Fish Kill in the Village Pond
Students at a school beside a village pond record water data for a science project. On 12 June, after three days with afternoon temperatures near 38 °C and a night-time thunderstorm that washed fertiliser from a maize field into the pond, they find 40 dead tilapia at dawn. Dawn readings: water temperature 31 °C, dissolved oxygen (DO) 3.1 mg/L, pH 7.2. Afternoon readings the same day: temperature 33 °C, DO 9.8 mg/L, pH 8.6. Ten days earlier, before the rain, the same pond read 27 °C, DO 6.4 mg/L at dawn, pH 7.4. A control pond 300 m away, with no farmland beside it, reads 4.9 mg/L at dawn and shows no deaths. The club has 12,000 taka and can fund exactly one measure: a night-time aerator, or clearing the water hyacinth that covers about a third of the pond surface. The teacher asks for a decision with evidence, not opinion, and warns that whatever they choose will make it harder to test the fertiliser explanation later.
1. Using the readings, explain why the fish died at dawn rather than in the afternoon.
Read the worked answer
The fish died when oxygen was lowest, and the data locate that moment precisely: DO fell from 9.8 mg/L in the afternoon to 3.1 mg/L before sunrise on the same day, a swing of 6.7 mg/L in under 24 hours.
Three factors combine. First, gas solubility falls as temperature rises; at 31–33 °C the water holds markedly less oxygen than at the 27 °C of ten days earlier, even with no biology involved. Second, the fertiliser washed in by the storm feeds algae; by day those algae photosynthesise and flood the water with oxygen (hence 9.8 mg/L and the pH rise to 8.6, as photosynthesis removes dissolved carbon dioxide), but at night photosynthesis stops and respiration continues, so the same bloom strips oxygen out. A bigger bloom therefore produces both the highest afternoon peak and the deepest dawn crash. Third, the control pond, with no farmland beside it, never fell below 4.9 mg/L, which points to the runoff rather than to heat alone.
The afternoon reading is the trap: high DO at 3 p.m. is not good news if it collapses before dawn.
2. Design a two-week monitoring plan that would actually test whether the fertiliser caused the low dawn oxygen.
Read the worked answer
The plan must compare places and times while changing one thing at a time.
Sample DO at 05:00–06:00 daily - the critical hour revealed by the data - at three points in the pond (where runoff enters, the middle, and the outlet) and at the same hour in the control pond. Record water temperature and pH at each reading, because both move with the algal cycle. Measure nitrate and phosphate once at the start and once at the end, so the fertiliser link is chemical as well as physical. Log rainfall and daytime maximum temperature daily as context variables.
Crucially, do not run the aerator during these two weeks. An aerator would raise dawn DO by an amount the students cannot separate from the effect of the fertiliser, and the experiment would answer nothing. If the aerator must run for animal welfare, restrict it to the control pond or accept that the test is weakened and say so in the report.
The prediction is testable in advance: if fertiliser is the cause, dawn DO should be lowest at the inflow point and should fall further after any additional rain, while the control pond stays above 4.5 mg/L. Students should write this prediction down before collecting data.
3. Should the club spend the 12,000 taka on the night aerator or on clearing the water hyacinth? Justify the choice and state its cost.
Read the worked answer
Buy the aerator, run at night, because it attacks the mechanism actually observed: an oxygen deficit in the hours before dawn. The data show that daytime oxygen is already plentiful (9.8 mg/L), so aeration during daylight would add nothing; night-time aeration lifts the minimum, which is what kills fish.
The hyacinth option looks attractive but is weaker on the evidence. Floating mats block light and stunt the submerged plants that would otherwise oxygenate the water, and the cleared weed rots, consuming oxygen - so the first effect of clearing may be another dip. It also regrows, meaning the 12,000 taka is spent again next season.
The honest cost: running the aerator will mix the water and raise dawn DO, which destroys the clean baseline the club wanted for the fertiliser test in the second task. The club must choose between protecting the fish and finishing the experiment; the defensible answer is to protect the fish now, record that decision openly, and run the comparison later against the control pond, which remains untreated.