would be a replicated whole-lake or paired-lake experiment, or a before-after-control-impact design: reduce phosphorus loading in one basin (buffer strips, septic repair, alum treatment) while monitoring a comparable reference basin, with chlorophyll, Secchi depth, and deep dissolved oxygen measured over multiple years. A series of in-lake enclosures that include sediment and run through the stratified season would also help.
Management implication: because the evidence points to phosphorus, the priority is reducing phosphorus (fertilizer timing and rates, buffer strips, septic maintenance), not controlling nitrogen, and monitoring should continue to verify that the bloom responds.