Reading a stream survey: nutrients, algae and a dawn fish kill
A valley in Malta runs from a hillside down to the sea. For 20 years a pig farm has discharged nutrient-rich waste into the stream 50 m below a spring. A student surveys four sites on the same day in September: A, 100 m above the discharge; B, 50 m below it; C, 300 m below it; and D, 1 km below it, near the mouth. She measures nitrate concentration and dissolved oxygen (DO) twice, at 06:00 and at 15:00.
| Site | Nitrate / mg dm⁻³ | DO at 06:00 / mg dm⁻³ | DO at 15:00 / mg dm⁻³ |
|---|---|---|---|
| A | 3 | 8.5 | 9.0 |
| B | 18 | 3.2 | 11.5 |
| C | 14 | 2.4 | 13.2 |
| D | 6 | 5.1 | 8.2 |
Water temperature is 22 °C at all four sites at both times. Residents reported a fish kill at site C at about 04:30 that morning. At 15:00 the stream bed at C is bright green with thick algal mats.
1. Explain the dissolved oxygen pattern at site C across the day, and explain why the fish died at about 04:30 rather than in the afternoon.
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The nitrate from the farm acts as a fertiliser, so algae grow heavily - hence the bright green mats. Algae are plants: in daylight they photosynthesise and release oxygen faster than the whole community respires it, so DO climbs to 13.2 mg dm⁻³, above what the water would hold in equilibrium with air at 22 °C. At night photosynthesis stops, but the algae, and the bacteria decomposing dead algae, keep respiring and consuming oxygen, so DO falls steadily all night.
By 04:30, after roughly ten hours without any photosynthetic oxygen input, DO was at its lowest point of the day - below the 2.4 mg dm⁻³ measured at 06:00, since 06:00 came after first light. Fish need several mg dm⁻³ to survive; below about 2–3 mg dm⁻³ they suffocate. Warm water makes this worse because it holds less dissolved oxygen to begin with.
This is why the kill was not in the afternoon: at 15:00 the water was oxygen-rich. The danger period in a nutrient-enriched stream is always the hours before dawn.
2. Nitrate falls from 18 mg dm⁻³ at B to 6 mg dm⁻³ at D. Does this show the stream is recovering? Justify your answer using the data.
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No - it shows the nutrient has moved, not disappeared.
The nitrate is taken up by algae and rooted plants as they grow, and some is converted to nitrogen gas by bacteria in the sediment. That removes it from the water, but the nitrogen is now stored in algal biomass.
Three pieces of evidence say the stream has not recovered. First, D still has 6 mg dm⁻³ nitrate, double the 3 mg dm⁻³ at the unpolluted site A. Second, DO at D at 06:00 is 5.1 mg dm⁻³, well below the 8.5 mg dm⁻³ at A - the oxygen demand has followed the algae downstream. Third, the mechanism itself predicts this: algae die and drift, so the decay and the bacterial oxygen consumption occur further down, not at the source.
A real recovery would look like nitrate close to 3 mg dm⁻³ and a dawn DO close to 8 mg dm⁻³ at all sites below the farm. The student should also record algal cover at D, which would test the claim directly.
3. Suggest one management measure for the farm, and predict its effect on DO at site C at 06:00. Explain the reasoning, including any delay you would expect.
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Measure: stop the waste entering the stream - store the slurry in a sealed tank and spread it on fields at agronomically sensible rates, or pass the discharge through a constructed wetland/buffer strip of reeds before it reaches the stream. Either keeps the nitrate out of the water.
Prediction: with less nitrate, algal growth is limited and the algal mats at C shrink. Less algal biomass means less respiration at night and less dead material for bacteria to decompose, so the overnight oxygen consumption falls. DO at 06:00 at C should climb from about 2 mg dm⁻³ towards the 8 mg dm⁻³ seen at the clean site A, and the dawn oxygen crash that killed the fish should stop.
Delay: the improvement will not be immediate. Existing algal mats must die, drift away and be decomposed, and the sediment's oxygen demand can persist for weeks. The first sign would be a narrowing of the gap between the 06:00 and 15:00 readings, before the absolute 06:00 value rises.